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J. Biol. Chem., Vol. 278, Issue 35, 33000-33010, August 29, 2003
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**
From the
Banting and Best Department of Medical
Research and Department of Molecular and Medical Genetics, University of
Toronto, Toronto, Ontario M5G 1L6, Canada, the
¶Department of Molecular Biology and
Microbiology, Tufts University School of Medicine, Boston, Massachusetts
02111, the ||Department of Biological Chemistry and
Molecular Pharmacology, Harvard Medical School, Boston, Massachusetts 02115,
and **Affinium Pharmaceuticals, Toronto, Ontario M5J
1V6, Canada
Received for publication, April 29, 2003 , and in revised form, June 19, 2003.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONEXPERIMENTAL PROCEDURESRESULTSDISCUSSIONREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL PROCEDURESRESULTSDISCUSSIONREFERENCES |
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Transcription by RNAPII has been functionally linked with maturation of the newly synthesized mRNA in both lower and higher eukaryotes (10–12). In particular, mutations in either cis-elements required for cleavage (13) or in genes encoding cleavage factors (14, 15) result in defective termination by RNAPII, indicating that termination of transcription by yeast RNAPII is coupled with 3'-end formation. Direct associations between the C-terminal domain (CTD) of the largest subunit (Rpb1) of RNAPII and CFIA (16) or CPF (17–19) have been described. Rna15, a component of CFIA, has been found to interact with Sub1 and Mbp1, two factors that have roles in initiation of transcription by RNAPII (20, 21). Consistent with these observations, chromatin immunoprecipitation (ChIP) assays have revealed that the recruitment of yeast 3'-end processing proteins in CFI and CPF is co-transcriptional (12, 22) and requires the RNAPII CTD (12).
In this study we used a systematic proteomic approach to identify components of the yeast cleavage and polyadenylation machinery and factors that link 3'-end processing to the transcription machinery. We present a comprehensive description of the polypeptides associated with CPF and describe various protein-protein interactions between subunits of CPF. In addition, we describe functional assays suggesting new roles for various components of CPF.
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EXPERIMENTAL PROCEDURES |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL PROCEDURESRESULTSDISCUSSIONREFERENCES |
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derivatives of
BY4741(REF2+ and ref2::Kan)
(Research Genetics), constructed by integration and counter-selection of the
pNKY1009 trp deleter vector
(24). TAP-tagged strains
(including C-terminal truncations) were constructed as described
(25,
26) and subsequently verified
by Western blotting using a rabbit anti-Werner syndrome protein primary
antibody and anti-rabbit horseradish peroxidase-coupled secondary antibody.
Purifications of TAP-tagged proteins were performed from 4l of cells grown in
YPD to an A600 of 1.5 essentially as described
(25,
27) with minor
modifications.
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RNA Analysis—Total RNA was isolated by the hot phenol method (30). For each primer extension reaction, 10 µg of total RNA and 200 fmol of 5'-32P-labeled primer were denatured in 10 µlof1x First Strand Buffer (FSB) (Invitrogen) (50 mM Tris-HCl, pH 8.3, 75 mM KCl, 3 mM MgCl) for 2 min at 90 °C and annealed for1hina41 °C water bath. The reaction was started by adding 10 µl of 1x First Strand Buffer containing 20 mM dithiothreitol, 0.1 mM dNTPs, 20 units of RNasin (Promega), 100 units of SuperScriptII-RNaseH (Invitrogen) and stopped after 1 h with an equal volume RNA loading dye containing 90% formamide. Samples were separated on a 6% acrylamide, 7 M urea polyacrylamide gel. The amount of reverse-transcribed U6 loaded was adjusted to match the signal given by snoRNA cDNAs. The primers specific for sequences downstream of the mature snoRNAs have been described (31). The primer used for assessing readthrough of snR33 was 5'-GCAATGGTGCAGATTGTGTCAACTC-3', whereas the primers used for reverse transcription at the CUP1 locus were CUP1INT (5'-TCATTTCCCAGAGCAGCATGACTTCTTGG-3') and CUP1POST 3'-END (5'-GGATTCTATACAGAGTTGTAAGTTAGGC-3'). Gene expression profiling using a yeast ORF oligonucleotide set was performed essentially as described (32).
-Galactosidase Assay—Three or four independent
colonies transformed with various reporter vectors (pHZ182-no
terminator; pL101-ADH2 terminator; pL501-GAL7 terminator)
(33,
34) were grown in synthetic
medium lacking uracil containing 2% galactose to an A600
0.6 and assayed as described
(35).
Chromatin Immunoprecipitation—Cells were grown in YPD to an A600 of 0.6–0.8 and processed essentially as described previously (36). Chromatin solution was incubated overnight at 4 °C with 20 µl (50% slurry) of rabbit IgG-agarose beads (Sigma) pre-washed with TE buffer (10 mM Tris-HCl, pH 8.0, 1 mM EDTA). The immunoprecipitate was washed stringently, and the recovered chromatin as well as the input chromatin was de-cross-linked and amplified by PCR as described (36). Various segments of a gene were amplified after ChIP as illustrated in Fig. 6. The 3'-ends of snoRNA genes amplified are as follows: +118 to +306 for snR13 and +84 to +278 for snR71. Numbering is with reference to the translational start for ORFs or the 5'-end of the RNA for snoRNA genes. The most downstream primers used for PCR amplification of snR13 and snR71 3'-ends annealed up to 96 nucleotides before the translational start of TRS31 and up to 121 nucleotides inside the coding region of the convergently transcribed LIN1 gene, respectively.
