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In Saccharomyces cerevisiae, hexose uptake is mediated by HXT proteins which belong to a superfamily of monosaccharide facilitators. We have identified three more genes that encode hexose transporters (HXT5, 6, 7). Genes HXT6 and HXT7 are almost identical and located in tandem 3' adjacent to HXT3 on chromosome IV. We have constructed a set of congenic strains expressing none or any one of the seven known HXT genes and followed growth and flux rates for glucose utilization. The hxt null strain does not grow on glucose, fructose or mannose, and both glucose uptake and flux rate were below the detection level. Expression of either HXT1, 2, 3, 4, 6 or 7 is basically sufficient for aerobic growth on these sugars. In most of the constructs, glucose was the preferred substrate compared to fructose or mannose. There is a considerable variation in flux and growth rates with 1% glucose, dependent on the expression of the individual HXT genes. Expression of either HXT2, 6 or 7 in the null background is sufficient for growth on 0.1% glucose, while growth of strains with only HXT1, 3 or 4 requires higher (> or = 1%) glucose concentrations. These results demonstrate that individual HXT proteins can function independently as hexose transporters, and that most of the metabolically relevant HXT transporters from S. cerevisiae have been identified.
Reifenberger et al. (Sat,) studied this question.