Physiology of mutants with reduced expression of plasma membrane H+‐ATPase

Two mutations containing insertions and deletions in the promoter in the plasma membrane H+‐ATPase gene (PMA1) of Saccharomyces cerevisiae have been introduced into the genome by homologous recombination, replacing the wild‐type gene. The resulting strains have 15 and 23% of the wild‐type ATPase content. Decreased levels of ATPase correlate with decreased rates of proton efflux and decreased uptake rates of amino acids, methylamine, hygromycin B and tetraphenylphosphonium. This supports a central role of the enzyme in yeast bioenergetics. However, the final accumulation gradient of tetraphenylphosphonium is not affected by the mutations and that of methylamine and 2‐aminoisobutyric acid is only decreased in the most extreme mutant. Apparently, kinetic constraints seem to prevent the equilibration of yeast active transports with the electrochemical proton gradient. As expected from their transport defects, the ATPase‐deficient mutants are more resistant to hygromycin B and more sensitive to acidification than wild‐type yeast. Mutant cells are very elongated, suggesting a structural role of the ATPase in the yeast surface.