GENERAL RESISTANCE TO STEROL BIOSYNTHESIS INHIBITORS IN SACCHAROMYCES-CEREVISIAE

Screening for resistance to fenpropimorph was undertaken in order to i solate yeast mutants affected in the regulation of the ergosterol path way. Among the mutants isolated, one bearing the recessive fen1-1 muta tion was characterized by a 1.5-fold increase in the ergosterol level and a general resistance to sterol biosynthesis inhibitors. The fen1-1 mutation was linked to MAT locus on chromosome III. The measurement o f enzyme activities involved in the ergosterol pathway revealed that i sopentenyl diphosphate (IPP) isomerase activity was specifically incre ased 1.5-fold as compared to the wild type strain. However, overexpres sion of IPP isomerase in the wild type strain was not by itself suffic ient to lead to sterol increase or resistance to sterol biosynthesis i nhibitors, showing that IPP isomerase is not a limiting step in the pa thway. The fen1-1 mutation permits viability in aerobiosis of yeast di srupted for sterol-14 reductase in absence of exogenous ergosterol sup plementation, whereas the corresponding strain bearing the wild type F EN1 allele grows only in anaerobiosis. This result shows that ignoster ol is able to efficiently replace ergosterol as bulk membrane componen t and that the fen1-1 mutation eliminates the specific ergosterol requ irement in yeast.