The plastid ribosomal proteins - Identification of all the proteins in the30 S subunit of an organelle ribosome (chloroplast)

Identification of all the protein components of a plastid (chloroplast) rib osomal 30 S subunit has been achieved, using two-dimensional gel electropho lesis, high performance liquid chromatography purification, N-terminal sequ encing, polymerase chain reaction-based screening of cDNA library, nucleoti de sequencing, and mass spectrometry (electrospray ionization, matrixassist ed laser desorption/ionization time-of-flight, and reversed-phase HPLC coup led with electrospray ionization mass spectrometry), 25 proteins were ident ified, of which 21 are orthologues of all Escherichia coli 30 S ribosomal p roteins (S1-S21), and 4 are plastid-specific ribosomal proteins (PSRPs) tha t have no homologues in the mitochondrial, archaebacterial, or cytosolic ri bosomal protein sequences in data bases. 12 of the 25 plastid 30 S ribosoma l proteins (PRPs) are encoded in the plastid genome, whereas the remaining 13 are encoded by the nuclear genome. Post-translational transit peptide cl eavage sites for the maturation of the 13 cytosolically synthesized PRPs, a nd post-translational N-terminal processing in the maturation of the 12 pla stid synthesized PRPs are described. Post-translational modifications in se veral PRPs were observed: alpha-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, 514, S18, and S19, leading to two dis tinct forms differing in mass and/or charge (the corresponding modification s are not observed in E, coli). The four PSRPs in spinach plastid 30 S ribo somal 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 th e hypothesis that PSRPs form a ''plastid translational regulatory module" o n the 30 S ribosomal subunit structure for the possible mediation of nuclea r factors on plastid translation.