Six ribosomal proteins are specific to higher plant chloroplast ribosomes [Subramanian, A.R. (1993) Trends Biochem. Sci. 18 , 177–180]. Three of them have been fully characterized [Yamaguchi, K., von Knoblauch, K. & Subramanian, A. R. (2000) J. Biol. Chem. 275 , 28455–28465; Yamaguchi, K. & Subramanian, A. R. (2000) J. Biol. Chem. 275 , 28466–28482]. The remaining three plastid-specific ribosomal proteins (PSRPs), all on the small subunit, have now been characterized (2D PAGE, HPLC, N -terminal/internal peptide sequencing, electrospray ionization MS, cloning/ sequencing of precursor cDNAs). PSRP-3 exists in two forms (α/β, N-terminus free and blocked by post-translational modification), whereas PSRP-2 and PSRP-4 appear, from MS data, to be unmodified. PSRP-2 contains two RNA-binding domains which occur in mRNA processing/stabilizing proteins (e.g. U1A snRNP, poly(A)-binding proteins), suggesting a possible role for it in the recruiting of stored chloroplast mRNAs for active protein synthesis. PSRP-3 is the higher plant orthologue of a hypothetical protein (ycf65 gene product), first reported in the chloroplast genome of a red alga. The ycf65 gene is absent from the chloroplast genomes of higher plants. Therefore, we suggest that Psrp-3/ycf65 , encoding an evolutionarily conserved chloroplast ribosomal protein, represents an example of organelle-to-nucleus gene transfer in chloroplast evolution. PSRP-4 shows strong homology with Thx, a small basic ribosomal protein of Thermus thermophilus 30S subunit (with a specific structural role in the subunit crystallographic structure), but its orthologues are absent from Escherichia coli and the photosynthetic bacterium Synechocystis . We would therefore suggest that PSRP-4 is an example of gene capture (via horizontal gene transfer) during chloro-ribosome emergence. Orthologues of all six PSRPs are identifiable in the complete genome sequence of Arabidopsis thaliana and in the higher plant... [ABSTRACT FROM AUTHOR]