Summary
The bacterium Streptomyces davaonensis synthesizes the antibiotic roseoflavin in the stationary phase of growth. The starting point for roseoflavin biosynthesis is riboflavin (vitamin B2) and four enzymes (RibCF, RosB, RosA and RosC) are necessary to convert a vitamin (riboflavin) into a potent, broad‐spectrum antibiotic (roseoflavin). In S. davaonensis, seven enzymatic functions are required to synthesize the roseoflavin precursor riboflavin from the central building blocks GTP and ribulose 5‐phosphate. When compared with other bacterial and in particular Streptomyces genomes the S. davaonensis genome contains an unusual high number (21) of putative riboflavin biosynthetic genes (rib genes), including a rib gene encoding an additional riboflavin synthase originating from an Archaeon. We show by complementation analyses and enzyme assays that 17 out of these 21 putative rib genes indeed encode for riboflavin biosynthetic enzymes. Biochemical analyses of selected enzymes support this finding. Transcriptome analyses show that all of the rib genes are expressed either in the exponential or in the stationary phase of growth and thus do not represent silent genes. We conclude that the Rib enzymes produced in the stationary phase represent a physiological adaptation to support roseoflavin biosynthesis.
is a red riboflavin analog with broad antimicrobial activity against Gram-positive bacteria (Otani et al., 1974). Under laboratory conditions RoF is produced in the stationary phase of growth by two different Gram-positive bacterial species, S. davaonensis (formerly S. davawensis (Landwehr et al., 2018)) and Streptomyces cinnabarinus (Jankowitsch et al., 2012). Other RoF producers are not known. RoF is taken up by target cells by riboflavin importers. Thus, RoF only acts against bacteria having riboflavin importers, which appear to be present mostly in Gram-positive bacteria (Grill
Corrinoid-dependent enzyme systems rely on the super-reduced state of the protein-bound corrinoid cofactor to be functional, for example, in methyl transfer reactions. Due to the low redox potential of the [Co II ]/[Co I ] couple, autoxidation of the corrinoid cofactor occurs and leads to the formation of the inactive [Co II ]-state. For the reactivation, which is an energydemanding process, electrons have to be transferred from a physiological donor to the corrinoid cofactor by the help of a reductive activator protein. In this study, we identified reduced flavodoxin as electron donor for the ATP-dependent reduction of protein-bound corrinoid cofactors of bacterial O-demethylase enzyme systems. Reduced flavodoxin was generated enzymatically using pyruvate:ferredoxin/flavodoxin oxidoreductase rather than hydrogenase. Two of the four flavodoxins identified in Acetobacterium dehalogenans and Desulfitobacterium hafniense DCB-2 were functional in supplying electrons for corrinoid reduction. They exhibited a midpoint potential of about À400 mV (E SHE , pH 7.5) for the semiquinone/ hydroquinone transition. Reduced flavodoxin could be replaced by reduced clostridial ferredoxin. It was shown that the low-potential electrons of reduced flavodoxin are first transferred to the iron-sulfur cluster of the reductive activator and finally to the protein-bound corrinoid cofactor. This study further highlights the importance of reduced flavodoxin, which allows maintaining a variety of enzymatic reaction cycles by delivering low-potential electrons.
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