Reduced near-UV sensitivity in Phycomyces mutants affected in the biosynthesis of 6,7-dimethyl-8-ribitylllumazine

Riboflavin-requiring mutants of Phycomyces blakesleeanus with defects in the genes, ribA, ribB, ribC and ribD were analyzed with respect to their contents of flavins, 6,7-dimethyl-8-ribityllumazine (DMRL) and pterins as well as their phototropic sensitivity. Strains were grown on minimal medium enriched with 10-6 M riboflavin (RB), and the concentrations of the respective pigments in sporangiophores were determined by HPLC. In strains A607 ribC401 and A641 ribC402 madA7 a loss of DMRL correlated with a loss of near-UV sensitivity. In general terms, the results suggest the participation of DMRL in photoreception, which does not necessarily imply DMRL as a photoreceptor chromophore. In more specific terms, the result could be understood on the basis of a UV/blue-light photorecptor, which includes besides a flavin also a lumazine-like chromophore. Mutants C318 ribAl and C323 ribA4 accumulated DMRL, the immediate precursor of RB, as well as biopterin and neopterin. Mutant C322 ribB contained normal amounts of DMRL and pterins. Mutant C324 ribD5 had reduced amounts of neopterin and biopterin. The fact that some of the RB-requiring mutants displayed abnormal amounts of pterins indicates a common regulation for the flavin and the pterin pathway. (Author)