ComEB protein is dispensable for transformation but must be translated for optimal synthesis of ComEC

Abstract

We show that the ComEB protein is not required for transformation in Bacillus subtilis, despite its expression from within the comE operon under competence control. We show further that the synthesis of the putative channel protein ComEC is translationally coupled to the upstream comEB open reading frame, so that translation of comEB and a suboptimal ribosomal binding site embedded in its sequence are needed for proper comEC expression. Translational coupling appears to be a common mechanism in three major competence operons for the adjustment of protein amounts independent of transcriptional control, probably ensuring the correct stoichiometries for assembly of the transformation machinery. comEB and comFC respectively encode cytidine deaminase and a protein resembling type 1 phosphoribosyl transferases and we speculate that nucleotide scavenging proteins are produced under competence control for efficient reutilization of the products of degradation of the non-transforming strand during DNA uptake.