Immunological similarities between specific chloroplast ribosomal proteins from Chlamydomonas reinhardtii and ribosomal proteins from Escherichia coli

Polyclonal antibodies were elicited against seven of the 33 differentproteins of the large subunit of the chloroplast ribosome fromChlamydomonas reinhardtii. Three of these proteins are synthesized in thechloroplast and four are made in the cytoplasm and imported. In westernblots, six of the seven antisera are monospecific for their respectivelarge subunit ribosomal proteins, and none of these antisera cross-reactedwith any chloroplast small subunit proteins from C. reinhardtii. Antiserato the three chloroplast-synthesized ribosomal proteins cross-reacted withspecific Escherichia coli large subunit proteins of comparable charge andmolecular weight. Only one of the four antisera to the chloroplastribosomal proteins synthesized in the cytoplasm cross-reacted with an E.coli large subunit protein. None of the antisera cross-reacted with any E.coli small subunit proteins. On the assumption of a procaryotic,endosymbiotic origin for the chloroplast, those chloroplast ribosomalproteins still synthesized within the organelle appear to have retainedmore antigenic sites in common with E. coli ribosomal proteins than havethose which are now the products of cytoplasmic protein synthesis. Antiserato this cytoplasmically synthesized group of chloroplast ribosomal proteinsdid not recognize any antigenic sites among C. reinhardtii cytoplasmicribosomal proteins, suggesting that the genes for the cytoplasmicallysynthesized chloroplast ribosomal proteins either are not derived from thecytoplasmic ribosomal protein genes or have evolved to a point where noantigenic similarities remain.