Investigation of azole resistance in the Ustilago maydis sterol demethylase mutant erg 40

Resistance to azole fungicides in Ustilago maydis (DC) Corda has been examined using the mutant erg 40, a newly isolated mutant Tri^R-1 and erg 40 revertants. Azole-induced growth arrest of the wild type did not support an obvious role for 3,6-diol in the mode of action has is clear for Saccharomyces cerevisiae Meyer ex Hansen. The level of microsomal P450 of erg 40 was identical to that of the parent, and reversion analysis showed no evidence of mutation in the sterol Δ⁵⁽⁶⁾ desaturase, as would be expected for a S. cerevisiae mutant accumulating 14α-methylfecosterol. Resistance appeared to be due to a single mutation in P450 14αdm. It is proposed that the orthologous forms of fungal sterol Δ⁵⁽⁶⁾ desaturases have varied responses when attempting to utilise 14α-methylated substrates.