RibR, a possible regulator of theBacillus subtilisriboflavin biosynthetic operon,in vivointeracts with the 5′-untranslated leader ofribmRNA

RibR is a minor cryptic flavokinase (EC 2.7.1.26) of the Gram-positive bacterium Bacillus subtilis with an unknown cellular function. The flavokinase activity appears to be localized to the N-terminal domain of the protein. Using the yeast three-hybrid system, it was shown that RibR specifically interacts in vivo with the nontranslated wild-type leader of the mRNA of the riboflavin biosynthetic operon. This interaction is lost partially when a leader containing known cis-acting deregulatory mutations in the so-called RFN element is tested. The RFN element is a sequence within the rib-leader mRNA reported to serve as a receptor for an FMN-dependent 'riboswitch'. In RibR itself, interaction was localized to the carboxy-terminate part of the protein, a segment of unknown function that does not show similarity to other proteins in the public databases. Analysis of a ribR-defective strain revealed a mild deregulation with respect to flavin (riboflavin, FMN and FAD) biosynthesis. The results indicate that the RNA-binding protein RibR may be involved in the regulation of the rib genes.