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.
Many bacteria have the capacity to utilize a large number of sugars and sugar derivatives, including sugar alcohols (polyols). For example, hexitols such as mannitol or glucitol are wellestablished carbon sources for numerous bacteria, including the Gram-negative and Gram-positive model organisms Escherichia coli and Bacillus subtilis. They can either be taken up by ion symporters, such as GutP of B. subtilis (1), or by ABC transporters (2) or can be transported and concomitantly phosphorylated by the phosphoenolpyruvate (PEP):carbohydrate phosphotransferase system (PTS) (3, 4). The three pentitols arabinitol, xylitol, and ribitol are less frequently utilized by bacteria. The first pentitol is also known under the name arabitol and the last one as adonitol. D-Ribitol is present in plants (for example, in Adonis vernalis) (5), and D-ribitol-5-phosphate (D-ribitol-5-P) is also a constituent of teichoic and lipoteichoic acids of certain Gram-positive organisms. Evidence for a D-ribitol transporter and D-ribitol-specific metabolic enzymes was first provided for the bacterium Enterobacter aerogenes (previously called Anaerobacter aerogenes and Klebsiella aerogenes). The D-ribitol dehydrogenase of this organism was purified and charact...