In addition to the transport flux and the flux through the initial pentose­converting enzymes, the “pulling” effect [55] of the flux through enzymatic reactions downstream of xylitol, as well as through glycolysis, appears to be equally important for ethanolic pentose fermentation. It was early recognized that the presence of glucose during xylose fermentation enhanced the gly­colytic activity [122-124]. Furthermore, it was recently shown that no xylitol was formed in the glucose-xylose coconsumption phase during xylose fer­mentation with recombinant S. cerevisiae in mineral medium [54], nor in lignocellulose hydrolysates which contain hexose sugars [6,12,14].

4.7

Other Modifications

Transcription factors involved in glucose repression have also been modi­fied in order to affect ethanolic xylose fermentation. The gene MIG1, or both MIG1 and MIG2, were deleted in an XR-XDH-XK-carrying strain of S. cere­visiae [125] to generate strains which were constantly glucose de-repressed during glucose-xylose cofermentation. This engineering strategy had little ef­fect on ethanol formation. It rather led to increased xylitol formation [125] (strains CPB. CR2 and CPB. MBH2, Table 3). Similarly, when truncated ver­sions of the MIG1 gene were expressed in xylose-utilizing strains of S. cere — visiae, growth and ethanol formation were only marginally affected [126]. The bacterial phosphoketolase pathway, which converts xylulose-5-phosphate directly to glyceraldehyde-3-phosphate and acetyl-P, has also been introduced in S. cerevisiae to enhance ethanolic xylose fermentation [127,128]. The xyl­itol yield decreased without any increase in the ethanol yield [128] (strain TMB3001c-p6XFP/p4PTA/p5EHADH2, Table 2). In contrast, heterologous ex­pression of a bacterial hemoglobin gene to render the cells a more oxidized state in oxygen-limited conditions was successful [129]. Improved ethanolic xylose fermentation was observed. This strategy is, however, only applicable in oxygenated cultures [129].

4.8