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Title: Isolation of an Operon Involved in Xylitol Metabolism from a Xylitol-utilizing Pantoea ananatis Mutant

Author
item SAKAKIBARA, YOSHIKIYO - JAPAN
item Saha, Badal

Submitted to: Journal of Bioscience and Bioengineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/20/2008
Publication Date: 9/1/2008
Citation: Sakakibara, Y., Saha, B.C. 2008. Isolation of an Operon Involved in Xylitol Metabolism from a Xylitol-utilizing Pantoea ananatis Mutant. Journal of Bioscience and Bioengineering. 106(4):337-344.

Interpretive Summary: Microbial production of xylitol (a low calorie anticariogenic sweetener) using pentose (five carbon) sugars (xylose, arabinose) derived from agricultural residues is attractive for reducing the manufacturing cost. Some bacteria cannot utilize xylitol as a growth substrate. A mutant bacterial strain had been isolated that could utilize xylitol. In this study, the operon (set of genes) involved in xylitol metabolism by the mutant strain has been isolated and partially characterized. The research will help to develop a recombinant bacterium that can produce xylitol from pentose sugars.

Technical Abstract: 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 E. 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 function of the operon remains unknown, the operon appears to be involved in uptake and metabolism of various sugar alcohols. A gene encoding a DeoR-type transcriptional regulator, xytR, is located upstream of xdh 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.