MECHANISM OF ACTION OF THE PHYTOTOXIN SYRINGOMYCIN - A RESISTANT MUTANT OF SACCHAROMYCES-CEREVISIAE REVEALS AN INVOLVEMENT OF CA2+ TRANSPORT

The bacterial phytotoxin syringomycin affects plasma-membrane-associated functions of plants and yeast. These include increases in transmembrane K+, H+ and Ca2+ fluxes and membrane potential. Mutants of Saccharomyces cerevisiae 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-CaCl2 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 Ca2+-sensitivity and syringomycin-resistance. R4-3G had higher net Ca-45(2+) uptake rates than strain 8A-1B and higher intracellular Ca2+ levels in medium containing 1 mM-CaCl2. The altered Ca-45(2+) uptake rates of the mutant were not influenced by syringomycin and were not related to altered capabilities for Ca2+ 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 Ca2+ 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 Ca2+ transport.