Streptomyces davawensis synthesizes the antibiotic roseoflavin, one of the few known natural riboflavin analogs, and is roseoflavin resistant. It is thought that the endogenous flavokinase (EC 2.7.1.26)/flavin adenine dinucleotide (FAD) synthetase (EC 2.7.7.2) activities of roseoflavin-sensitive organisms are responsible for the antibiotic effect of roseoflavin, producing the inactive cofactors roseoflavin-5-monophosphate (RoFMN) and roseoflavin adenine dinucleotide (RoFAD) from roseoflavin. To confirm this, the FAD-dependent Sus scrofa D-amino acid oxidase (EC 1.4.3.3) was tested with RoFAD as a cofactor and found to be inactive. It was hypothesized that a flavokinase/FAD synthetase (RibC) highly specific for riboflavin may be present in S. davawensis, which would not allow the formation of toxic RoFMN/RoFAD. The gene ribC from S. davawensis was cloned. RibC from S. davawensis was overproduced in Escherichia coli and purified. ). Both RibC enzymes synthesized RoFAD and RoFMN. The functional expression of S. davawensis ribC did not confer roseoflavin resistance to a ribC-defective B. subtilis strain.Riboflavin (vitamin B 2 ) analogs have the potential to serve as basic structures for the development of novel anti-infectives (3). Consequently, both the mode of action of riboflavin analogs and the mechanism of resistance to these compounds are of substantial interest. Only very few natural riboflavin analogs are known (24). Moreover, the only known organism to produce a riboflavin analog with antibiotic activity is the grampositive bacterium Streptomyces davawensis (32).S. davawensis synthesizes the anti-vitamin 8-dimethyl-amino-8-demethyl-D-riboflavin, or roseoflavin ( Fig. 1), from riboflavin (27) via 8-amino-and 8-methylamino-8-demethyl-D-riboflavin (22). Roseoflavin is toxic to gram-positive but also to gramnegative bacteria if the compound is able to enter the cell (16). The molecular basis for roseoflavin toxicity is not clear. In all organisms, riboflavin serves as the direct precursor for the cofactors (ribo)flavin-5Ј-monophosphate (FMN) and flavin adenine dinucleotide (FAD) (12). FAD and (to a lesser extent) FMN are active components of flavoproteins, being involved in a wide range of redox and other reactions (15). FMN is synthesized from riboflavin by flavokinase (EC 2.7.1.26), and FAD is produced from FMN by FAD synthetase (EC 2.7.7.2) (2). In microorganisms, both enzymatic activities seem to be present in sufficient amounts, since only traces of free riboflavin are detectable in the cytoplasm (11). Accordingly, cytoplasmic roseoflavin may quickly be converted to the corresponding FMN/ FAD analogs roseoflavin-5Ј-monophosphate (RoFMN) and roseoflavin adenine dinucleotide (RoFAD). The synthesis of these inactive cofactors may explain the antibiotic activity of roseoflavin ( Fig. 1) (31, 36).S. davawensis is roseoflavin resistant, and the mechanism of self-resistance could involve a flavokinase/FAD synthetase with a high substrate specificity for riboflavin. Such an enzyme would not accept roseoflavin as...