SUMMARY: Mutants of have been selected for resistance to the polyene antibiotics, etruscomycin, filipin, nystatin, pimaricin and rimocidin. All mutants are resistant to nystatin and there are several patterns of cross-resistance. The mutants were allocated to four unlinked genes and A fifth gene, was recovered as a double mutant with a strain carrying the mutation. The mutation does not cause resistance, and are allelic to the previously described and (Ahmed & Woods, 1967). There are correlations between the polyene used for mutant isolation and (i) the extent of cross-resistance; (ii) the selection of mutants at particular genes. Ultraviolet absorption spectra of non-saponifiable material extracted from representative mutants showed that all five genes affect the sterol composition of the cell. The major sterols found in are ergosterol and 24,(28) dehydroergosterol, the latter is not found in any of the mutants whilst ergosterol is lacking in and only present at very low levels in The spectra of extracts of and indicate the presence of new sterols (Woods, 1971). Mutants of excrete sterol into the growth medium. Studies on double mutants indicate epistasis between genes with respect to sterol pattern and suggest that they are metabolically related in the sequence . A rapid qualitative technique for the preparation of non-saponifiable extracts of yeast for sterol analysis is described.


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