Cleavage, but Not Read-through, Stimulation Activity Is Responsible for Three Biologic Functions of Transcription Elongation Factor S-II*

Transcription elongation factor S-II stimulates cleavage of nascent transcripts generated by RNA polymerase II stalled at transcription arrest sites. In vitro experiments have shown that this action promotes RNA polymerase II to read through these transcription arrest sites. This S-II-mediated cleavage is thought to be necessary, but not sufficient, to promote read-through in the in vitro systems. Therefore, Saccharomyces cerevisiae strains expressing S-II mutant proteins with different in vitro activities were used to study both the cleavage and the read-through stimulation activities of S-II to determine which S-II functions are responsible for its biologic functions. Strains expressing mutant S-II proteins active in both cleavage and read-through stimulation were as resistant as wild type strains to 6-azauracil and mycophenolic acid. 6-Azauracil also induced IMD2 gene expression in both these mutant strains and the wild type. Furthermore, strains having a genotype con-sisting of one of these S-II mutations and the spt4 null mutation grew as well as the spt4 null mutant at 37 °C, a restrictive temperature for a strain bearing double null mutations of spt4 and

Transcription elongation factor S-II, originally purified from mouse Ehrlich ascites tumor cells, is an RNA polymerase IIstimulating factor in promoter-independent RNA synthesis (1). It has been found in all eukaryotes thus far investigated, and its primary structure is highly conserved (2)(3)(4)(5)(6)(7). S-II is a unique transcription elongation factor that promotes transcript elongation through transcription arrest sites found in genes (8 -11). The molecular mechanism of this phenomenon has been investigated extensively in in vitro systems, and it has been shown that S-II stimulates the nuclease activity of RNA polymerase II, which then cleaves the nascent transcript. Then the 3Ј-end of the nascent RNA is realigned with the catalytic site of RNA polymerase II, and the transcription elongation complex tries reading through the arrest site again (11)(12)(13)(14). The cleavage stimulation activity of S-II can be separated from its readthrough stimulating activity; although the cleavage stimulation activity is essential, it is not sufficient to promote readthrough of RNA polymerase II in vitro (15).
There have been several reports describing the function of S-II in Saccharomyces cerevisiae. In several yeast strains bear null mutations of the genes encoding the transcription elongation machinery, such as spt4⌬, rpb9⌬, and ctk1⌬, S-II is indispensable for cell proliferation under certain culture conditions (16 -18). Although Kulish et al. (19) reported that S-II promotes read-through of RNA polymerase II in vivo as well as in vitro, it remains to be elucidated whether S-II functions in vivo through its cleavage-stimulating activity, its read-throughstimulating activity, or both. S-II also plays a role in the development of 6-AU 1 or MPA resistance in yeast (2,20) and in the expression of the pyrimidine-specific 5Ј-nucleotidase SDT1 gene; it is also involved in the induction of the IMP dehydrogenase IMD2 gene and the polyamine transporter TPO1 gene in response to treatment with 6-AU or MPA (21)(22)(23). Stimulation of expression of these genes is thought to be necessary for developing 6-AU and MPA resistance in S. cerevisiae.
In this study, the two S-II activities were investigated in an effort to determine which are responsible for the biologic functions, such as drug resistance and transcription induction, in S. cerevisiae. Site-directed mutations were introduced into the S-II gene to generate mutant yeast strains that express S-II mutant proteins with different levels of cleavage and readthrough stimulation activities in vitro. These strains were then tested to determine their sensitivity to 6-AU and MPA, their ability to induce the IMD2 gene in response to 6-AU treatment, and their temperature-sensitive growth retardation in combination with the spt4 null mutation. The results show that cleavage-stimulating, but not read-through-stimulating, activity correlates with the biologic functions of S-II, suggesting that the former activity of S-II is responsible for its functions in vivo.

