The bacterial phytotoxin syringomycin affects plasma-membrane-associated functions of plants and yeast. These include increases in transmembrane K, H and Ca fluxes and membrane potential. Mutants of resistant to growth inhibition by syringomycin were isolated and characterized. Many of the mutant isolates were unable to grow in yeast extract/peptone/dextrose medium supplemented with 400 mM-CaCl which permitted the growth of the parent strain (8A-1B). Genetic analyses of one of these mutants, strain R4-3G, showed a single recessive mutation that simultaneously led to Ca-sensitivity and syringomycin-resistance. R4-3G had higher net Ca uptake rates than strain 8A-1B and higher intracellular Ca levels in medium containing 1 mM-CaCl. The altered Ca uptake rates of the mutant were not influenced by syringomycin and were not related to altered capabilities for Ca efflux. R4-3G had similar syringomycin-stimulated increases in K efflux but lower syringomycin-stimulated increases in membrane potential than 8A-1B. It is concluded that Ca transport is important in the response of yeast to syringomycin and that the toxin-stimulated membrane potential increase, but not K efflux, is closely associated with Ca transport.


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