Roles of d-Lactate Dehydrogenases in the Anaerobic Growth of MR-1 on Sugars.

MR-1 is a facultative anaerobe that respires using a variety of electron acceptors. Although this organism is incapable of fermentative growth in the absence of electron acceptors, its genome encodes LdhA (a putative fermentative NADH-dependent d-lactate dehydrogenase [d-LDH]) and Dld (a respiratory quinone-dependent d-LDH). However, the physiological roles of LdhA in MR-1 are unclear. Here, we examined the activity, transcriptional regulation, and traits of deletion mutants to gain insight into the roles of LdhA in the anaerobic growth of MR-1. Analyses of d-LDH activity in MR-1 and the deletion mutant confirmed that LdhA functions as an NADH-dependent d-LDH that catalyzes the reduction of pyruvate to d-lactate. and assays revealed that expression was positively regulated by the cyclic-AMP receptor protein, a global transcription factor that regulates anaerobic respiratory pathways in MR-1, suggesting that LdhA functions in coordination with anaerobic respiration. Notably, we found that a deletion mutant of all four NADH dehydrogenases (NDHs) in MR-1 (ΔNDH mutant) retained the ability to grow on -acetylglucosamine under fumarate-respiring conditions, while an additional deletion of or deprived the ΔNDH mutant of this growth ability. These results indicate that LdhA-Dld serves as a bypass of NDH in electron transfer from NADH to quinones. Our findings suggest that the LdhA-Dld system manages intracellular redox balance by utilizing d-lactate as a temporal electron sink under electron acceptor-limited conditions. NADH-dependent LDHs are conserved among diverse organisms and contribute to NAD regeneration in lactic acid fermentation. However, this type of LDH is also present in nonfermentative bacteria, including members of the genus , while their physiological roles in these bacteria remain unknown. Here, we show that LdhA (an NADH-dependent d-LDH) works in concert with Dld (a quinone-dependent d-LDH) to transfer electrons from NADH to quinones during sugar catabolism in MR-1. Our results indicate that d-lactate acts as an intracellular electron mediator to transfer electrons from NADH to membrane quinones. In addition, d-lactate serves as a temporal electron sink when respiratory electron acceptors are not available. Our study suggests novel physiological roles for d-LDHs in providing nonfermentative bacteria with catabolic flexibility under electron acceptor-limited conditions.