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J. Bacteriol. doi:10.1128/JB.01895-06
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

L-Sorbose reductase and its transcriptional regulator involved in L-sorbose utilization of Gluconobacter frateurii

Department of Biological Chemistry, Faculty of Agriculture, Applied Molecular Bioscience, Graduate School of Medicine, Yamaguchi University, Yamaguchi 753-8515, Japan; Department of Microbiology, Faculty of Science, King Mongkut's University of Technology Thonburi, Prachauthit Road., Tungkru, Bangkok 10140, Thailand

^* To whom correspondence should be addressed. Email: hirot{at}yamaguchi-u.ac.jp.

	Abstract

Upstream of the gene for FAD-dependent D-sorbitol dehydrogenase, sldSLC, a putative transcriptional regulator was found in Gluconobacter frateurii THD32 (NBRC 101656). In this study, the whole of the sboR gene and the adjacent gene, sboA, were cloned and analyzed. The sboR mutation did not affect FAD-SLDH activity in the membrane fractions. The SboA enzyme expressed and purified from the Escherichia coli transformant showed NADPH-dependent L-sorbose reductase (NADPH-SR) activity, which was different from the NADPH-SR previously reported from G. suboxydans IFO 3291 in molecular size and amino acid sequence. The mutant defective in sboA showed significantly reduced growth on L-sorbose, indicating that this enzyme is required for efficient growth on L-sorbose. The sboR mutant grew on L-sorbose even better than the wild-type strain, and higher NADPH-SR activity was detected in cytoplasm fractions. RT-PCR experiments indicate that sboRA is composed of an operon. These data suggest that sboR is involved in the repression of sboA but not in the induction of sldSLC on D-sorbitol, and another activator is required for the induction of these genes by D-sorbitol or L-sorbose.