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RESULTS |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL PROCEDURESRESULTSDISCUSSIONREFERENCES |
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To test its association with the mRNA cleavage machinery, Ref2 was derivatized at its C terminus with a tandem affinity purification (TAP) tag (25), containing two IgG-binding modules of protein A and a calmodulin-binding peptide, by recombining tag-encoding DNA into the chromosomal REF2 gene. Hence, a normal expression level of the Ref2 protein was maintained, allowing isolation of physiologically relevant protein complexes. Ref2-containing protein complexes were then purified sequentially on IgG and calmodulin columns and analyzed by SDS-PAGE and silver staining as presented in Fig. 1A (left panel), which shows that Ref2-Tap is specifically associated with a large multiprotein complex. Moreover, the association is not likely mediated by RNA as RNase A treatment (10 µg/ml) had little effect on the pattern of associated proteins (Fig. 1A, lanes 3 and 4). Upon identification of the polypeptides co-purified with Ref2 by matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry (MS) (38) and peptide-mass fingerprinting (39), the Ref2-containing complex, holo-CPF, proved to be a larger version of the originally described CPF complex. The original CPF complex contained CFII subunits (Cft1, Cft2, Ysh1, and Pta1) and Pap1 and PF1 subunits (Fip1, Yth1, and four other unidentified polypeptides, simply designated p58, p53, p37, and p35) (4, 40). p53 was later identified as the polyadenylation factor subunit (Pfs) 2 (40), required for both mRNA cleavage and polyadenylation. The Ref2-containing complex shown in Fig. 1 contained all the originally characterized components of CPF as well as seven new components also recently identified by several other groups (17, 41–44). Three of them were similar in size to the uncharacterized p58, p37, and p35 components of PFI and were identified as Ykl059c, Ykl018w, and Glc7, respectively. In addition, our holo-CPF complex included three other polypeptides not originally shown to be associated with the yeast cleavage and polyadenylation machinery, namely Ygr156w, Ssu72, and Yor179c. We have given the name Syc1 (Similar to Ysh1 C-terminal) to Yor179c. Ygr156w and Ykl018w are also known as Pti1 and Swd2.
Very similar holo-CPF complexes have been described recently (17, 41–43). There are only small discrepancies between their findings and ours. For example, we and Gavin et al. (41) identified Ref2 as a band that migrates just above Pap1 rather than below it (17). On the other hand, we and Dichtl et al. (17) positioned Fip1 just below Pfs2 and above Pti1 rather than below (41). In addition, Dichtl et al. (17) did not identify Syc1 but instead identified a protein of unknown function of similar size encoded by the non-essential gene, YDL094c. Walsh et al. (42) described a CPF complex lacking Ssu72, Pti1, and Syc1. He et al. (43) very recently also described a complex identical in composition to ours.
Ssu72 is a factor that affects start site selection during transcription
initiation. Ssu72 (suppressor of sua7, gene 2) interacts both
genetically and physically with the general transcription factor TFIIB,
encoded by the SUA7 gene, and the second largest subunit of RNAPII
(45,
46). Its presence in the
holo-CPF complex was confirmed by tagging and purifying the Ssu72 protein, as
shown in Fig. 1A
(lane 2). The ssu72-1 and sub1 alleles
interact genetically in an allele-specific manner with the same set of
sua7 alleles, suggesting that there could be a functional
relationship between them
(47). Interestingly, Sub1 was
proposed to interact with Rna15, a subunit of CFIA, and suppress termination
by RNAPII (20).
Ygr156w (Pti1) is encoded by an essential gene (48), contains an RNA recognition motif, has weak similarity with human CSTF2 outside its RNA recognition motif, and was shown very recently to have a role in 3'-end formation (49). The N- and C-terminal regions of Pti1 and Rna15 are also weakly similar. Glc7 (GlyCogen-deficient 7) is the only essential type I Ser/Thr protein phosphatase of S. cerevisiae and participates in many cellular biochemical pathways by interacting with a myriad of regulators (50). Ykl059c, probably p58 in the original CPF, is an essential protein recently described as Mpe1 (mutant PCF11 extragenic suppressor) and required for both steps of pre-mRNA 3'-end processing (51). Ykl018w (Swd2), also a component of the Set1 complex (52, 53), is indispensable for viability in yeast (48) and contains multiple WD repeats (seven, according to the Protein Structure Analysis server at bmerc-www.bu.edu/psa/request.htm), a motif found in proteins involved in a variety of cellular functions and often components of multiprotein complexes (54). Yor179c (Syc1) is a non-essential protein (48), whose amino acid sequence is highly similar to that of the C-terminal region of the CFII component Ysh1 (35% identity and 52% similarity over 199 amino acids).
