Two-Component Flavin-Dependent Riboflavin Monooxygenase Degrades Riboflavin in Devosia riboflavina.

The actinobacterium splits riboflavin (vitamin B) into lumichrome and d-ribose. However, such degradation by other bacteria and the involvement of a two-component flavin-dependent monooxygenase (FMO) in the reaction remain unknown. Here we investigated the mechanism of riboflavin degradation by the riboflavin-assimilating alphaproteobacterium (formerly ). We found that adding riboflavin to bacterial cultures induced riboflavin-degrading activity and a protein of the FMO family that had 67% amino acid identity with the predicted riboflavin hydrolase (RcaE) of MF109. The genome clustered genes encoding the predicted FMO, flavin reductase (FR), ribokinase, and flavokinase, and riboflavin induced their expression. This finding suggests that these genes constitute a mechanism for utilizing riboflavin as a carbon source. Recombinant FMO (rFMO) protein of oxidized riboflavin in the presence of reduced flavin mononucleotide (FMN) provided by recombinant FR (rFR), oxidized FMN and NADH, and produced stoichiometric amounts of lumichrome and d-ribose. Further investigation of the enzymatic properties of rFMO indicated that rFMO-rFR coupling accompanied O consumption and the generation of enzyme-bound hydroperoxy-FMN, which are characteristic of two-component FMOs. These results suggest that FMO is involved in hydroperoxy-FMN-dependent mechanisms to oxygenize riboflavin and a riboflavin monooxygenase is necessary for the initial step of riboflavin degradation. Whether bacteria utilize either a monooxygenase or a hydrolase for riboflavin degradation has remained obscure. The present study found that a novel riboflavin monooxygenase, not riboflavin hydrolase, facilitated this process in The riboflavin monooxygenase gene was clustered with flavin reductase, flavokinase, and ribokinase genes, and riboflavin induced their expression and riboflavin-degrading activity. The gene cluster is uniquely distributed in species and actinobacteria, which have exploited an environmental niche by developing adaptive mechanisms for riboflavin utilization.