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J Biosci Bioeng. 2008 Oct;106(4):337-44. doi: 10.1263/jbb.106.337.

Isolation of an operon involved in xylitol metabolism from a xylitol-utilizing Pantoea ananatis mutant.

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Fermentation Biotechnology Unit, National Center for Agricultural Utilization Research, Agricultural Research Service, U.S. Department of Agriculture, 1815 N. University st., Peoria, IL 61604, USA.


An operon involved in cryptic xylitol metabolism of Pantoea ananatis was cloned by transposon tagging. A xylitol negative mutant with a transposon insertion in the xylitol 4-dehydrogenase gene (xdh) was isolated and genomic DNA around the transposon was sequenced. Consequently, six consecutive genes, xytB-G are located downstream of xdh in the same strand. These seven genes are cotranscribed as a single transcript in a P. ananatis xylitol-utilizing mutant, suggesting that they comprise an operon. In addition to xdh, xytF also encodes oxidoreductase that is a member of the short-chain dehydrogenase/reductase family. Recombinant Escherichia coli that heterologously expresses the Xdh protein converts xylitol to xylulose as expected. On the other hand, the recombinant XytF protein has activity with l-arabitol but not with xylitol. XytB, xytD and xytE have significant sequence similarities to genes encoding the substrate-binding, ATP-binding and permease subunits, respectively, of ATP-binding cassette transporters. Although the physiological role of the operon remains unknown, the operon appears to be involved in uptake and metabolism of a various sugar alcohols. A gene encoding a DeoR-type transcriptional regulator, xytR, is located upstream of the operon in the opposite strand and a single nucleotide substitution that could cause a nonsense mutation is present in the xytR gene of the xylitol-utilizing mutant. This result suggests that the product of xytR negatively controls expression of the operon like other DeoR regulators.

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