Pti1, Swd2, Mpe1, and Syc1 Are Genuine Components of CPF—In order to confirm our findings, polypeptides identified in holo-CPF that were not originally known to be subunits of CPF were individually tagged and purified as described above. As the salt concentration during our purifications had a significant impact on the yield of some purified polypeptides2 (also see below), we carried out a series of purifications at 100 and 150 mM NaCl with tagged Pti1, Swd2, Mpe1, and Syc1 as well as Ssu72 and Ref2. A strain containing tagged Pta1, a known CFII subunit of CPF, was used as a reference. The results, shown in Fig. 1B, indicated that the new putative CPF-associated factors described here and elsewhere (17, 41–44) are genuine components of CPF. Full-length Pti1 and Mpe1 were purified less efficiently than the other CPF components, but their C-terminal truncated versions were purified efficiently and showed the complete holo-CPF composition2 (Fig. 3), suggesting that the tags on the intact proteins might cause some steric hindrance. As also described elsewhere (42), Glc7-TAP co-purified with Pta1, a component of CFII, which we identified by MS, and many other polypeptides.2
Interestingly, the purification of tagged Syc1 yielded the six new polypeptides present in holo-CPF (Ref2, Pti1, Swd2, Glc7, Ssu72, and Syc1) (see also Refs. 17 and 41–44), but none of the previously identified CFII or PFI components except Pta1 (Fig. 1B, lanes 14 and 15, and Fig. 2B, lane 2). Accordingly, we have named this subcomplex APT (Associated with Pta1). It is possible that there are two complexes, APT and core-CPF (containing only CFII, Pap1, and the PFI components Mpe1, Pfs2, Fip1, and Yth1) (40), that dynamically associate to form holo-CPF. Alternatively, the binding to solid supports of tagged Syc1 during its purification may lead to the dissociation of the APT complex from the other components of CFII and PFI. Regardless of which interpretation is correct, this observation indicates that the six new CPF-associated polypeptides described here and elsewhere (17, 41–44) are able to associate with each other and Pta1 in the APT subcomplex.
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CF IA Associates Weakly with CPF—In the course of these experiments, we also obtained evidence that CFIA associates with CPF. First, we found that Rna14, a subunit of CFIA, was present and identified by MALDI-TOF MS, as indicated by the asterisks in lanes 2 and 12 of Fig. 1B, when Pta1 and Ssu72 were tagged and purified in buffers containing 100 mM NaCl, although the yield of Rna14 was clearly substoichiometric. A similar result was found for Pti1-TAP (Fig. 3C, lane 2). Second, when the CFIA subunits, Pcf11 (see Fig. 2A) and Rna15,2 were tagged and purified in buffers containing 100 mM NaCl, substoichiometric amounts of various CPF subunits were visible by silver staining and three of them, Ysh1, Pta1, and Pfs2, were identified by MS along with the CFIA-specific subunits Rna14, Rna15, Pcf11, and Clp1. Based on these data, it seems likely that CFIA is weakly associated in vivo with core-CPF and holo-CPF and tends to be separated from them during our purification procedure. An association between CFIA and core CPF is supported by other biochemical and genetic data (40, 55). A complex containing holo-CPF and CFIA would have all the known components required for a standard, accurate cleavage and polyadenylation reaction, except CFIB (Hrp1/Nab4) and the poly(A)-binding protein Pab1 (see Fig. 2C). Interestingly, these latter two proteins, as well as the nuclear import protein Kap104, co-purify in buffers containing 100 mM salt (but not 150 mM) when Hrp1/Nab4 is used as the tagged protein.3
Distinct Core-CPF and Holo-CPF Complexes—That there are distinct core-CPF and holo-CPF complexes is supported by the experiment shown in Fig. 2B. In this experiment, originally identified (lanes 6–9) and recently identified (lanes 2–5) components of holo-CPF were tagged and purified in parallel. It is apparent that when various components of core-CPF were tagged and purified, the purified complexes contained mainly CFII (Cft1, Cft2, Ysh1, and Pta1), Pap1, and PFI (Mpe1, Pfs2, Fip1, and Yth1), the components of core-CPF. The APT-specific components were clearly obtained in substoichiometric yield compared with the core-CPF-specific polypeptides. In contrast, when the APT-specific polypeptides were tagged and purified, all the holo-CPF polypeptides were obtained in roughly similar amounts, although the silver-staining procedure precluded a strictly quantitative analysis. In these instances, it was also noticeable that Pta1 was obtained in higher yield than the other CFII-specific subunits Cft1, Cft2, and Ysh1 (compare lanes 2–5 with 6–9), suggesting that Pta1 may be associated with both core-CPF and APT. It is striking that holo-CPF can be obtained by simply combining core CPF with the APT subcomplex. A model describing these findings is presented in Fig. 2C.
Ref2 Recruits Glc7 and Swd2 to the Holo-CPF—The Ref2
requirement for in vitro cleavage of pre-mRNA with suboptimal
polyadenylation sites has been linked to its last 200 amino acids
(37). We investigated the
importance of the C terminus of Ref2 for the integrity of holo-CPF by
constructing and purifying truncated TAP-tagged versions of Ref2 missing the
last 100 amino acids (Ref2100-TAPp) or last 200 amino acids
(Ref2200-TAPp). As seen in Fig.
