The Bacillus subtilis chromosome region encoding homologues of the Escherichia coli mssA and rpsA gene products.

A gene was found in Bacillus subtilis which encodes a protein highly homologous to the Escherichia coli rpsA gene product, the S1 ribosomal protein. The B. subtilis protein contains the domain responsible for binding to ribosomes and two S1 motifs, instead of four as found in the E. coli protein. The B. subtilis protein is similar in this way to the equivalent protein of plant chloroplast ribosomes, supposed to be the counterpart of E. coli S1. The gene is expressed during vegetative growth in B. subtilis at the transcriptional and translational levels, as judged by Northern hybridization and expression in a translational fusion with a reporter gene. In contrast to the E. coli situation, it can be inactivated without dramatic effects on cell viability. Southern hybridization of the B. subtilis DNA fragment encoding this gene revealed specific homologous fragments in all other Gram-positive bacteria tested. The hybridization pattern with B. stearothermophilus suggests the presence of at least two homologous genes in this bacterium. We show that in B. subtilis the ORF preceding the rpsA homologue encodes a protein which is highly similar to the product of the E. coli mssA gene which is located upstream of rpsA. Again, in contrast to the E. coli situation, where these genes are co-transcribed, in B. subtilis they are separated by a transcription terminator and the mssA homologue is transcribed during sporulation. We suggest that during the evolution very similar structures and genetic organization of these two genes were conserved but acquired different functions in Gram-negative and Gram-positive bacteria.