Identification of a Dehydrogenase Required for Lactose Metabolism in Caulobacter crescentus

Caulobacter crescentus , which thrives in freshwater environments with low nutrient levels, serves as a model system for studying bacterial cell cycle regulation and organelle development. We examined its ability to utilize lactose (i) to gain insight into the metabolic capacities of oligotrophic bacteria and (ii) to obtain an additional genetic tool for studying this model organism, aiming to eliminate the basal enzymatic activity that hydrolyzes the chromogenic substrate 5-bromo-4-chloro-3-indolyl-β-d -galactopyranoside (X-gal). Using a previously isolated transposon mutant, we identified a gene,lacA , that is required for growth on lactose as the sole carbon source and for turning colonies blue in the presence of X-gal. LacA, which contains a glucose-methanol-choline (GMC) oxidoreductase domain, has homology to the flavin subunit ofPectobacterium cypripedii 's gluconate dehydrogenase. Sequence comparisons indicated that two genes nearlacA ,lacB andlacC , encode the other subunits of the membrane-bound dehydrogenase. In addition to lactose, all threelac genes are involved in the catabolism of three other β-galactosides (lactulose, lactitol, and methyl-β-d -galactoside) and two glucosides (salicin and trehalose). Dehydrogenase assays confirmed that thelac gene products oxidize lactose, salicin, and trehalose. This enzymatic activity is inducible, and increasedlac expression in the presence of lactose and salicin likely contributes to the induction. Expression oflacA also depends on the presence of thelac genes, implying that the dehydrogenase participates in induction. The involvement of a dehydrogenase suggests that degradation of lactose and other sugars inC. crescentus may resemble a proposed pathway inAgrobacterium tumefaciens .