The Plastid Ribosomal Proteins
IDENTIFICATION OF ALL THE PROTEINS IN THE 30 S SUBUNIT OF AN
ORGANELLE RIBOSOME (CHLOROPLAST)*
Kenichi
Yamaguchi,
Klaus
von Knoblauch, and
Alap R.
Subramanian
From the Department of Biochemistry, The University of Arizona,
Tucson, Arizona 85721 and the Max-Planck-Institut für
Molekulare Genetik, Berlin-Dahlem, Germany 14195
Identification of all the protein components of a
plastid (chloroplast) ribosomal 30 S subunit has been achieved, using
two-dimensional gel electropholesis, high performance liquid
chromatography purification, N-terminal sequencing,
polymerase chain reaction-based screening of cDNA library,
nucleotide sequencing, and mass spectrometry (electrospray ionization,
matrix-assisted laser desorption/ionization time-of-flight, and
reversed-phase HPLC coupled with electrospray ionization mass
spectrometry). 25 proteins were identified, of which 21 are orthologues
of all Escherichia coli 30 S ribosomal proteins (S1-S21),
and 4 are plastid-specific ribosomal proteins (PSRPs) that have no
homologues in the mitochondrial, archaebacterial, or cytosolic
ribosomal protein sequences in data bases. 12 of the 25 plastid 30 S
ribosomal proteins (PRPs) are encoded in the plastid genome, whereas
the remaining 13 are encoded by the nuclear genome. Post-translational
transit peptide cleavage sites for the maturation of the 13 cytosolically synthesized PRPs, and post-translational N-terminal
processing in the maturation of the 12 plastid synthesized PRPs are
described. Post-translational modifications in several PRPs were
observed: 
-N-acetylation of S9, N-terminal processings leading to five mature forms of S6 and two mature forms of S10, C-terminal and/or internal modifications in S1, S14, S18, and S19,
leading to two distinct forms differing in mass and/or charge (the
corresponding modifications are not observed in E. coli). The four PSRPs in spinach plastid 30 S ribosomal subunit (PSRP-1, 26.8 kDa, pI 6.2; PSRP-2, 21.7 kDa, pI 5.0; PSRP-3, 13.8 kDa, pI 4.9;
PSRP-4, 5.2 kDa, pI 11.8) comprise 16% (67.6 kDa) of the total protein
mass of the 30 S subunit (429.3 kDa). PSRP-1 and PSRP-3 show sequence
similarities with hypothetical photosynthetic bacterial proteins,
indicating their possible origins in photosynthetic bacteria. We
propose the hypothesis that PSRPs form a "plastid translational
regulatory module" on the 30 S ribosomal subunit structure for the
possible mediation of nuclear factors on plastid translation.
*
The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in
accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
The nucleotide sequence(s) reported in this paper has been submitted to the GenBankTM/EMBL Data Bank with accession number(s) AF242547, AF245665, AF250383.
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