Ferredoxin and ribosomal protein S10 are encoded on the cyanelle genome of Cyanophora paradoxa

Organization of a large gene cluster encoding ribosomal proteins in the cyanobacterium Synechococcus sp. strain PCC 6301: comparison of gene clusters among cyanobacteria, eubacteria and chloroplast genomes

The structure of a large gene cluster containing 22 ribosomal protein (r-protein) genes of the cyanobacterium Synechococcus sp. strain PCC6301 is presented. Based on DNA and protein sequence analyses, genes encoding r-proteins L3, L4, L23, L2, S19, L22, S3, L16, L29, S17, L14, L24, L5, S8, L6, L18, S5, L15, L36, S13, S11, L17, SecY, adenylate kinase (AK) and the alpha subunit of RNA polymerase were identified. The gene order is similar to that of the E. coli S10, spc and alpha operons. Unlike the corresponding E. coli operons, the genes for r-proteins S4, S10, S14 and L30 are not present in this cluster. The organization of Synechococcus r-protein genes also resembles that of chloroplast (cp) r-protein genes of red and brown algal species. This strongly supports the endosymbiotic theory that the cp genome evolved from an ancient photosynthetic bacterium.

Chloroplast ribosomes and protein synthesis

Consistent with their postulated origin from endosymbiotic cyanobacteria, chloroplasts of plants and algae have ribosomes whose component RNAs and proteins are strikingly similar to those of eubacteria. Comparison of the secondary structures of 16S rRNAs of chloroplasts and bacteria has been particularly useful in identifying highly conserved regions likely to have essential functions. Comparative analysis of ribosomal protein sequences may likewise prove valuable in determining their roles in protein synthesis. This review is concerned primarily with the RNAs and proteins that constitute the chloroplast ribosome, the genes that encode these components, and their expression. It begins with an overview of chloroplast genome structure in land plants and algae and then presents a brief comparison of chloroplast and prokaryotic protein-synthesizing systems and a more detailed analysis of chloroplast rRNAs and ribosomal proteins. A description of the synthesis and assembly of chloroplast ribosomes follows. The review concludes with discussion of whether chloroplast protein synthesis is essential for cell survival.

Eukaryote-eukaryote endosymbioses: insights from studies of a cryptomonad alga

It has been proposed that those plants which contain photosynthetic plastids surrounded by more than two membranes have arisen through secondary endosymbiotic events. Molecular evidence confirms this proposal, but the nature of the endosymbiont(s) and the number of endosymbioses remain unresolved. Whether plastids arose from one type of prokaryotic ancestor or multiple types is the subject of some controversy. In order to try to resolve this question, the plastid gene content and arrangement has been studied from a cryptomonad alga. Most of the gene clusters common to photosynthetic prokaryotes and plastids are preserved and seventeen genes which are not found on the plastid genomes of land plants have been found. Together with previously published phylogenetic analyses of plastid genes, the present data support the notion that the type of prokaryote involved in the initial endosymbiosis was from within the cyanobacterial assemblage and that an early divergence giving rise to the green plant lineage and the rhodophyte lineage resulted in the differences in plastid gene content and sequence between these two groups. Multiple secondary endosymbiotic events involving a eukaryotic (probably rhodophytic alga) and different hosts are hypothesized to have occurred subsequently, giving rise to the chromophyte, cryptophyte and euglenophyte lineages.