Molecular mechanism of SugR-mediated sugar-dependent expression of the ldhA gene encoding L-lactate dehydrogenase in Corynebacterium glutamicum.

This paper reports on the transcriptional regulation mechanism of the Corynebacterium glutamicum ldhA gene encoding L: -lactate dehydrogenase responsible for production of L: -lactate. DNA affinity purification allowed us to identify SugR, a global repressor of genes involved in sugar uptake and glycolysis, as a protein binding to the ldhA promoter region. Whereas ldhA gene expression and ldhA promoter activity were completely repressed during growth of wild-type cells in the absence of sugar, no such repression was observed in sugR mutant cells, indicating that SugR represses ldhA transcription. Electrophoretic mobility shift assays and DNase I footprint analyses revealed that two direct repeats, centered at position-17 with respect to the transcriptional start point, are required for SugR binding to the ldhA promoter and that fructose-1-phosphate (F-1-P) is the strongest negative effector of repressor activity of SugR. Furthermore, the ldhA promoter activity during growth on either fructose or sucrose, under which F-1-P is generated, is higher than on glucose, supporting the results of the in vitro binding assays. Thus, C. glutamicum ldhA is repressed by SugR in the absence of sugar and the expression level is dependent on the extent of derepression, which varies in response to sugars provided.