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.