Genetic interactions in the regulation of carotenogenesis in Phycomyces

The regulation of carotenogenesis in the fungus Phycomyces blakesleanus is mediated by several gene products and influenced by light, retinol, and other external agents. De-regulation of the biosynthetic pathway is found in the deep-yellow carS mutants and the white carA mutants. The carA, carS recombinants are yellow; their β-carotene content is intermediate between that of strains carrying each of the two mutations. Mutagen exposure of a deep-yellow carS mutant led to the isolation of white double-mutants which become yellow in the presence of retinol. They carry two very closely linked mutations in the carS gene. When separated in different organisms, each of these mutations results in a deep-yellow phenotype. Our results support the idea that two gene products recognize the beta rings of β-carotene and shut off biosynthesis when enough β-carotene has been made. The modified gene product present in the white carS mutants stops synthesis in the absence of β-carotene.