Cross-talk between the L-sorbose and D-sorbitol (D-glucitol) metabolic pathways in Lactobacillus casei

A gene encoding sorbitol-6-phosphate dehydrogenase (SorF) belonging to the sorbose operon (sorFABCDG) has been characterized in Lactobacillus casei. Inactivation of this gene revealed the presence of another sorbitol-6-phosphate dehydrogenase that was induced by D-sorbitol (D-glucitol). The gene encoding this activity (gutF) has also been isolated, sequenced and disrupted. The sorbitol-6-phosphate dehydrogenase genes (sorF, gutF) were required for growth on L-sorbose and D-sorbitol, respectively. Biochemical and transcriptional analyses of the wild-type and mutant strains demonstrated that L-sorbose and D-sorbitol induced sorF and the gene encoding the sorbose operon activator (sorR), while the expression of gutF was only activated by D-sorbitol. Furthermore, these studies indirectly suggested that a common metabolite of the L-sorbose and D-sorbitol metabolic pathways (probably D-sorbitol 6-phosphate) would act as the effector of SorR. The same effector would also be the inducer of gutF, although the two pathways seem to be subject to distinct regulatory mechanisms.