3A, lane 6, Ref2100-TAPp lost contact
with all CPF subunits except Glc7, consistent with the observation that there
is a two-hybrid interaction between Ref2 and Glc7
(56). An additional 100-amino
acid deletion disrupted the interaction between Ref2 and Glc7
(Fig. 3A, lane
7), implying that residues 333–433 of Ref2 are required for its
interaction with Glc7. A putative type I protein phosphatase-binding site,
similar to the consensus site (KR)X
(RHQKAMNST)(VI)(RHSATMK)(FW)X{0,3}(END)
(58), is found in this region
(amino acids 368–376). These experiments not only show that Ref2 and
Glc7 interact directly in vivo, but also strongly suggest that Ref2
is probably the sole interacting partner for Glc7 in CPF, as no other protein
co-purifies along with full-length Glc7 and Ref2100-TAP. If this were
true, one would expect that holo-CPF would lack Glc7 in a strain lacking Ref2.
In order to test this hypothesis, we TAP-tagged and purified Pti1 in a
ref2 background. As shown in
Fig. 3C, lane
3, the type I protein phosphatase was no longer present in CPF,
validating our initial finding. In addition, the lack of Swd2 in this complex
suggests Ref2 interacts with Swd2 and excludes a strong interaction between
Pti1 and either Glc7 or Swd2.
The requirement of Ref2 for Swd2 recruitment to CPF is further supported by
the fact that Swd2-TAP purification in a ref2 background
yields no CPF components (Fig.
3D, lane 2). Instead, a recently described
Set1-containing complex (52,
53) contained Swd2. This
suggests that the two complexes compete for their common component, Swd2. As
Swd2-TAP purification in a REF2+ strain yielded mainly
CPF, most Swd2 is probably bound to the cleavage machinery in
vivo.
Pti1 and Pta1 Are Important Scaffold Subunits in CPF—In order to analyze the role of Pti1 in the architecture of CPF, we used the same strategy as for Ref2. Intriguingly, deletion of the last 1172 or 160 (see Fig. 3A, lane 2) amino acids of Pti1 did not result in any alteration of the composition of CPF, ruling out a structural role for the missing C-terminal end. However, when a further 88 amino acids were deleted, leaving only amino acids 1–174, a dramatic change took place, the only CPF components still co-purifying with Pti1 being Ref2, Swd2, and Glc7 (Fig. 3A, lane 3). A TAP-tagged strain with the C-terminal 248 amino acids of Pti1 deleted, leaving amino acids 1–174 of Pti1 fused to TAP, was viable. Because it was recently found (49) that deleting amino acids 185–260 of Pti1 was lethal, the TAP tag on Pti1-(1–174) may stabilize Pti1 and partially preserve its function.
An interpretation of these data is shown in
Fig. 4. Our results exclude
stable interactions between Ref2, Glc7, or Swd2 on the one hand, and CPF
components other than Pti1. Rather, it would appear that Pti1 is a central
adaptor that bridges different modules in the 1-MDa holo-CPF complex. As
Ref2, Glc7, Pti1, and Swd2 form a structural module in which Ref2 interacts
with Glc7 and Pti1 interacts with neither Swd2 nor Glc7, it follows that Ref2
contacts both Swd2 and amino acids 1–174 of Pti1. This conclusion is in
accord with the requirement of Ref2 for the presence of Swd2 in CPF. An
interaction between Pti1 and Ref2 by two-hybrid analysis was also reported
very recently (44).
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Similarly, when a version of Pta1 with a 200-amino acid C-terminal
truncation was purified, an important change in the CPF composition occurred
(Fig. 3B). In this
case, mainly Ref2, Pti1, Swd2, Glc7, Yth1, and Ssu72 co-purified with the
tagged Pta1200. As Ref2, Glc7, and Swd2 do not interact with Pta1 (see
the explanations above), and the purification of Syc1 does not result in the
co-purification of Yth1 (Fig.
1B and
2B), it seemed very
likely that Pti1 either directly contacts Pta1 or uses Ssu72 as a bridge to
Pta1 (see Fig. 4). As well,
this result implied that the Pta1 subunit of CFII may interact with the Yth1
subunit of PFI.
The roles of Pti1 and Pta1 in our model for the organization of CPF (Fig. 4), as inferred from our data on the co-purification of various proteins from whole cell extracts, were also examined by in vitro protein interaction assays using recombinant proteins. Full-length Pti1 interacted with both Ssu72 and Pta1, in good agreement with the predictions from the TAP data (Fig. 3E). In addition, we found that amino acids 1–90 of Pti1 were dispensable for its interaction with Pta1 (Fig. 3F). As a truncated form of Pti1 containing amino acids 1–262 efficiently bound both Pta1 and Ssu72 in vivo, and amino acids 1–174 were not sufficient for Pti1 to interact with Pta1 or Ssu72 in vivo or in vitro (Fig. 3, A and F), it seems likely that the region of Pti1 containing amino acids 178–262 is required for its interaction with both Ssu72 and Pta1.
Similar Pti1-binding experiments were also carried out with a number of other proteins. Binding interactions were found with Clp1 (Fig. 3E) and Pcf11,3 both of which are subunits of CFIA, suggesting that Pti1 interactions with Clp1 and Pcf11 may at least partly mediate the weak interaction between CFIA and CPF (see Fig. 2C). No interaction or only weak interactions, which may be nonspecific, were found for Pti1 with Rna14, Rna15, Yth1, Pap1, Brr5, Fip1,3 Mpe1, and Sub1 (Fig. 3E).
