Sterols in erg mutants of Phycomyces: metabolic pathways and physiological effects

Phycomyces is a fungal producer of beta-carotene and other beneficial metabolites. Several erg mutants of Phycomyces, originally selected to study the effects of membrane alteration on physiological responses, have now been used to gain information about sterol biosynthesis in filamentous fungi. One mutant, H23, and its progeny were found to be blocked at episterol C-5 dehydrogenase and did not produce ergosterol or any other sterol with a conjugated Delta(5,7) diene system. This mutant showed abnormal phototropism, which was correlated with the altered sterol composition. Another mutant, H25, seems to be a regulatory mutant. All analyzed mutants synthesized ergosta-7,22,24(28)-trien-3beta-ol, demonstrating for the first time that the sterol C-22 dehydrogenase of Phycomyces is capable of recognizing sterols with a 24(28) unsaturated side chain. New evidence regarding the biogenesis of neoergosterol and phycomysterols, the potential sparking function of cholesterol, as well as the regulation of sterol biosynthesis in this fungus is also reported. Given these results, a pathway for sterol biosynthesis in Phycomyces is proposed.