The S. cerevisiae genome contains the gene XKS1 coding for XK [26,27], but the XK activity in wild-type S. cerevisiae is too low to support ethanolic xylose fermentation in strains engineered with a xylose pathway [26,99,100]. It is only when additional copies of XKS1 are expressed that recombinant xylose­utilizing S. cerevisiae produce ethanol from xylose [79] (strain TMB3001, Tables 1-4), [100] (strain H1691, Table 1), [101] (strain 1400 (pLNH32), Table 2; strain H2490, Tables 3 and 4). However, nonphysiological or unreg­ulated kinase activity may cause a metabolic disorder [102]. It was indeed experimentally demonstrated that only fine-tuned overexpression of XKS1 in S. cerevisiae led to improved xylose fermentation to ethanol [103,104]. Sim­ilarly, it was shown that arabinose-utilizing recombinant S. cerevisiae strains expressed a mutated L-ribulokinase gene with lower specific activity, indicat­ing that a low kinase activity had been selected as advantageous for arabinose utilization [71].

4.4