l -Sorbose Reductase and Its Transcriptional Regulator Involved in l -Sorbose Utilization of Gluconobacter frateurii

Abstract

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