2 CLASSES OF PLANT CDNA CLONES DIFFERENTIALLY COMPLEMENT YEAST CALCINEURIN MUTANTS AND INCREASE SALT TOLERANCE OF WILD-TYPE YEAST

The salt-sensitive phenotype of yeast cells deficient in the phosphopr otein phosphatase, calcineurin, was used to identify genes from the hi gher plant Arabidopsis thaliana that complement this phenotype, cDNA c lones corresponding to two different sequences, designated STO (salt t olerance) and STZ (salt tolerance zinc finger), were found to increase d tolerance of calcineurin mutants and of wild-type yeast to both Liand Na+ ions, STZ is related to Cys(2)/His(2)-type zinc-finger protein s found in higher plants, and STO is similar to the Arabidopsis CONSTA NS protein in regions that may also be zinc fingers, Although neither protein has sequence similarity to any protein phosphatase, STO was ab le to at least partially compensate for all tested additional phenotyp ic effects of calcineurin deficiency, and STZ compensated for a subset of these effects, Salt tolerance produced by STZ appeared to be parti ally dependent on ENA1/PMR2, a P-type ATPase required for Li+ and Naefflux in yeast, whereas the effect of STO on salt tolerance was indep endent of ENA1/PMR2, STZ and STO were found to be expressed in Arabido psis roots and leaves, whereas only STO message was detectable in flow ers. An apparent increase in the level of STZ mRNA was observed in res ponse NaCl exposure in Arabidopsis seedlings, but the level of STO mRN A was not altered by this treatment.