We have previously shown that the mutation of the Schizosaccharomyces pombe PPZ-like protein phosphatase encoded by the gene pzh1 + results in increased tolerance to sodium and in hypersensitivity to potassium ions. A similar phenotype has also been reported for deletants in the spm1/pmk1 gene, encoding a mitogen-activated protein (MAP) kinase. We have found that the sodium tolerance phenotype of pzh1 deletants is stronger than that of spm1 mutants, and both effects are additive. Therefore, most probably both gene products mediate different pathways on sodium tolerance. In our hands, mutation of the kinase does not alter the tolerance to potassium, but it yields cells more tolerant to magnesium ions. While in budding yeast the mutations are synthetically lethal, fission yeast cells lacking both the phosphatase and the kinase genes are viable. Interestingly, their ability to export H + to the medium is greatly impaired (although not that of pzh1 or spm1 single mutants). We have observed that, although the amount of the H + -ATPase in the plasma membrane is not altered, the activity of the enzyme is lower than normal and cannot be induced by glucose. These observations suggest that the activity of the H + -ATPase in fission yeast might be regulated by phospho-dephosphorylation mechanisms that might involve the pzh1 + and spm1 + gene products.z 1998 Federation of European Biochemical Societies.

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