Genetics of carotene biosynthesis in Phycomyces

Genetic analysis of carotenogenesis in Phycomyces is hampered by the inability of most mutants to complete the sexual cycle. Heterokaryons between complementing mutants or between a mutant and a helper strain are, however, fertile. Using this method crosses have been carried out between mutants representing the different known genes and phenotypes.The gene for phytoene dehydrogenase, carB, is closely linked to carRA, a bifunctional gene specifying both lycopene cyclase and a product involved in substrate transfer. The regulatory gene carS lies on the same chromosome, but not close to the structural genes. The situation is thus reminiscent of an operon.Two chemoinsensitive mutants, showing little or no response to the effects of retinol and 2-(4-chlorophenyl)thiotriethylamine HCl on carotenogenesis, carry mutations in separate genes. One of these is probably identical with carRA; the other is a new gene termed carl.