A YEAST STEROL AUXOTROPH (ERG25) IS RESCUED BY ADDITION OF AZOLE ANTIFUNGALS AND REDUCED LEVELS OF HEME

Genetic disruption of the Saccharomyces cerevisiae C-4 sterol methyl o xidase ERG25 gene leads to sterol auxotrophy, We have characterized a suppression system that requires two mutations to restore viability to this disrupted strain, One suppressor mutation is erg11, which is blo cked in 14 alpha-demethylation of lanosterol and is itself an auxotrop h, The second suppressor mutation required is either slu1 or slu2 (sup pressor of lanosterol utilization), These mutations are leaky versions of HEM2 and HEM4, respectively; addition of exogenous hemin reverses the suppressing effects of slu1 and slu2, Suppression of erg25 by erg1 1 slu1 (or erg11 slu2) results in a slow-growing strain in which lanos terol, the first sterol in the pathway, accumulates, This result indic ates that endogenously synthesized lanosterol can substitute for ergos terol and support growth, In the triple mutants, all but 1 (ERG6) of t he 13 subsequent reactions of the ergosterol pathway are inactive, Azo le antibiotics (clotrimazole, ketoconazole, and itraconazole) widely u sed to combat fungal infections are known to do so by inhibiting the E RG11 gene product, the 14 alpha-demethylase. In this investigation, we demonstrate that treatment of the sterol auxotrophs erg25 slu1 or erg 25 slu2 with azole antibiotics paradoxically restores viability to the se strains in the absence of sterol supplementation via the suppressio n system we have described.