Our observations that Pti1 directly bound Pta1 and Ssu72 and that tagged Pti1 co-purified efficiently only with subunits of CPF, rather than with subunits of CFIA, suggest that Pti1 is an integral component of holo-CPF. It was recently suggested, however, that Pti1 is in an altered form of CFIA, in which it replaces Rna15, on the basis of weak sequence similarity between Pti1 and Rna15, as well as in vitro protein-protein interactions between Pti1 and either Pcf11 or Rna14 (49). It seems to us unlikely, however, that Pti1 is a component of CFIA, as Pti1 did not co-purify with tagged Pcf11 in amounts high enough to be detected by silver staining in our purifications2 (see also Fig. 2A). Components of a tagged protein complex must be stably associated in order to survive the dilutions associated with two affinity purifications. On the other hand, binding in vitro to a concentrated ligand may reflect weak binding with a Kd value as high as 10–5 M. Therefore, we favor the conclusion that Pti1 is a stable component of CPF that associates only weakly with subunits of CFIA.
Ssu72 Is Important for Preventing Readthrough in Vivo to Downstream Genes—The association of a putative transcription initiation factor, Ssu72, with the cleavage and polyadenylation machinery was very intriguing. We asked whether Ssu72 plays a role in mRNA 3'-end formation by taking advantage of the fact that a strain containing a C-terminally TAP-tagged Ssu72 showed a mutant phenotype (slow growth at 30 °C and temperature sensitivity at 37 °C). A strain containing N-terminally TAP-tagged Ssu72 (N-TAP-Ssu72) has no growth defect (at 30 or 37 °C) and N-TAP-Ssu72 associates with the same set of polypeptides as Ssu72-C-TAP.2 The growth defect induced by ssu72-C-TAP was rescued by the wild type gene expressed from a low copy vector (see Fig. 5A).
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To test whether SSU72 might be involved in 3'-end formation
for polyadenylated transcripts, we used a previously described system
(34) that allows one to
evaluate the efficiency of mRNA cleavage and/or transcription termination by
RNAPII in vivo by monitoring the levels of -galactosidase. In
this assay, briefly summarized in Fig.
5B, a vector containing the lacZ gene fused
in-frame to an exon that follows a modified rp51 intron is used to
assess the expected levels of transcription (100% readthrough). In a second
vector, a wild type polyadenylation signal is inserted within the
rp51 intron to ensure cleavage and polyadenylation, as well as
termination, and prevent transcription of the downstream lacZ gene.
Inefficient transcription termination and/or mRNA cleavage is detected when
the downstream lacZ gene becomes transcribed and -galactosidase
is synthesized.
Our C-TAP allele of ssu72 showed a significant readthrough (21 and
10%) at semipermissive temperature (28 °C) for two polyadenylation
signals, derived from the ADH2 and GAL7 genes, respectively,
that efficiently suppress readthrough (<2%) in a wild type strain (see
Fig. 5C). An allele
(pcf11-9) of a CFIA subunit known to be involved in cleavage and
termination (14) was used a
positive control and caused substantially greater readthrough (70 and 40%). As
a signal becomes apparent only after all or most of the lacZ gene is
transcribed, it is possible that readthrough values may be underestimated
(i.e. it is possible that a population of transcription complexes may
terminate before the end of lacZ is reached). At this point, we
cannot readily distinguish whether the role of Ssu72 in preventing readthrough
is direct or else is mediated indirectly by cleavage of the transcript
followed by exonucleolytic degradation of the downstream RNA. Indeed, Ssu72
was recently shown to be required for the cleavage reaction for at least some
pre-mRNA substrates (43). This
same study, using different mutations in Ssu72, did not detect an effect of
Ssu72 on termination by RNAPII using a nuclear run-on assay, which is a more
direct measure of termination by RNAPII
(43). Therefore, it seems
likely that the readthrough effect of our ssu72-C-TAP allele reflects
inefficient cleavage of the transcript and consequent stabilization of the
downstream readthrough RNA. It is possible, of course, that Ssu72 is required
for termination downstream of some poly(A) signals but not the poly(A) signal
of the CYC1 gene studied by He et al.
(43). Mutations we tested in
other members of the CPF complex that showed various defects in vivo
(e.g. growth rate) or ex vivo (TAP purification)
(ref2, pta1200TAP, and
pti1248TAP) did not show a significant level of
readthrough for either of the polyadenylation signals that we
tested.2
As an additional test for the participation of Ssu72 in normal termination and/or 3'-end formation, we examined whether readthrough occurred at the CUP1 locus, commonly used as a sensor for deficient mRNA 3'-end formation. By using reverse transcription analysis, we were able to show that transcripts extended beyond the normal cleavage site (59) to a higher extent (more than 2-fold) in our ssu72-C-TAP mutant even at the semi-permissive temperature of 30 °C (see Fig. 5D).