EXPERIMENTAL PROCEDURES
S. cerevisiae Mutant Strains-All yeast strains used in this study are summarized in Table I. HKY01 and HKY02 are S-II null mutant strains (courtesy of Dr. T. Ito) derived from YPH499 (24); S-II gene was dis-* The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
¶ To whom correspondence should be addressed. Tel.: 81-285-51-2382; Fax: 81-285-51-2226; E-mail: nakanfnt@daiichipharm.co.jp. rupted by URA3 or HIS3 gene introduction in these strains, respectively. Plasmid pH1180 was constructed by inserting the 3.4-kb PvuII fragment of the S. cerevisiae S-II gene into pBluescript KS(ϩ) (Stratagene, La Jolla, CA). To screen for mutant clones, silent base-exchange mutations were introduced to form the following new restriction enzyme recognition sites in each mutant: KasI sites for Mt1 and Mt2; SacII sites for Mt3, Mt4, Mt8, and Mt9; an SphI site for Mt5; a BspT104I site for Mt6; and a PvuI site for Mt7. pMt1b was constructed by inserting the PstI-NcoI fragment of the S-II gene amplified by PCR with the primers 5Ј-AAACTGCAGGATCTGGCGCCAGCGCCCTTAGC-TGCAAAGATAGAAG-3Ј and 5Ј-GGGGCCAGTTGTTCAGGTCCTAAC-TGTATGCC-3Ј (TU-23) into the PstI-NcoI site of pH1180. pMt1 was constructed by inserting the PstI-KasI fragment of the S-II gene amplified by PCR with the primers 5Ј-CACAGTGTAGTCAGTCCGCA-TAAGAGCATTCATCATGG-3Ј (TU-22) and 5Ј-TAAGGGCGCTGGCG-CCAGATCCTTGGCATCG-3Ј into the PstI-KasI site of pMt1b. pMt2b was constructed by inserting the PstI-NcoI fragment of the S-II gene amplified by PCR with the primers TU-22 and 5Ј-CATGCCATGGTGG-CGCCAGCTGCGGCGTATAAGGCTTGCTTGGC-3Ј into the PstI-NcoI site of pH1180. pMt2 was constructed by inserting the KasI-NcoI fragment of the S-II gene amplified by PCR with the primers 5Ј-CAACGC-ACAGGGCGCCACCATAGAAAGG-3Ј and TU-23 into the KasI-NcoI site of pMt2b. pMt3b was constructed by inserting the MfeI-NcoI fragment of the S-II gene amplified by PCR with the primers 5Ј-CTATCA-ATTGCAAACACAATCCGCGGATTTGCCATTGACCAC-3Ј and TU-23 into the MfeI-NcoI site of pYSII-2 (2). pMt3 was constructed by inserting the PstI-NcoI fragment of pYSII-2 to the PstI-NcoI site of pH1180. pMt4b was constructed by inserting the PstI-NcoI fragment of the S-II gene amplified by PCR with the primers 5Ј-AAACTGCAGATCCGCG-GATGAACCATTGACCACTGCTTGTACATGTG-3Ј and TU-23 into the PstI-NcoI site of pH1180. pMt4 was constructed by inserting the PstI-SacII fragment of the S-II gene amplified by PCR with the primers TU-22 and 5Ј-GGTTCATCCGCGGATCTTGTTTGCAATTG-3Ј into the PstI-SacII site of pMt4b. pMt5b was constructed by inserting the PstI-NcoI fragment of the S-II gene amplified by PCR with the primers 5Ј-AAACTGCAGAAGCATGCGGTAACAGATGGGCTTTCTCTTAGAA-TAG-3Ј and TU-23 into the PstI-NcoI site of pH1180. pMt5 was constructed by inserting the PstI-SphI fragment of the S-II gene amplified by PCR with the primers TU-22 and 