Overexpression of the HAL1 gene improves the tolerance of Saccharomyce
s cerevisiae to NaCl by increasing intracellular K+ and decreasing int
racellular Na+. The effect of HAL1 on intracellular Na+ was mediated b
y the PMR2/ENA1 gene, corresponding to a major Na+ efflux system. The
expression level of ENA1 was dependent on the gene dosage of HAL1 and
overexpression of HAL1 suppressed the salt sensitivity of mil mutants
in calcineurin and Hal3p, other known regulators of ENA1 expression. T
he effect of HAL1 on intracellular K+ was independent of the TRK1 and
TOK1 genes, corresponding to a major K+ uptake system and to a K+ effl
ux system activated by depolarization, respectively. Overexpression of
HAL1 reduces K+ loss from the cells upon salt stress: a phenomenon me
diated by an unidentified K+ efflux system. Overexpression of HAL1 did
not increase NaCl tolerance in galactose medium. NaCl poses two types
of stress, osmotic and ionic, counteracted by glycerol synthesis and
sodium extrusion, respectively. As compared to glucose, with galactose
as carbon source glycerol synthesis is reduced and the expression of
ENA1 is increased. As a consequence, osmotic adjustment through glycer
olsynthesis, a process not affected by HAL1, is the limiting factor fo
r growth on galactose under NaCl stress. (C) 1997 by John Wiley & Sons
, Ltd.