We next investigated the possibility that Ssu72 may also be important for
3'-end formation of another class of transcripts generated by RNAPII,
namely snoRNAs. It has been shown that CFIA subunits, but not the originally
identified CPF components, seem to be involved in snoRNA processing
(60,
61). By using primer extension
analysis to quantify the amounts of readthrough fusion transcripts, we found
that our ssu72-C-TAP allele shows significant readthrough when
compared with wild type at several snoRNA loci, snR13, -39b, -50, -71
and -128, that we tested (see Fig.
5E), but no discernible alteration at several other loci
(snR3, -45, and -47),2 suggesting that the
requirement of Ssu72 for efficient snoRNA 3'-end formation may be
gene-specific. The mutations in CPF subunits, which showed no defect in the
readthrough assay for mRNA cleavage signals shown in
Fig. 5, B and
C, namely ref2,
pta1200TAP, and pti1248TAP, were
also negative for readthrough at the snR13 locus2
(Fig. 5E).
A recent study (31)
suggested that genes situated downstream of snoRNA loci may appear
up-regulated in an expression profiling microarray experiment if a mutation
causes a deficiency in 3'-end formation for the upstream snoRNAs. In
order to identify snoRNA transcripts whose correct 3'-end formation
potentially requires Ssu72 or other components of APT, we performed expression
profiling experiments with ssu72CTAP, pta1200 (after
2 h at 38 °C), and ref2 (30 °C) mutants, and the
corresponding wild type strains using an oligonucleotide set representing 6200
yeast ORFs. Consequently, our oligonucleotide array can detect putative snoRNA
readthrough products that contain ORFs transcribed from the same strand only,
unlike the ORF array used previously
(31). It is quite likely that
there is also readthrough we failed to detect downstream of other snoRNA genes
in which the downstream transcript has the opposite orientation.
Interestingly, in the ssu72 mutant, a considerable set of ORFs
located downstream of snoRNA genes and transcribed in the same direction were
dramatically up-regulated. Approximately half (7 of 15) of the genes whose
expression was increased at least 7-fold were such genes (see
Fig. 5F). Overall, 16
of 234 genes up-regulated more than 2-fold in ssu72CTAP were
downstream of snoRNA loci. In three of these cases, readthrough from the
mature snoRNA transcript to the downstream region was confirmed by reverse
transcription (Fig.
5E), strongly suggesting that the increased expression of
post-snoRNA ORFs is a direct result of readthrough. As well, two genes found
to be up-regulated more than 2-fold in ref2 and pta1 mutants
by microarray analysis were located downstream of snoRNA loci (snR33 and -43
and snR5 and -33, respectively). Primer extension analysis revealed that
readthrough from the snR33 locus into the downstream gene was substantial in
the ref2 and pta1 mutants and somewhat less pronounced in
the ssu72 mutant (Fig.
5G).
Ssu72 and Other Subunits of APT Associate with snoRNA Genes and Primarily with the 3'-Ends of mRNA-encoding Genes—ChIP after formaldehyde cross-linking has been used to demonstrate that some cleavage and polyadenylation factor subunits bind to transcriptionally active genes (12, 36). We investigated the possibility that Ssu72 might directly interact with 3'-ends of genes transcribed by RNAPII as do various subunits of CFIA or core-CPF.4 The level of cross-linking of N- and C-terminally TAP-tagged Ssu72 (wild type and mutant, respectively) along a constitutive, highly expressed gene, PYK1, was analyzed as described previously (36) (see Fig. 6, B and C). The levels of cross-linking were compared with those obtained for a control region from chromosome V devoid of open reading frames as well as sn/snoRNA genes. Either tagged variant of Ssu72 cross-linked at the 5'-end, middle and 3'-end of the PYK1 gene at levels higher than the control sequence, but cross-linking to the 3'-end region containing the polyadenylation signal (62) was substantially enriched (see Fig. 6, B and C). Other subunits of APT, namely Ref2, Pti1, Swd2, Glc7, Syc1 (weakly), as well as Pta1, showed a pattern of cross-linking to the PYK1 gene similar to that of Ssu72 (see Fig. 6B), suggesting that the APT subunits of holo-CPF co-localize at least at some genes. The preferential localization of APT subunits to the region around the polyadenylation signal is likely to be quite general, because we have preferentially cross-linked APT subunits to the 3'-ends of the PMA1 and PYK1 genes (Fig. 6D), as well as the ADH12,4 gene. Grs1, a tRNA synthetase proposed to be required for cleavage-independent transcription termination by RNAP II (63), did not cross-link above the background on the PYK1 and PMA1 genes (Fig. 6, B and D). TAP tagging and purification of Grs1 suggested that Grs1 is not strongly associated with any other protein.2
As mutations in SSU72 and SUB1 can specifically suppress alleles of SUA7 deficient in transcription start site selection, it has been proposed that they are functionally related, at least at the level of transcription initiation. On the other hand, Sub1 has also been reported to play a negative role in transcription termination by RNAPII (20) and, more recently, a positive role in mRNA 3'-end processing (43). In ChIP experiments on the PYK1 gene, we found that although Sua7 is detectable only at the promoter, as expected for a general initiation factor, Sub1, the yeast homolog of the human co-activator PC4 (64, 65), is substantially enriched both at the promoter and in the poly(A) signal region (Fig. 6B), which is in good agreement with its proposed roles. The lower level of Sub1 cross-linking to the middle of the PYK1 gene suggests that Sub1 probably has independent roles at the promoter and 3'-end.
