The chromosomal location of genes for elongation factor Tu and ribosomal protein S10 in the cyanobacterium Spirulina platensis provides clues to the ancestral organization of the str and S10 operons in prokaryotes.

The structural gene (rps10) encoding ribosomal protein S10 of the cyanobacterium Spirulina platensis has been localized both on chromosomal DNA and the previously characterized recombinant plasmid pSp7 harbouring the 3'-terminal portion of the gene for elongation factor G (fus) and the gene for elongation factor Tu (tuf). Alignment of the predicted S10 sequence of S. platensis with the homologous sequences from cyanelles, bacteria, archaea and eukarya showed that the cyanobacterial S10 shares a high degree of sequence homology (74% amino acid identity) with the cyanellar protein. Unlike the situation in Escherichia coli, the rps10 gene of S. plantensis is unlinked to the S10 operon genes, being adjacent to the str operon genes. Since a similar organization could be observed in cyanelles of Cyanophora paradoxa and in all archaea so far analysed, this probably represents the ancestral state.