SSU1 mediates sulphite efflux in Saccharomyces cerevisiae

Ssu1p, a plasma membrane protein involved in sulphite metabolism in Sacchar omyces cerevisiae, was found to be required for efficient sulphite efflux. An SSU1 null mutant accumulated significantly more sulphite than wild-type, whereas cells expressing multicopy SSU1 accumulated significantly less. Ce lls expressing FZF1-4, a dominant allele of a transcriptional activator of SSU1 that confers sulphite resistance, also accumulated less sulphite. beta -galactosidase activity in the FZF1-4 strain carrying an SSU1::lacZ fusion was found to be 8.5-fold higher than in a strain carrying wild-type FZF1, c onfirming that the heightened resistance was correlated with hyperactivatio n of SSU1. Multicopy SSU1 was also found to increase the sulphite resistanc e of a number of unrelated sulphite-sensitive strains by a factor of 3- to 8-fold. Rates of efflux of free sulphite from cells expressing multicopy SS U1 or FZf1-4 were significantly greater than that from wild-type or from a SSU1 null mutant. Rates of efflux of bound sulphite from wild-type, a SSU1 null mutant, a FZF1-4 mutant, or cells expressing multicopy SSU1 were not s ignificantly different, suggesting that Ssu1p specifically mediates efflux of the free form of sulphite. Copyright (C) 2000 John Wiley & Sons, Ltd.