Consistent with the involvement of Ssu72 in snoRNA 3'-end formation, Ssu72, like Nrd1 and Nab3, which were previously shown to be involved in this process (31), also cross-linked at two snoRNA loci, snR13 and snR71, which showed readthrough in an ssu72-C-TAP mutant strain (see Fig. 6E). SnoRNA loci are too small for the ChIP technique to distinguish whether Ssu72, Nrd1, and Nab3 are localized primarily at the 3'-ends. Similarly, the other APT subunits Ref2, Pti1, Swd2, and Pta1 were bound to the snR13 and snR71 genes (see Fig. 6E) even though the lack of Ref2 did not result in increased readthrough at these genes. Apparent cross-linking of APT subunits to snR71 could, in principle, reflect cross-linking to the 3'-end of the downstream LIN1 gene. Cross-linking of APT to snR13, however, should not reflect cross-linking to the promoter of the downstream TRS31 gene, because the TRS31 promoter is quite distant from the PCR primers and because APT cross-links poorly to promoters (Fig. 6, B and D, and data not shown). Therefore, the APT subcomplex of holo-CPF appears associated as a whole with snoRNA genes and primarily with the 3'-ends of mRNA-encoding genes even when some subunits of APT are not required to efficiently prevent readthrough. Conversely, Nrd1 and Nab3 are associated with the PYK1, PMA1 (Fig. 6, B and D), and ADH12 genes even though these proteins are not known to participate in 3'-end formation for polyadenylated transcripts. Interestingly, Nrd1 and Nab3 are associated with the PYK1 and PMA1 genes primarily near their promoter regions and taper off toward the 3'-ends, which is the converse of the pattern observed for subunits of the APT complex. Our finding that Nrd1 and Nab3 are associated with mRNA-encoding genes, as well as snoRNA genes, is supported by several lines of evidence. First, Nab3 was identified as a nuclear protein that binds polyadenylated RNAs and is also required for the formation of various mRNAs (66). Second, Nrd1 binds the CTD of the largest subunit, Rpb1, of RNAPII suggesting Nrd1 might have a more general role in RNAPII transcription (67). Third, Nrd1 and Nab3 are required for maintaining normal levels of the Nrd1 mRNA and possibly other mRNAs (31).
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DISCUSSION |
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In particular, we describe a subcomplex of holo-CPF, APT, which contains the Pta1 subunit of CFII and six additional polypeptides (Ref2, Pti1, Swd2, Glc7, Ssu72, and Syc1) whose association with CPF was not discovered until recently (9, 17, 41–44). By the systematic tagging and purification of each subunit, we showed that five of the six polypeptides associated with Pta1 in APT exist in vivo mostly as members of holo-CPF-APT complexes. The exception is Glc7, which associates with many polypeptides2 (42). Because Pta1 is present in both CFII and APT, dimerization of Pta1 could, conceivably, mediate the association of APT with CFII. Because only some, perhaps one-quarter to one-half, of the CPF molecules in a yeast extract are associated with APT, it is possible that APT is only engaged on some genes transcribed by RNAPII. We also found that there is a weak, rather than a stable (41), interaction between CPF and CFIA, enabling us to provide an outline for the assembly of the yeast cleavage and polyadenylation machinery (Fig. 2C).
There is evidence that the mammalian cleavage and polyadenylation specificity factor interacts with the general transcription factor TFIID at promoters and then is transferred to the elongating RNAPII (68). It has been proposed that the 3'-end machinery may engage the transcription machinery in a similar way in the yeast S. cerevisiae (12, 46). Our tandem affinity purification of Ssu72 and ChIP data argue against a stable interaction between Ssu72 and Sua7 (yTFIIB). However, a transient or weak interaction at the promoter level is still entirely possible. Our data suggest that APT is initially recruited at promoters, as Ssu72 and the other APT components that we tested cross-linked at the PYK1 and PMA1 promoters significantly above the background.
Regardless of when APT is initially recruited to the transcription unit, our observation that there is enhanced cross-linking in the region that contains the poly(A) signal makes it likely that an encounter with the polyadenylation signal (at the DNA or RNA level) triggers an accumulation of APT at the 3'-UTR. The simplest hypothesis that explains our ChIP results is that a weak, unstable interaction between APT and the RNAPII transcription initiation and/or elongation complexes that first form when RNAPII is near the promoter is stabilized when the polyadenylation signal is transcribed. Interactions between RNAPII and various subunits of CFIA (16) or CPF (17–19, 46) have been described, suggesting that RNAPII per se could initially recruit the 3'-end processing machinery. As some of these interactions are mediated by a hyperphosphorylated CTD, and RNAPII must be hypophosphorylated in order to initiate transcription (69), it seems plausible that regulating the phosphorylation state of Rpb1 may influence 3'-end processing. The yeast CTD phosphatase, Fcp1 (70), may modulate 3'-end formation for RNAPII transcripts by opposing Ctk1, which phosphorylates Ser-2 of the CTD heptad repeats YSPTSPS (71) and was recently suggested to participate in this process (49). However, Glc7, the protein phosphatase subunit of the APT subcomplex in CPF could also potentially play a role in 3'-end formation by dephosphorylating RNAPII or some other substrate, like Pap1 (72). Intriguingly, Ssu72 was proposed recently (73, 74) to have a tyrosine phosphatase activity.
