In naturally xylose-utilizing bacteria, D-xylose is isomerized to D-xylulose [18] by xylose isomerase (XI). Xylulose is then phosphorylated to xylulose 5-phosphate [19], which is an intermediate of the pentose phosphate pathway (PPP). A similar pathway has been found in an anaerobic fungus [20]; how­ever, most naturally xylose-utilizing fungi contain a more complex pathway consisting of reduction-oxidation reactions involving the cofactors NAD(P)H and NAD(P)+ (Fig. 1). Xylose is reduced to xylitol [21-23] by a NAD(P)H — dependent xylose reductase (XR), and xylitol is then oxidized to D-xylulose by a NAD+-dependent xylitol dehydrogenase (XDH) [22,24,25]. As in bacte­ria, xylulose is phosphorylated to D-xylulose 5-phosphate by a xylulokinase (XK) [26,27]. Despite the inability of S. cerevisiae to utilize xylose, the genes encoding the reductive-oxidative xylose pathway enzymes XR, XDH, and XK are present in its genome [26,28,29]; however, they are expressed at too low levels to allow xylose utilization. Even when the genes were overexpressed, no growth on xylose could be detected [30]. Neither was it possible through adaptation protocols to upregulate the expression of these genes to levels high enough to allow significant xylose fermentation [31].

2.2