IDENTIFICATION OF NOVEL HXT GENES IN SACCHAROMYCES-CEREVISIAE REVEALSTHE IMPACT OF INDIVIDUAL HEXOSE TRANSPORTERS ON GLYCOLYTIC FLUX

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 conge nic strains expressing none or any one of the seven known HXT genes an d followed growth and flux rates for glucose utilization. The hxt null strain does not grow on glucose, fructose or mannose, and both glucos e uptake and flux rate were below the detection level. Expression of e ither 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 va riation in flux and growth rates with 1% glucose, dependent on the exp ression of the individual HXT genes. Expression of either HXT2, 6 or 7 in the null background is sufficient for growth on 0.1% glucose, whil e growth of strains with only HXT1, 3 or 4 requires higher (greater th an or equal to 1%) glucose concentrations. These results demonstrate t hat individual HXT proteins can function independently as hexose trans porters, and that most of the metabolically relevant HXT transporters from S. cerevisiae have been identified.