While our manuscript was in preparation, a report (17) showing that Ssu72 plays a role in suppressing pausing by RNAPII upstream of a polyadenylation signal was published. Evidence was also presented that Ssu72 contributes to transcription termination but only for the CUP1 gene. Our results support and complement the findings of that report by suggesting that Ssu72 may be more generally important as a positive effector in suppressing readthrough by RNAPII not only at protein-encoding genes but also at snoRNA-encoding genes. Similar results were reported very recently (74). Although our experiments do not distinguish between the effects of Ssu72 on cleavage and termination, one possibility is that Ssu72, which interacts directly with RNAPII, acts directly as a termination factor for RNAPII at some sites. The observation that the temperature sensitivity of an ssu72-2 strain can be partly suppressed by 6-azauracil (17), which depletes UTP and GTP pools and enhances pausing by RNAPII, is consistent with a role for Ssu72 in termination by RNAPII. Alternatively, Ssu72 may only indirectly decrease the level of terminator readthrough by enhancing cleavage at the poly(A) site (43), followed by exonucleolytic digestion of the downstream RNA, or else by directly recruiting the exosome or some other RNA-processing nuclease to the transcript. This latter possibility does not agree particularly well with the apparent specificity of readthrough among snoRNA loci caused by the ssu72-C-TAP mutation (Fig. 5E). Our findings physically position Ssu72 (and other recent CPF components in APT) mainly at the 3'-ends of the protein-encoding genes PYK1, PMA1, and ADH1. Similar observations have been made for other cleavage and polyadenylation factors.4
It is interesting that Ssu72 is also necessary for forming the
3'-ends of many snoRNAs. In this sense, the effect of Ssu72 resembles
those of Nrd1 and Nab3 (31)
and subunits of the CFIA complex
(60,
61). We were also able to
identify one snoRNA, namely snR33, for which other subunits of the
APT complex, Ref2 and Pta1, are important for preventing readthrough into
downstream genes. Because Sen1 is involved in the snoRNA 3'-end
formation (31), the apparently
limited involvement of Pta1 and Ref2 in normal snoRNA 3'-end formation
could possibly be explained by the 2-fold up-regulation of the Sen1 mRNA
we observed in microarray experiments. Very recently, Dheur et al.
(44) showed that Ref2 and Pti1
are important for snoRNA 3'-end maturation and proposed a role for these
proteins in uncoupling of cleavage and polyadenylation during small RNA
3'-end formation. Therefore, four of the seven APT subunits are required
for the correct formation of the 3'-ends of snoRNAs which suggests a
role for APT as a modulator of uncoupling cleavage from polyadenylation of the
RNAPII transcripts.
Our experiments did not distinguish whether effects of Ssu72 on the
expression of downstream genes were caused by transcriptional readthrough or
stabilization of the downstream RNA as a consequence of failure to cleave at
the upstream cleavage signal. Indeed, Ssu72 appears to be required for
cleavage but not termination on the CYC1 gene
(43). Nevertheless, Ssu72 may
contribute to termination on some snoRNA genes (and perhaps some
protein-coding genes) by influencing pausing by RNAPII
(17). How Ssu72 could suppress
pausing by RNAPII upstream of a termination signal and enhance termination
once the termination signal has been transcribed would require further
explanation, but there is a precedent in the activity of the NusA protein of
E. coli. Like Ssu72, NusA interacts with RNAP, and NusA normally
enhances pausing and termination by RNAP, but the presence of nut
site RNA and the bacteriophage 
N protein causes NusA to suppress
termination (75). In a similar
way, perhaps the presence of the polyadenylation signal in RNA and the
proteins that recognize it cause the activity of Ssu72 to change from
suppression of pausing to enhancement of termination.
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FOOTNOTES |
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Supported by a C. H. Best doctoral fellowship.
Scholar of the Leukemia and Lymphoma Society.
To whom correspondence should be addressed: Banting and Best Dept. of Medical
Research, University of Toronto, 112 College St., Rm. 210, Toronto, Ontario
M5G 1L6, Canada. Tel.: 416-978-4141; Fax: 416-978-8528, E-mail:
jack.greenblatt{at}utoronto.ca.
1 The abbreviations used are: CF, cleavage factor; CPF, cleavage and
polyadenylation factor; RNAPII, RNA polymerase II; UTR, untranslated regions;
PF I, polyadenylation factor I; CTD, C-terminal domain of the largest subunit
of RNA polymerase II; Pap1, poly(A) polymerase; ChIP, chromatin
immuno-precipitation; TAP, tandem affinity purification; MALDI-TOF, matrix
assisted laser desorption ionization-time of flight; MS, mass spectrometry;
ORF, open reading frame; snoRNA, small nucleolar RNA; GST, glutathione
S-transferase.
2 E. Nedea and J. Greenblatt, unpublished data.
3 X. He and C. Moore, unpublished data.
4 M. Kim, S. H. Ahn, and S. Buratowski, unpublished data.
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