The transport of pentose sugars in S. cerevisiae occurs through hexose trans­porters [80,81], albeit with an affinity one to two orders of magnitude lower than for hexose sugars [47,82]. Therefore, pentose transport was early con­sidered a rate-controlling step for ethanolic pentose fermentation [47]. Nev­ertheless, a metabolic control analysis study demonstrated that transport controlled xylose conversion only in strains with high XR activity, and only at low xylose concentrations [83]. Few reports exist on expression of pen­tose transporters in pentose-utilizing S. cerevisiae strains [80,84]. The most effective approach has been the overexpression of the galactose permease Gal2 in recombinant arabinose-fermenting S. cerevisiae [71]. This is in part due to the difficulty in actively expressing heterologous membrane proteins. In a recent breakthrough, the first active heterologous expression of a glu — cose/xylose facilitated diffusion transporter and a glucose/xylose symporter from Candida intermedia [85] in S. cerevisiae [86] was reported. So far the fermentation performance of these strains has not been reported. Never­theless, the expression of heterologous xylose transporters opens up new metabolic engineering strategies to further increase the rate of xylose utiliza­tion in xylose-fermenting S. cerevisiae strains.

4.2