5Ј-CATCTGTTACCGCATGCT-TCACATGTACAG-3Ј into the PstI-SphI site of pMt5. pMt6 to -9 were obtained by using a QuikChange site-directed mutagenesis kit (Stratagene) and pH1180 as a template. Primers used for mutants were 5Ј-GCCAGGTATGCTATAATTTATTCGAACGTCATATC-3Ј and 5Ј-GA-TATGACGTTCGAATAAATTATAGCATACCTGGC-3Ј for pMt6, 5Ј-CA-GTCACCGATCGAGCTACATGTGGTAAATG-3Ј and 5Ј-CATTTACCA-CATGTAGCTCGATCGGTGACTG-3Ј for pMt7, 5Ј-CAATTGCAAACAC-AATCCGCGGATAATCCATTGACC-3Ј and 5Ј-GGTCAATGGATTATC-CGCGGATTGTGTTTGCAATTG-3Ј for pMt8, and 5Ј-CAATTGCAAAC-ACAATCCGCGGATGAACC-3Ј and 5Ј-GGTTCATCCGCGGATTGTGT-TTGCAATTG-3Ј for pMt9. After the presence of the mutations on the plasmid clones was confirmed by DNA sequencing, the BamHI-PvuI (pMt2) or PvuII (all other constructs) fragments were excised, and an EZ Yeast Transformation Kit (Qbiogene, Inc., Carlsbad, CA) was used to introduce the fragments separately into HKY01 to create the S-II mutant strains designated Mt1 to Mt9. When the mutant S-II fragment is introduced by homologous recombination into the S-II locus, it loses the URA3 gene instead and is transformed to 5-fluoroorotic acid-resistant. Therefore, we isolated 5-fluoroorotic acid-resistant HKY01 transformants and performed genomic PCR to screen for the desired mutants with the primers 5Ј-CACTCGATGATGGGACTACG-3Ј and 5Ј-GCGCT-AGTAAGACAGATAGG-3Ј followed by restriction enzyme digestion. The introduction of the mutant sequences into the S-II locus was confirmed by Southern blot analysis. S-II genes were amplified from each S-II mutant strain, and sites of mutation were confirmed by DNA sequencing. Each mutant strain bore only the introduced mutation.
A PCR-based gene disruption method (25) was used to prepare all spt4⌬ strains. PCR with the plasmid pRS416 (Stratagene) as a template was used to prepare the DNA fragment containing the URA3 gene used to disrupt the SPT4 gene. The primers used were 5Ј-ATTCATTACTA-TTATACATGTGATATCAGAACGGAAGGTTAGATTGTACTGAGAGT-GCAC-3Ј and 5Ј-TTACACCTGGCCACATTCAGTTTGGCAAAAGCGA-ACGAGGCTGTGCGGTATTTCACACCG-3Ј. The resulting fragment was separately introduced to YPH499, HKY02, and Mt1 to -9 using an EZ Yeast Transformation Kit. URA ϩ transformants were selected on SD (uraϪ) agar plates (0.67% (w/v) yeast nitrogen base without amino acids; 2% (w/v) glucose; 0.1 mg/ml each histidine, adenine sulfate, tryptophan, and lysine; and 0.25 mg/ml leucine; 2% bactoagar), and Southern blot analysis was used to confirm gene disruption.
Anti-yeast S-II Antibodies-Recombinant yeast wild type S-II protein was expressed in Escherichia coli and purified as described elsewhere (2). Anti-yeast S-II antiserum was prepared at Asahi Techno Glass Corp. (Chiba, Japan) by injecting a New Zealand White rabbit with 0.3 mg of the purified recombinant yeast S-II protein. Anti-yeast S-II antibodies were affinity-purified from the antiserum as previously described (26).
