Conservation and evolution of the nucleus-encoded and chloroplast-specific ribosomal proteins in pea and spinach

Two cDNA clones have been isolated from a lambda g11 cDNA library constructed with poly(A)+ mRNAs prepared from spinach seedlings. These nuclear cDNAs encode chloroplast (cp) ribosomal (r) proteins designated L24 and L40. These r-proteins have been identified in the cp 50S r-subunit by immunoblot analysis, amino acid (aa) composition and N-terminal aa sequencing. The L24 r-protein contains a central eubacterial homologous core with the N- and C-terminal polypeptide extensions. The L40 r-protein has no homologous counterpart in bacterial ribosomes. The two nuclear encoded r-proteins have their homologues in pea, a legume, showing that specific elements of cp ribosomes are conserved in higher plants. Surprisingly, the cp-specific r-protein L40 has a higher aa substitution rate than that of other eubacterial-like cp r-proteins identified previously in pea and spinach.