Characterization and functional analysis of the poxB gene, which encodes pyruvate oxidase in Lactobacillus plantarum

The pyruvate oxidase gene (poxB) from Lactobacillus plantarum Lp80 was cloned and characterized. Northern blot and primer extension analyses revealed that transcription of poxB is monocistronic and under the control of a vegetative promoter. poxB mRNA expression was strongly induced by aeration and was repressed by glucose. Moreover, Northern blotting performed at different stages of growth showed that poxB expression is maximal in the early stationary phase when glucose is exhausted. Primer extension and in vivo footprint analyses revealed that glucose repression of poxB is mediated by CcpA binding to the cre site identified in the promoter region. The functional role of the PoxB enzyme was studied by using gene overexpression and knockout in order to evaluate its implications for acetate production. Constitutive overproduction of PoxB in L. plantarum revealed the predominant role of pyruvate oxidase in the control of acetate production under aerobic conditions. The DeltapoxB mutant strain exhibited a moderate (20 to 25%) decrease in acetate production when it was grown on glucose as the carbon source, and residual pyruvate oxidase activity that was between 20 and 85% of the wild-type activity was observed with glucose limitation (0.2% glucose). In contrast, when the organism was grown on maltose, the poxB mutation resulted in a large (60 to 80%) decrease in acetate production. In agreement with the latter observation, the level of residual pyruvate oxidase activity with maltose limitation (0.2% maltose) was less than 10% of the wild-type level of activity.