PISATIN RESISTANCE IN DICTYOSTELIUM-DISCOIDEUM AND NEUROSPORA-CRASSA - COMPARISON OF MUTANT PHENOTYPES

The pea phytoalexin pisatin, at its inhibitory concentration, was show n to have two distinct inhibitory effects on amoebae of the cellular s lime mould Dictyostelium discoideum. One effect was cytolytic and was demonstrable even in non-growing cells whereas the second effect was o bserved only under conditions favourable to growth. Pretreatment with a sublethal concentration of pisatin induced the amoebae to acquire re sistance to both these effects. Mutations in nysC that alter membrane sterols and confer resistance to the polyene antibiotics nystatin and pimaricin blocked resistance to the growth-associated inhibitory effec t but did not affect acquisition of resistance to the cytolytic effect . The nysB sunD double mutant HK412 displayed a partially constitutive resistance to the cytolytic effect but, like the nysC mutants, was bl ocked in the acquisition of resistance to the growth-associated inhibi tory effect. Pisatin-treated cells incubated in pisatin-free medium lo st their ability to grow on pisatin-containing medium much more rapidl y than they lost resistance to the cytolytic effect of pisatin. These results suggest that the induction of pisatin resistance may involve t he turning-on of independent resistance mechanisms against the two inh ibitory effects of pisatin. This could account for our inability to is olate pisatin-resistant mutants in a single step. The Neurospora crass a erg1 and erg3 mutants that have altered membrane sterols and are nys tatin resistant displayed sensitivity to pisatin. The pisatin-sensitiv ity phenotype of the erg mutants was used in selections to identify co mplementing plasmids from an ordered Neurospora genomic library. The a ssociation of pisatin sensitivity with membrane sterol alterations in both D. discoideum and N. crassa supports the hypothesis that mechanis ms underlying nondegradative pisatin resistance are evolutionarily con served.