LldR is a lactate-responsive transcription factor (TF) that transcriptionally regulates the lldPRD operon consisting of lactate permease and lactate dehydrogenase. The lldPRD operon facilitates the utilisation of lactic acid in bacteria. However, the role of LldR in whole genomic transcriptional regulation, and the mechanism involved in adaptation to lactate remains unclear. We used genomic SELEX (gSELEX) to comprehensively analyse the genomic regulatory network of LldR to understand the overall regulatory mechanism of lactic acid adaptation of the model intestinal bacterium Escherichia coli . In addition to the involvement of the lldPRD operon in utilising lactate as a carbon source, genes related to glutamate-dependent acid resistance and altering the composition of membrane lipids were identified as novel targets of LldR. A series of in vitro and in vivo regulatory analyses led to the identification of LldR as an activator of these genes. Furthermore, the results of lactic acid tolerance tests and co-culture experiments with lactic acid bacteria suggested that LldR plays a significant role in adapting to the acid stress induced by lactic acid. Therefore, we propose that LldR is an l-/d-lactate sensing TF for utilising lactate as a carbon source and for resistance to lactate-induced acid stress in intestinal bacteria.

Flagella are vital bacterial organs that allow microorganisms to move to favorable environments. However, their construction and operation consume a large amount of energy. The master regulator FlhDC mediates all flagellum-forming genes in E. coli through a transcriptional regulatory cascade, the details of which remain elusive. In this study, we attempted to uncover a direct set of target genes in vitro using gSELEX-chip screening to re-examine the role of FlhDC in the entire E. coli genome regulatory network. We identified novel target genes involved in the sugar utilization phosphotransferase system, sugar catabolic pathway of glycolysis, and other carbon source metabolic pathways in addition to the known flagella formation target genes. Examining FlhDC transcriptional regulation in vitro and in vivo and its effects on sugar consumption and cell growth suggested that FlhDC activates these new targets. Based on these results, we proposed that the flagella master transcriptional regulator FlhDC acts in the activation of a set of flagella-forming genes, sugar utilization, and carbon source catabolic pathways to provide coordinated regulation between flagella formation, operation and energy production.