Summary: A 2.5 kb DNA fragment of the gene was cloned by complementation of the mutations that simultaneously lead to resistance to the phytotoxin syringomycin and sensitivity of growth to high Caconcentrations. 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. was identical to , which is suggested to encode C-5 sterol desaturase required for ergosterol biosynthesis. The protein product of was identified by Western blot analysis as a protein of 40 kDa in the particulate fraction. Gene disruption experiments demonstrated that elimination of is not lethal, but results in membrane C-5 desaturated sterol deficiencies, resistance to syringomycin and sensitivity to high Ca. The 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 Caand an increased rate of cellular Cainflux.


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