Genetics of L-sorbose transport and metabolism in Lactobacillus casei

Genes encoding L-sorbose metabolism of Lactobacillus casei ATCC 393 have be en identified on a 6.8-kb chromosomal DNA fragment. Sequence analysis revea led seven complete genes and a partial open reading frame transcribed as tw o units, The deduced amino acid sequences; of the first transcriptional uni t (sorRE) showed high similarity to the transcriptional regulator and the L -sorbose-1-phosphate reductase of the sorbose (sor) operon from Klebsiella pneumoniae. The other genes are transcribed as one unit (sorFABCDG) in oppo site direction to sorRE, The deduced peptide sequence of sorF showed homolo gy with the D-sorbitol-6-phosphate dehydrogenase encoded in the sor operon from K. pneumoniae and sorABCD to components of the mannose phosphotransfer ase system (PTS) family but especially to domains EIIA, EIIB, EIIC and EIID of the phosphoenolpyruvate-dependent L-sorbose PTS from K. pneumoniae. Fin ally, the deduced amino acid sequence of a truncated gene (sorG) located do wnstream of sorD presented high similarity with ketose-1,6-bisphosphate ald olases, Results of studies on enzyme activities and transcriptional analysi s revealed that the two gene clusters, sorRE and sorFABCDG, are induced by L-sorbose and subject to catabolite repression by D-glucose, Data indicatin g that the catabolite repression is mediated by components of the PTS eleme nts and by CcpA, are presented. Results of sugar uptake assays in L. casei wild-type and sorBC mutant strains indicated that Lsorbose is taken up by L -sorbose-specific enzyme II and that L, casei contains an inducible D-fruct ose-specific PTS, Results of growth analysis of those strains and a man sor BC double mutant suggested that L-sorbose is probably also transported by t he D-mannose PTS, We also present evidence, from studies on a sorR mutant, suggesting that the sorR gene encodes a positive regulator of the two sor o perons. Sequence alignment of SorR SorC (K. pneumoniae), and DeoR (Bacillus subtilis) revealed that they might constitute a new group of transcription al regulators.