d -Xylose Metabolism in Hypocrea jecorina : Loss of the Xylitol Dehydrogenase Step Can Be Partially Compensated for by lad1 -Encoded l -Arabinitol-4-Dehydrogenase

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Abstract
With the goal of the genetic characterization of the d -xylose pathway in Hypocrea jecorina (anamorph: Trichoderma reesei ), we cloned the xdh1 gene, encoding NA D- xylitol dehydrogenase, which catalyzes the second step of fungal d -xylose catabolism. This gene encodes a 363-amino-acid protein which has a mass of 38 kDa, belongs to the zinc-containing alcohol dehydrogenase family, exhibits high sequence identity to the published sequences of xylitol dehydrogenases from yeast origins, but contains a second, additional binding site for Zn 2+ . The enzyme catalyzed the NA D- dependent oxidation of xylitol and d -sorbitol and the NADH-dependent reduction of d -xylulose and d -fructose. No activity was observed with NADP, l -arabinose, or l -arabinitol. A single 1.4-kb transcript was formed during growth on xylan, d -xylose, l -arabinose, l -arabinitol and, at a lower abundance, xylitol, d -galactose, galactitol, and lactose but not on d -glucose and glycerol. xdh1 deletion mutants exhibited 50% reduced growth rates on d -xylose, whereas growth rates on xylitol remained unaltered. These mutants contained 30% of the xylitol dehydrogenase activity of the parent strain, indicating the presence of a second xylitol dehydrogenase. This activity was shown to be due to lad1 -encoded l -arabinitol-4-dehydrogenase, because H. jecorina xdh1 lad1 double-deletion strains failed to grow on d -xylose or xylitol. In contrast, lad1 deletion strains of H. jecorina grew normally on these carbon sources. These results show that H. jecorina contains a single xylitol dehydrogenase which is encoded by xdh1 and is involved in the metabolism of d -xylose and that lad1 -encoded l -arabinitol-4-dehydrogenase can compensate for it partially in mutants with a loss of xdh1 function.