The yeast PMR21£NAI gene encodes an ATPase involved in sodium extrusion and induced by NaCI. At low salt concentrations (0.3 1VID induction is mediated by the HOG-MAP kinase pathway, a system activated by non-specific osmotic stre&~. At high salt concentrations (0.8 M) induction i~ mediated by the protein phosphatase caleineurin and is specific for sodium. Protein kinase A and Sis2ptHal3p modulate PMR21£NAI expression as negative and positive factors, respectively but Sis2p/Hal3p does not perticipate in the transduction of the salt signal. Salt stress dect'eases the level of cAMP and the resulting decrease in protein kinase A activity may contribute t6 HOGmediated Induction.Key words: Signal transduction; Salt stress; MAP kinase; Calcineurin; cAMP; Saccharomyces cerevisiae 1. Introductit, n Living cells can withstand a variety of stress conditions such as extreme temperatures, water deficit, high salt concentrations, starvation etc. Under these circumstances organisms modify their cellular machinery in order to counteract the deleterious effect of damaging agents. This constitutes a stress response and involves modulation of enzymatic activities as well as changes in gene expression [1,2]. In ~'ecent years considerable interest has been raised by the mechanisms of plant tolerance to drought and salinity, two major factors limiting agricultural productivity [3]. The signal transduction pathways operating during osmotic and salt stress are only partially understood and this gap of knowledge is a limiting factor for urgent biotechnological applications [2].The yeast Saccharomyces cerevisiae can be utilized as a convenient model system in salinity studies [2]. Salt stress has two major harmful effects for cells: the loss of turgor pressure and the toxicity of Na ÷ ions to cellular metabolism. The adaptation of yeast to these conditions requires the modification of the plasma membrane transport systems to exclude Na + ions from the cytosol and the onset of glycerol synthesis to restore turgor pressure. This adaptation is largely based on changes in the expression of key genes such as PMR21ENAI (in the following referred to as ENAI), encoding an ATPase involved in sodium extrusion [4], and GPDI, encoding~a dehydrogenase involved in gljcerol synthesis [5]. Many other genes are also induced by saP, stress whose contribution to the adaptatio,a, if any, is not well understood [2].In the present work we have investigated the environmental signals and tran,,~lucing pathways mediating the induction of *Corresponding author'. Fax: (34) (6) Materials and methodsYPD and synthetic medium were prepared as described [12]. YPD contained 2% glucose, 1% yeast extract and 2% peptone. Synthetic medium contained 2% glucose, 0.7% yeast nitrogen base without amino acids (Difco), 50 mM MES [2.(N-morpholino) ethanosulfonic acid] adjusted to pH 6.0 with Tris and the amino acids, purine and pyrimidine bases required by the strains.Strains YPH499 (MATs ura3 leu2 his3 trpl lys2 ado2) and JBY10 (isogonic to YPH499 with hogl.Al:: TRPI) ...

The response of yeast to osmotic stress has been proposed to rely on the HOG-MAP kinase signalling pathway and on transcriptional activation mediated by STRE promoter elements. However, the osmotic induction of HAL1, an important determinant of salt tolerance, is HOG independent and occurs through the release of transcriptional repression. We have identified an upstream repressing sequence in HAL1 promoter (URS HAL1 ) located between -231 and -156. This promoter region was able to repress transcription from a heterologous promoter and to bind proteins in nonstressed cells, but not in salt-treated cells. The repression conferred by URS HAL1 is mediated through the Ssn6-Tup1 protein complex and is abolished in the presence of osmotic stress. The Ssn6-Tup1 co-repressor is also involved in the regulation of HOG-dependent genes such as GPD1, CTT1, ALD2, ENA1 and SIP18, and its deletion can suppress the osmotic sensitivity of hog1 mutants. We propose that the Ssn6-Tup1 repressor complex might be a general component in the regulation of osmostress responses at the transcriptional level of both HOG-dependent and -independent genes.