초록
<P>Xylose reductase (XR) is the first enzyme in <SMALL>D</SMALL>-xylose metabolism, catalyzing the reduction of <SMALL>D</SMALL>-xylose to xylitol. Formation of XR in the yeast <I>Candida tropicalis</I> is significantly repressed in cells grown on medium that contains glucose as carbon and energy source, because of the repressive effect of glucose. This is one reason why glucose is not a suitable co-substrate for cell growth in industrial xylitol production. XR from the ascomycete <I>Neurospora crassa</I> (NcXR) has high catalytic efficiency; however, NcXR is not expressed in <I>C</I>. <I>tropicalis</I> because of difference in codon usage between the two species. In this study, NcXR codons were changed to those preferred in <I>C</I>. <I>tropicalis</I>. This codon-optimized NcXR gene (termed NXRG) was placed under control of a constitutive glyceraldehyde-3-phosphate dehydrogenase (GAPDH) promoter derived from <I>C</I>. <I>tropicalis</I>, and integrated into the genome of xylitol dehydrogenase gene (<I>XYL2</I>)-disrupted <I>C</I>. <I>tropicalis</I>. High expression level of NXRG was confirmed by determining XR activity in cells grown on glucose medium. The resulting recombinant strain, LNG2, showed high XR activity (2.86 U (mg of protein)<SUP>−1</SUP>), whereas parent strain BSXDH-3 showed no activity. In xylitol fermentation using glucose as a co-substrate with xylose, LNG2 showed xylitol production rate 1.44 g L<SUP>−1</SUP> h<SUP>−1</SUP> and xylitol yield of 96% at 44 h, which were 73 and 62%, respectively, higher than corresponding values for BSXDH-3 (rate 0.83 g L<SUP>−1</SUP> h<SUP>−1</SUP>; yield 59%).</P>