Utilization of d -Ribitol by Lactobacillus casei BL23 Requires a Mannose-Type Phosphotransferase System and Three Catabolic Enzymes

Abstract

ABSTRACT Lactobacillus casei strains 64H and BL23, but not ATCC 334, are able to ferment d -ribitol (also called d -adonitol). However, a BL23-derived ptsI mutant lacking enzyme I of the phosphoenolpyruvate:carbohydrate phosphotransferase system (PTS) was not able to utilize this pentitol, suggesting that strain BL23 transports and phosphorylates d -ribitol via a PTS. We identified an 11-kb region in the genome sequence of L. casei strain BL23 ( LCABL_29160 to LCABL_29270 ) which is absent from strain ATCC 334 and which contains the genes for a GlpR/IolR-like repressor, the four components of a mannose-type PTS, and six metabolic enzymes potentially involved in d -ribitol metabolism. Deletion of the gene encoding the EIIB component of the presumed ribitol PTS indeed prevented d -ribitol fermentation. In addition, we overexpressed the six catabolic genes, purified the encoded enzymes, and determined the activities of four of them. They encode a d -ribitol-5-phosphate ( d -ribitol-5-P) 2-dehydrogenase, a d -ribulose-5-P 3-epimerase, a d -ribose-5-P isomerase, and a d -xylulose-5-P phosphoketolase. In the first catabolic step, the protein d -ribitol-5-P 2-dehydrogenase uses NAD + to oxidize d -ribitol-5-P formed during PTS-catalyzed transport to d -ribulose-5-P, which, in turn, is converted to d -xylulose-5-P by the enzyme d -ribulose-5-P 3-epimerase. Finally, the resulting d -xylulose-5-P is split by d -xylulose-5-P phosphoketolase in an inorganic phosphate-requiring reaction into acetylphosphate and the glycolytic intermediate d -glyceraldehyde-3-P. The three remaining enzymes, one of which was identified as d -ribose-5-P-isomerase, probably catalyze an alternative ribitol degradation pathway, which might be functional in L. casei strain 64H but not in BL23, because one of the BL23 genes carries a frameshift mutation.