NAD-Independent L-Lactate Dehydrogenase Is Required for L-Lactate Utilization in Pseudomonas stutzeri SDM

Background: Various Pseudomonas strains can use L-lactate as their sole carbon source for growth. However, the L-lactate-utilizing enzymes in Pseudomonas have never been identified and further studied. Methodology/Principal Findings: An NAD-independent L-lactate dehydrogenase (L-iLDH) was purified from the membrane fraction of Pseudomonas stutzeri SDM. The enzyme catalyzes the oxidation of L-lactate to pyruvate by using FMN as cofactor. After cloning its encoding gene (lldD), L-iLDH was successfully expressed, purified from a recombinant Escherichia coli strain, and characterized. An lldD mutant of P. stutzeri SDM was constructed by gene knockout technology. This mutant was unable to grow on L-lactate, but retained the ability to grow on pyruvate. Conclusions/Significance: It is proposed that L-iLDH plays an indispensable function in Pseudomonas L-lactate utilization by catalyzing the conversion of L-lactate into pyruvate.