Western Blot Analysis-YPH499 was transformed with pRS416 (Stratagene), designated TU100, and used as the wild type strain for this experiment. Mutant yeast strains Mt1 to Mt9 were also transformed with pRS416. The resulting nine S-II mutant strains, TU100, and an S-II mull mutant strain HKY01 were separately grown in SD (uraϪ) medium at 30°C until they reached midlog phase growth, and then cells were collected by centrifugation. A CellLytic Y solution (Sigma) containing 1 mM phenylmethylsulfonyl fluoride, 5 mM benzamidine, 10 g/ml leupeptin, and 2 g/ml pepstatin A was used to lyse each strain separately. Fifty micrograms of protein from the crude lysate was loaded in each lane of a SDS-polyacrylamide gel and then blotted to a Indirect Immunofluorescence-The same yeast strains used in the Western blot analysis were separately incubated in SD (uraϪ) medium at 30°C until they reached midlog phase growth, and then 0.18 volume of formaldehyde and 0.13 volume of 1 M potassium acetate buffer (pH 6.5) were added to the medium. Each culture was incubated at 23°C for 1 h. Cells were then collected by centrifugation and resuspended in 0.1 M potassium acetate buffer (pH 6.5) containing 5% formaldehyde and incubated at 23°C for 1 h. The cells were then collected by centrifugation and washed once with SHA buffer (1 M sorbitol, 0.1 M HEPES-Na, pH 7.2, and 5 mM NaN 3 ) and resuspended in SHA. Cells were then treated with Zymolyase 100T (Seikagaku Corp., Tokyo, Japan) and transferred onto ADCELL slides (Erie Scientific Co.; Portsmouth, NH). Cells were fixed in methanol at Ϫ20°C for 6 min and then in acetone at Ϫ20°C for 30 s. Fixed cells were incubated with affinity-purified antiyeast S-II antibodies or preimmune rabbit IgG followed by incubation with anti-rabbit IgG (goat) conjugated to the Alexa-488 fluorescent dye (Molecular Probes, Inc., Eugene, OR) for detection. The nuclei were stained with 4Ј,6-diamidino-2-phenylindole.
Drug Sensitivity Assay-The same yeast strains used in the Western blot analysis were separately incubated in SD (uraϪ) medium at 30°C for overnight and then diluted with fresh medium to A 600 ϭ 0.0425. The diluted cultures were incubated until the A 600 reached 0.17 and then diluted 10-, 100-, 1000-, or 10000-fold, and 7 l of each diluted culture was spotted onto SD (uraϪ) plates containing 75 g/ml 6-AU, 25 g/ml MPA, or no drug. The plates were incubated at 30°C for 4 days.
Northern Blot Analysis-The same yeast strains used in the Western blot analysis were separately incubated in SD (uraϪ) medium at 30°C until midlog phase, and then 6-AU to give 75 g/ml was added. After 0, 0.5, or 2 h, cells were collected by centrifugation, and a hot phenol method (27) was used to extract total RNA. Fifteen micrograms of total RNA was loaded onto each lane of an agarose gel containing formaldehyde for Northern blot analysis. The probes were the PCR products amplified from yeast genomic DNA with 5Ј-GTGGTATGTTGGCCGGT-ACTACCG-3Ј and 5Ј-TCAGTTATGTAAACGCTTTTCGTA-3Ј as primers for IMD2 (22) and 5Ј-TGATAACGGTTCTGGTATGTG-3Ј and 5Ј-T-AGTCAGTCAAATCTCTACCG-3Ј as primers for actin. The AlkPhos Direct Labeling System with CDP-Star (Amersham Biosciences) was used to label the probes with alkaline phosphatase, and the probes were hybridized with the blots at 55°C for 16 h. The blots were exposed to x-ray films, and the developed films were scanned with a GS-700 calibrated densitometer (Bio-Rad) to quantify the amount of transcript. The signal of the actin mRNA in each sample was used to normalize the band intensity, and the IMD2 signal of the TU100 following a 2-h induction by 6-AU was used as a relative measurement unit.
Other Methods-DNA manipulation was performed as described elsewhere (28). The method of Bradford (29) was used to determine protein concentration; bovine serum albumin was used as a standard.

