A 2.5 kb DNA fragment of the Saccharomyces cerevisiae S Y . 7 gene was cloned by complementation of the syrl mutations that simultaneously lead t o resistance to the phytotoxin syringomycin and sensitivity of growth to high Ca2+ concentrations. Sequencing of this fragment revealed a single open reading frame encoding a polypeptide of 365 amino acids. Four hydrophobic regions each separated by hydrophilic regions were present in the protein. SYR7 was identical to ERG3, which is suggested to encode C-5 sterol desaturase required for ergosterol biosynthesis. The protein product of SYR7 was identified by Western blot analysis as a protein of 40 kDa in the particulate fraction. Gene disruption experiments demonstrated that elimination of SYR7IERG3 is not lethal, but results in membrane C-5 desaturated sterol deficiencies, resistance to syringomycin and sensitivity to high Ca2+. The syrl mutant cells had significantly decreased ability for syringomycin binding. The results indicated that C-5 desaturated sterols are involved in the binding of syringomycin to the cell, and the lack of the sterols in the mutant membrane results in sensitivity to high Ca2+ and an increased rate of cellular Ca2+ influx.

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A brief exposure (ca 20 min) of the yeast Saccharomyces cerevisiae to the phytotoxin syringomycin was sufficient to kill the cell. The protective effect of sterols against this cytotoxicity of syringomycin was investigated. Syringomycin was much more toxic to growing cells than to stationary-phase cells. The cytotoxicity of syringomycin was reduced in an environment containing sterols. Cytotoxicity of syringomycin at 3 pg ml-' (ca 2-5 p~) was completely abolished by the simultaneous presence of 10 pwcholesterol in the medium. Cholesterol acetate had no protective effect. Ergosterol, sitosterol and stigmasterol also protected against syringomycin, but they were less effective than cholesterol. The protective effect of sterols against the action of syringomycin is consistent with our hypothesis that membrane ergosterol is a critical component for syringomycin-binding as suggested by recent genetic studies.