Regulatory mechanisms of 6,7-dimethyl-8-ribityllumazine formation in resting cells of a riboflavin-adenine-deficient mutant of Bacillus subtilis.

The regulatory mechanisms of riboflavin biosynthesis in the resting cells of a riboflavin-adenine-deficient mutant of Bacillus subtilis were examined. The growth and pH of the medium remained unchanged in spite of the administration of 0-200 microg/mL riboflavin to the medium during incubation of the resting cells for 17 h. However, the formation of 6,7-dimethyl-8-ribityllumazine (DMRL) and flavin adenine dinucleotide (FAD) was restricted and augmented and then reached its plateau, showing an inverse relation in the addition of riboflavin up to 50 microg/mL to the medium and a parallel relation in the supplementation of riboflavin up to 200 microg/mL to the medium. In the experiments, the amount of flavin mononucleotide (FMN) was negligible and riboflavin was not detected in the resting cells. The results indicated that not the repression of related enzymes but negative feedback inhibition by FAD, but not that by riboflavin or FMN, is operative in the biosynthetic pathway of riboflavin in the riboflavin adenine double-less mutant of Bacillus subtilis.