Construction of S-II Mutant Strains-The S. cerevisiae S-II
protein consists of three domains: domains I, II, and III. Domains II and III are joined by a short linker region (30). Awrey et al. (31) constructed a variety of the S. cerevisiae S-II mutant proteins that span domains II and III. They then determined the in vitro cleavage and read-through-stimulating activities of these mutant S-II proteins with an assay that used a human histone H3.3 gene fragment, which contains well characterized transcriptional arrest sites, as the template (31). To see whether both cleavage and read-through stimulating activities of S-II were responsible for its biologic functions, nine of these mutant S-II proteins, designated Mt1 to Mt9, were selected for further tests. Results from the in vitro assays conducted by Awrey et al. (31) show that four of the mutant proteins had strong cleavage stimulation but different read-through-stimulating activities (Mt1, -4, -5, and -7); four others (Mt2, -6, -8, and -9) had moderate cleavage stimulation but different readthrough-stimulating activities, and Mt3 had little of either activity (Table II). Mt6 has a mutation in domain II, which binds to RNA polymerase II. Mt1, -2, and -7 have mutations in the linker region, and the other mutant proteins have mutations in domain III, which is essential for transcript cleavage and read-through-stimulating activities as well as 6-AU sensitivity suppression (31,32). These mutations were introduced into the S-II locus of S. cerevisiae by homologous recombination.
First, Western blot analysis was used to determine the amount of mutant S-II proteins expressed, and indirect immunofluorescence was used to determine the cellular localization of the mutant S-II proteins; both methods used anti-yeast S-II specific antibodies. Results of the Western blot analysis show that all S-II mutant strains have expression patterns similar to that of the wild type strain (Fig. 1). In addition to a major band of 42 kDa, which was as large as the recombinant S-II, a faint band (35 kDa) was observed in all strains. Since this signal was not detected in the S-II null mutant HKY01 (Fig. 1), it is  (31). 0, inactive or partially active at 500:1 (S-II protein:RNA polymerase II); 1, active at 500:1; 2, active at 100:1; 3, active at 5:1 for in vitro activities. ϩ, resistant or strong induction; Ϯ, sensitive; Ϫ, very sensitive or marginal induction for in vivo activities.

In vitro
In vivo thought that the smaller band represents a degradation product of S-II. Results of preliminary studies suggest that the smaller form lacks the carboxyl terminus region 2 that is essential for RNA polymerase II stimulation, read-through stimulation in vitro, and 6-AU sensitivity suppression in vivo (32). The band intensity of the Mt4 S-II protein is reproducibly weaker than the S-II proteins from the other mutant strains (Fig. 1). These data suggest that, compared with the S-II proteins from the other mutant strains, the amount of S-II protein expressed by the Mt4 strain was lower, or the antigenicity of Mt4 S-II protein is weaker. However, it should be noted that results presented below indicate that the amount of Mt4 S-II protein expressed was apparently sufficient to perform its biologic functions (Table II).
The results of indirect immunofluorescence experiments to locate S-II are shown in Fig. 2. As shown in Fig. 2A, anti-S-II staining of the wild type strain TU100 showed strong fluorescence that overlapped with nuclear staining, as previously reported (33). In contrast, no fluorescence was observed in S-II null mutant strain HKY01 stained with anti-S-II (Fig. 2B) or in wild type (Fig. 2B) and Mt1 to Mt9 (data not shown) cells stained with the same concentration (1.7 g/ml) of preimmune rabbit IgG. The staining patterns of Mt1 to Mt9 with anti-S-II were indistinguishable from that of the wild type (Fig. 2C) and overlapped with nuclear staining (data not shown). These results indicate that all of the mutant S-II proteins are localized in the nucleus as is the wild type S-II protein.
