| Summary: | Bacterial metabolism shifts from aerobic respiration to fermentation at the transition from exponential to stationary growth phases in response to limited oxygen availability. <i>Corynebacterium glutamicum</i>, a Gram-positive, facultative aerobic bacterium used for industrial amino acid production, excretes <span style="font-variant: small-caps;">l</span>-lactate, acetate, and succinate as fermentation products. The <i>ldhA</i> gene encoding <span style="font-variant: small-caps;">l</span>-lactate dehydrogenase is solely responsible for <span style="font-variant: small-caps;">l</span>-lactate production. Its expression is repressed at the exponential phase and prominently induced at the transition phase. <i>ldhA</i> is transcriptionally repressed by the sugar-phosphate-responsive regulator SugR and <span style="font-variant: small-caps;">l</span>-lactate-responsive regulator LldR. Although <i>ldhA</i> expression is derepressed even at the exponential phase in the <i>sugR</i> and <i>lldR</i> 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 <i>ldhA</i> expression was investigated. The GlxR-binding site found in the <i>ldhA</i> promoter was modified to inhibit or enhance binding of GlxR. The <i>ldhA</i> promoter activity and expression of <i>ldhA</i> were altered in proportion to the binding affinity of GlxR. Similarly, <span style="font-variant: small-caps;">l</span>-lactate production was also affected by the binding site modification. Thus, GlxR was demonstrated to act as a transcriptional activator of <i>ldhA</i>.
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