Functional Characterization of the Histidine Kinase BaeS Reveals Critical Residues for BaeSR-Dependent Stress Signaling in Escherichia coli

Escherichia coli, a facultative anaerobic Gram-negative member of the Enterobacteriaceae, is an increasingly important opportunistic pathogen driven in part by rising resistance to clinically important antibiotics. Regulation of multidrug efflux systems by two-component signal transduction pathways, particularly the BaeSR system, plays a central role in this process. However, the functional residues governing signal transduction through the sensor kinase BaeS remain incompletely defined. In this study, we integrated domain prediction, homology-guided site-directed mutagenesis, in vitro protein purification, autophosphorylation assays, and reverse-transcription quantitative polymerase chain reaction (RT-qPCR)-based transcriptional analysis of selected BaeSR-regulated genes to delineate key residues required for BaeS function. Sequence analysis identified His250 as a candidate autophosphorylation site and Asn364 as a conserved residue within the catalytic domain. Biochemical characterization of purified wild-type BaeS and an H250A mutant demonstrated that His250 is indispensable for autophosphorylation. Consistently, RT-qPCR analysis showed that BaeS activation markedly induced the transcription of BaeSR-regulated efflux-associated genes, whereas genetic deletion of baeS or selective disruption of kinase activity by the N364A mutation abolished this response. Together, these findings establish His250 as a key residue for BaeS autophosphorylation and identify Asn364 as essential for inducible BaeSR signaling and activation of resistance-associated target genes, thereby establishing an experimental framework for elucidating BaeSR-mediated efflux regulation and informing future studies of resistance regulatory networks and potential intervention strategies centered on key signaling nodes.