Drug Sensitivity in S-II Mutant Strains-The S. cerevisiae S-II null mutant is known to be sensitive to 6-AU (2) and MPA (20). To investigate the biologic effects of S-II mutant proteins, the sensitivity of each S-II mutant strain to 6-AU and MPA was tested. Fig. 3 shows that mutant strains Mt1, -2, -4, -5, -6, -7, and -9 are as resistant to 6-AU as the wild type is; in contrast, Mt3 and Mt8 are as sensitive to 6-AU as is the S-II null mutant. Interestingly, despite its inability to promote read-through in vitro (Table II), Mt5 is as resistant to 6-AU as is the wild type. The sensitivities of mutant strains Mt1 to Mt9 to 25 g/ml MPA correlate well with the sensitivities to 6-AU (Fig. 3). Additionally, introduction of a centromeric plasmid that contained the S. cerevisiae S-II gene complemented the drug sensitivities of Mt3 and Mt8 (data not shown). These results suggest that the read-through-stimulating activity of S-II is not needed for the suppression of 6-AU and MPA sensitivity.
IMD2 Gene Induction in S-II Mutant Strains-The IMD2 gene encodes IMP dehydrogenase, a key enzyme in the de novo synthesis of guanine nucleotides (34). The activity of IMP dehydrogenase is inhibited by MPA and by 6-azauridine monophosphate, an active metabolite of 6-AU (20). IMD2 gene transcription can be induced by the addition of 6-AU or MPA to the culture medium (22), and this induction depends on S-II. This induction is thought to be important for the development of 6-AU and MPA resistance. Therefore, the ability of 6-AU to induce IMD2 gene transcription in the S-II mutant strains was tested. The wild type TU100 strain, the S-II null mutant HKY01 strain, and the mutant strains Mt1 to Mt9 transformed with pRS416 were incubated in a synthetic medium in the presence or absence of 75 g/ml 6-AU for up to 2 h, and Northern blot analysis was used to determine the amount of IMD2 mRNA produced by each strain. As shown in Fig. 4 mutant strains Mt1, -2, -4, -5, -6, -7, and -9 induced IMD2 gene expression as well as the wild type strain did, whereas mutant strains Mt3 and Mt8 were as transcriptionally inactive as the S-II null mutant. These data illustrate that the biologic ability to induce the IMD2 gene transcription correlates positively with the in vitro cleavage stimulation activity exhibited by the S-II mutant proteins.
Temperature Sensitivity of the Yeast Strains Bearing S-II Mutations and spt4 Null Mutation-SPT4 gene encodes a subunit of the transcription elongation factor DSIF (DRB sensitivity-inducing factor) (35). A null mutant of the DST1 gene, which encodes S. cerevisiae S-II, shows temperature sensitivity when combined with the spt4 null mutation, whereas the spt4 null mutant is not temperature-sensitive (17). To see whether mutant S-II proteins can suppress the temperature-sensitive phenotype of the dst1⌬spt4⌬ double null mutant strain, S-II mutant strains carrying the spt4 null mutation were constructed and tested for growth at 37°C. The results are shown in Fig. 5. The dst1⌬spt4⌬ double null mutant strain showed clear temperature sensitivity. Mt1spt4⌬, Mt4spt4⌬, Mt5spt4⌬, Mt7spt4⌬, and Mt9spt4⌬ grew as well as spt4⌬ at 37°C, indicating that the Mt1, -4, -5, -7, and -9 S-II mutant proteins suppress temperature sensitivity as well as does the wild type S-II protein. However, the read-through activities of these five S-II mutant proteins vary and do not correlate with their suppression activities. Mt2spt4⌬ and Mt6spt4⌬ showed moderate temperature sensitivity, indicating that the suppression activities of Mt2 and Mt6 proteins are weaker than that of the wild type S-II protein. Mt3spt4⌬ and Mt8spt4⌬ were temperaturesensitive, showing that the Mt3 and Mt8 proteins lack suppression activity. These results indicate that the biologic ability to suppress temperature-restricted growth correlates positively with the in vitro cleavage stimulation activities of the S-II mutant proteins. DISCUSSION At a transcriptional arrest site in a gene, S-II stimulates the nuclease activity of RNA polymerase II into cleaving several bases from the nascent transcript and causes the realignment of the 3Ј end of the nascent transcript with the active site of the polymerase, and the transcription elongation complex tries to read through the arrest site again (11)(12)(13)(14). Results from previous studies have shown