THE PHYSIOLOGICAL ROLES OF MEMBRANE ERGOSTEROL AS REVEALED BY THE PHENOTYPES OF SYR1 ERG3 NULL MUTANT OF SACCHAROMYCES-CEREVISIAE/

Ergosterol is a major sterol component of fungal plasma membranes. The effects of disrupting the Saccharomyces cevevisiae SYR1/ERG3 gene, wh ich encodes sterol C-5 desaturase, an enzyme of ergosterol biosynthesi s pathway, were markedly different for different S. cerevisiae strains and growth temperatures. The null mutation of SYR1 (Delta syr1) in st rain RAY-3A had only a slight effect on the growth rate at 28 degrees C. However, at this temperature, the same mutation caused poor growth in strain KA-311A and no growth in strain W303-1A. The Delta syr1 disr uptant of these strains were able to grow at 37 degrees C, as,yell as their parental strains. Moreover, the growth of the Delta syr1 disrupt ant of W303-1A and KA-311A strains were severely inhibited at 16 degre es C. These results indicated that ergosterol is essential for growth at low temperatures, and the effects of the gene disruption are variab le by the genetic background. The growth defect at low temperatures ap peared to be due to the defect of tryptophan uptake in the Delta syr1 mutants. The Delta syr1 mutants were sensitive to a wide variety of dr ugs, chemicals, and ions, suggesting that yeast ergosterol is importan t as permeability barrier against various chemical stresses.