Biosynthesis of riboflavin in Bacillus subtilis: function and genetic control of the riboflavin synthase complex

Abstract

Two riboflavin synthase activities (heavy and light) have been observed in earlier studies with B. subtilis. The heavy enzyme is a complex of one molecule of light enzyme (consisting of three α subunits) and approximately 60 β subunits. The formation of α and β subunits is coordinately controlled. Mutants apparently deficient in β subunits were isolated as riboflavin requires after mutagenesis of B. subtilis with ICR 191. The mutants could grow with diacetyl instead of riboflavin. Growth with diacetyl was associated with the accumulation of substantial amounts of the riboflavin precursor, 6,7-dimethyl-8-(D-ribityl)lumazine. It follows that the mutants are deficient in an enzyme activity required for the formation of the lumazine from the pyrimidine precursor. It is concluded that heavy riboflavin synthase is a bifunctional enzyme. The riboflavin synthase activity is mediated by the α subunits, whereas the β subunits are necessary for an earlier biosynthetic step.