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A new perspective on hexose transporter gene expression in Saccharomyces cerevisiae Greatrix, Bradley W.


Saccharomyces cerevisiae contains a family of twenty hexose transporter (HXT) and ZiYT-related genes. Hexose transporters facilitate the uptake of six-carbon sugars across the plasma membrane. Of the seventeen identified HXTs, 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 (both glucose and fructose) in excess of 40 % (w/v), depletion of nutrients and extremes of both temperature and pH. These conditions may affect the transcriptional activation of HXT genes for which no function has been assigned thus far. Using 7iYTpromoter-/acZ 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, HXT 13 and to a lesser extent HXT 15 were all induced in the presence of non-fermentable carbon sources. HXT1 encodes a low affinity transporter, and previous work by other groups revealed that the HXT1 promoter activity peaked at 4 % sugar, and that expression o f this gene remained constant up to 8 % sugar (w/v). We have confirmed these results, but also extended the range of sugar concentrations tested up to 40 % (w/v). Initially, it appeared that HXT1 mRNA was upregulated 10-fold at 40 % vs 2 % glucose. However, in short-term osmotic shock experiments, HXT1 promoter activity was actually down-regulated by 40 % glucose. To reconcile these results we tested the half-life of HXT1 mRNA under osmotic pressure, and found the transcript to be stabilized in 40 % glucose. This stabilization is not dependent on HOG1. Furthermore, the stabilization of HXT1 mRNA does not appear to be gene specific because 30 minutes after transcriptional arrest there is five-fold more mRNA in osmotically stressed versus non-stressed yeast cells. The implication of this observation is that a large portion of S. cerevisiae mRNA molecules may have a decreased rate of turnover during exposure to osmotic stress.

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