Leucine responsive regulatory protein is involved in methionine metabolism and polyamine homeostasis in acetic acid bacterium Komagataeibacter europaeus.

The leucine responsive regulatory protein (Lrp) is a global transcription factor that regulates the expression of genes involved in amino acid metabolism. To identify metabolic pathways and related genes under the control of Lrp in the acetic acid bacterium Komagataeibacter europaeus, the Kelrp null mutant (KGMA7110), which requires supplementation of all 20 amino acids for normal growth, was cultivated in minimal media containing or lacking particular amino acids. The results confirmed that KGMA7110 was auxotrophic for methionine and its catabolites S-adenosylmethionine (SAM) and spermidine (SPD). Quantitative reverse-transcription PCR analysis revealed lower metK (SAM synthetase) and mdtI (SPD efflux pump) expression in KGMA7110 than in wild-type KGMA0119. By contrast, these genes were significantly up-regulated in the Kelrp mutant lacking the putative C-terminal ligand-sensing domain (KGMA7203), indicating abnormal regulation of target genes by the KeLrp variant in KGMA7203. KGMA7110 (0.69±0.27 µM) and KGMA7203 (4.90±0.61 µM) excreted lower and higher quantities of SPD, respectively, than KGMA0119 (2.28±0.26 µM). This was attributed to imbalanced carbon flow caused by Kelrp disruption that respectively attenuated and stimulated metK and mdtI expression. These findings indicate that KeLrp plays a key role in SAM biosynthesis and intracellular polyamine homeostasis in K. europaeus.

Expression of the alaE gene is positively regulated by the global regulator Lrp in response to intracellular accumulation of l-alanine in Escherichia coli.

The alaE gene in Escherichia coli encodes an l-alanine exporter that catalyzes the active export of l-alanine using proton electrochemical potential. In our previous study, alaE expression was shown to increase in the presence of l-alanyl-l-alanine (Ala-Ala). In this study, the global regulator leucine-responsive regulatory protein (Lrp) was identified as an activator of the alaE gene. A promoter less ß-galactosidase gene was fused to an alaE upstream region (240 nucleotides). Cells that were lacZ-deficient and harbored this reporter plasmid showed significant induction of ß-galactosidase activity (approximately 17-fold) in the presence of 6 mM l-alanine, l-leucine, and Ala-Ala. However, a reporter plasmid possessing a smaller alaE upstream region (180 nucleotides) yielded transformants with strikingly low enzyme activity under the same conditions. In contrast, lrp-deficient cells showed almost no ß-galactosidase induction, indicating that Lrp positively regulates alaE expression. We next performed an electrophoretic mobility shift assay (EMSA) and a DNase I footprinting assay using purified hexahistidine-tagged Lrp (Lrp-His). Consequently, we found that Lrp-His binds to the alaE upstream region spanning nucleotide -161 to -83 with a physiologically relevant affinity (apparent KD, 288.7 ± 83.8 nM). Furthermore, the binding affinity of Lrp-His toward its cis-element was increased by l-alanine and l-leucine, but not by Ala-Ala and d-alanine. Based on these results, we concluded that the gene expression of the alaE is regulated by Lrp in response to intracellular levels of l-alanine, which eventually leads to intracellular homeostasis of l-alanine concentrations.