Characterization of Genes Involved in D-Sorbitol Oxidation in Thermotolerant Gluconobacter frateurii

Further upstream of sldSLC, genes for FAD-dependent D-sorbitol dehydrogenase in Gluconobacter frateurii, three additional genes (sldR, xdhA, and perA) are found: for a transcriptional regulator, NAD(P)-dependent xylitol dehydrogenase, and a transporter protein, a member of major facilitator superfamily, respectively. xdhA and perA but not sldR were found to be in the same transcriptional unit. Disruption of sldR resulted in a dramatic decrease in sldSLC promoter activity, indicating that it is an activator for sldSLC expression. The recombinant protein of XdhA expressed in Escherichia coli showed NAD-dependent dehydrogenase activities with xylitol and D-sorbitol, but a mutant strain defective in this gene showed similar activities with both substrates as compared to the wild-type strain. Nonetheless, the growth of the xdhA mutant strain on D-sorbitol and xylitol was retarded, and so was that of a mutant strain defective in perA. These results indicate that xdhA and perA are involved in assimilation of D-sorbitol and xylitol.