that this cleavage stimulation activity of S-II is necessary, but not sufficient, to promote read-through in vitro (15). To clarify this observation, S. cerevisiae S-II mutant strains were used to investigate the roles of the cleavage Yeast strains were incubated in SD (uraϪ) medium at 30°C until they reached midlog phase growth, and then 6-AU was added to 75 g/ml to each culture. After 0, 0.5, or 2 h, cells were collected, and total RNA was extracted for Northern blot analysis with IMD2 and actin genes as probes. WT, wild type TU100 strain; KO, S-II null mutant HKY01 strain. A, Northern blot analysis results. The numbers represent the S-II mutant strain numbers 1-9. B, graphic representation of data shown in A. The signals of the actin mRNA was used to normalize the band intensities; one mRNA amount unit represents the amount of IMD2 mRNA present in the wild type strain TU100 after 2 h of induction. and read-through stimulation activities of S-II in vivo. A 6-AU sensitivity assay (Fig. 3), RNA analysis for IMD2 induction by 6-AU (Fig. 4), and rescue from temperature-dependent growth inhibition of separate strains carrying one of the S-II mutations and an spt4 null mutation (Fig. 5) were used to assess S-II function. The results show that S-II mutant proteins that were active in both cleavage stimulation and read-through in vitro also suppressed 6-AU sensitivity, induced IMD2 gene expression, and suppressed dst1⌬spt4⌬ temperature-dependent growth inhibition. In contrast, those S-II mutant proteins inactive in vitro did not exhibit any of these biologic functions. Surprisingly, S-II mutant protein Mt5, which has strong cleavage stimulation activity but almost no read-through activity in vitro, performed all biologic functions. Overall, the cleavagestimulating activities, but not read-through stimulating activities of each mutant S-II protein, corresponded well with their biologic activities. The Mt2, Mt3, Mt6, and Mt8 S-II mutant proteins performed biologic functions not as well as wild type S-II protein (Table II). But the amounts of Mt2, -3, -6, and -8 proteins expressed were as great as the amount of wild type S-II (Fig. 1), and all mutant proteins were localized in the nucleus as the wild type protein was (Fig. 2). Thus, the low biologic activities of Mt2, -3, -6, and -8 S-II mutant proteins were not caused by insufficient protein expression or by cellular mislocalization. These results, summarized in Table II, strongly suggest that the cleavage stimulation activity of S-II alone is responsible for its biologic functions, whereas the readthrough stimulation activity of S-II is dispensable. Previously, S-II has been regarded as a transcriptional read-through factor, and its read-through activity has been assumed to be important for its biologic functions; however, the results of the present study do not support this notion. It is possible, however, that the S-II mutant proteins that are inactive in an in vitro read-through assay have sufficient read-through activity in the cell.
The results of the present study also imply the presence of some as yet unidentified transcriptional read-through factors. Since the mutant S-II protein Mt5, which lacks read-through activity in vitro, exhibited full biologic activity, it is possible that once the nascent transcript is cleaved by RNA polymerase II, other read-through factors might promote read-through in place of a read-through-incompetent S-II protein in Mt5 strain. Awrey et al. (31) suggested that a conformational change in the ternary elongation complex (RNA polymerase II-DNA template-nascent transcript) is required for read-through after the cleavage of the nascent transcript. It has been suggested that S-II and Rpb9 influence the conformational change necessary for read-through (31). Thus, Rpb9 is a good candidate for a read-through factor other than S-II. Mutant yeast strains sensitive to 6-AU that bear the Mt5 mutation may be good tools for identifying this putative read-through factor. Another possible approach would be to use an in vitro read-through assay system containing the S-II Mt5 protein to isolate read-through-promoting factors biochemically. These studies, which are currently under way, are the next logical steps in understanding the regulation of transcription elongation by RNA polymerase II in eukaryotic cells.