Differential regulation of genes involved in riboflavin biosynthesis in Brucella abortus

SERER I.; GARCÍA-ANGULO V.A.; BONOMI H.R.; GOLDBAUM F.A.

Buenos Aires

Conferencia; Brucellosis 2011, International Research Conference Including the 64th Brucellosis Research Conference; 2012

Brucella spp. presents an atypical riboflavin pathway, coding two genes for 6,7-dimethyl-8-ribityllumazine synthase, named ribH1 and ribH2. These enzymes synthesize lumazine, substrate for the biosynthesis of riboflavin (vitamin B2), which is the precursor of the essential cofactors FMN and FAD. The ribH1 gene is encoded in the chromosome I, in a ?classic? riboflavin operon organization as described in many other bacteria: nrdR-ribD-ribE-ribH1-nusB. In contrast, ribH2 is located in the chromosome II as a single ORF which displays an FMN riboswitch in its regulatory region and it is expressed during the intracellular phase. Only RibH2, and not RibH1, is involved in virulence during cell and mice infection. These data suggest differences in the regulatory mechanisms for the ribH genes. In this work, we achieved to demonstrate that ribH1 is transcribed polycistronically together with the riboflavin biosynthesis genes ribD and ribE, as well as the putative regulators nrdR and nusB genes. Overexpression of nusB produces a significative increase in the expression of ribH1, although RibH2 levels remain unchanged. On the other hand, RibH2 levels are diminished in high concentrations of riboflavin and FMN, while this does not occur with RibH1. In summary, these results indicate the existence of two possible independent regulatory mechanisms for the ribH1 and ribH2 genes: modulating the riboflavin operon transcription and the RibH2 translation, respectively. We are currently generating B. abortus null mutant strains in nrdR and nusB genes to assay their role in the expression of both lumazine synthase enzymes.