Transcriptional regulation of xylose utilization in Enterococcus mundtii QU 25

Enterococcus mundtii QU 25, a non-dairy lactic acid bacterium, produces optically pure l-lactic acid (≥99.9%) via homo-fermentation when cultured in the presence of xylose at high concentrations. However, as the xylose concentration decreases, a metabolic shift to hetero-lactic fermentation occurs in this strain. Furthermore, this strain preferentially metabolizes glucose when cultured in medium containing high concentrations of both glucose and xylose, indicating that a previously uncharacterized carbon-catabolite repression system may govern the regulation of these processes. Therefore, to increase the productivity of pure l-lactate by QU 25, it is necessary to investigate this regulatory process. In this study, we performed transcriptional analyses, including RNA sequencing to analyze the transcriptome of QU 25 cultivated in the presence of various glucose and/or xylose concentrations. Our results demonstrate that there was a gradual reduction in the expression of several genes in the xylose gene cluster as the glucose concentration increased, and that there was robust transcription of the genes involved in hetero-lactic fermentation under homo-lactic fermentation conditions. The former result indicates that transcriptional regulation of genes in the xylose gene cluster is involved in the catabolite repression observed in QU 25. The latter results show that the metabolic shift between homo- and hetero-lactic fermentation in QU 25 is not caused by the transcriptional regulation of related genes under the conditions tested. We therefore propose that a yet uncharacterized transcriptional regulation process is involved in the observed catabolite repression.