Expression of the HXT13, HXT15 and HXT17 genes in Saccharomyces cerevisiae and stabilization of the HXT1 gene transcript by sugar-induced osmotic stress

Saccharomyces cerevisiae contains a family of 17 hexose transporter (HXT) genes; only nine have assigned functions, some of which are still poorly defined. Despite extensive efforts to characterize the hexose transporters, the expression of HXT6 and HXT8-17 remains an enigma. In nature, S. cerevisiae finds itself under extreme nutritional conditions including sugars in excess of 40% (w/v), depletion of nutrients and extremes of both temperature and pH. Using HXT promoter–lacZ fusions, we have identified novel conditions under which the HXT17 gene is expressed; HXT17 promoter activity is up-regulated in media containing raffinose and galactose at pH 7.7 versus pH 4.7. We demonstrated that HXT5, HXT13 and, to a lesser extent, HXT15 were all induced in the presence of non-fermentable carbon sources. HXT1 encodes a low-affinity transporter and in short-term osmotic shock experiments, HXT1 promoter activity was reduced when cells were exposed to media containing 40% glucose. However, we found that the HXT1 mRNA transcript was stabilized under conditions of osmotic stress. Furthermore, the stabilization of HXT1 mRNA does not appear to be gene specific because 30 min after transcriptional arrest there is a fourfold more mRNA in osmotically stressed versus non-stressed yeast cells. A large portion of S. cerevisiae mRNA molecules may, therefore, have a decreased rate of turnover during exposure to osmotic stress indicating that post-transcriptional regulation plays an important role in the adaptation of S. cerevisiae to osmotic stress.