New Mutants of Phycomyces blakesleeanus for b-Carotene Production

The accumulation of b-carotene by the zygomycete Phycomyces blakesleeanus is increased by mutations in the carS gene. The treatment of spores of carS mutants with N-methyl-N*-nitro-N-nitrosoguanidine led to the isolation, at very low frequencies, of mutants that produced higher levels of b-carotene. Strain S556 produced about 9 mg of b-carotene per g of dry mass when it was grown on minimal agar. Crosses involving strain S556 separated the original carS mutation from a new, unlinked mutation, carF. The carF segregants produced approximately as much carotene as did carS mutants, but they were unique in their ability to produce zygospores on mating and in their response to agents that increase carotenogenesis in the wild type. The carotene contents of carF segregants and carF carS double mutants were increased by sexual interaction and by dimethyl phthalate but were not increased by light or retinol. Mixed opposite-sex cultures of carF carS mutants contained up to 33 mg of bcarotene per g of dry mass. Another strain, S444, produced more b-carotene than did S556 but was marred by slow growth, defective morphology, and bizarre genetic behavior. In all the strains tested, the carotene concentration was minimal during the early growth phase and became higher and constant for several days in older mycelia.