The Gene Encoding Fermentative l-Lactate Dehydrogenase in Is Positively Regulated by the Global Regulator GlxR.

Bacterial metabolism shifts from aerobic respiration to fermentation at the transition from exponential to stationary growth phases in response to limited oxygen availability. , a Gram-positive, facultative aerobic bacterium used for industrial amino acid production, excretes l-lactate, acetate, and succinate as fermentation products. The gene encoding l-lactate dehydrogenase is solely responsible for l-lactate production. Its expression is repressed at the exponential phase and prominently induced at the transition phase. is transcriptionally repressed by the sugar-phosphate-responsive regulator SugR and l-lactate-responsive regulator LldR. Although expression is derepressed even at the exponential phase in the and double deletion mutant, a further increase in its expression is still observed at the stationary phase, implicating the action of additional transcription regulators. In this study, involvement of the cAMP receptor protein-type global regulator GlxR in the regulation of expression was investigated. The GlxR-binding site found in the promoter was modified to inhibit or enhance binding of GlxR. The promoter activity and expression of were altered in proportion to the binding affinity of GlxR. Similarly, l-lactate production was also affected by the binding site modification. Thus, GlxR was demonstrated to act as a transcriptional activator of .