CpxR and HicB exert independent regulatory action on the gonococcal hicAB-encoded toxin-antitoxin system

The continued emergence of Neisseria gonorrhoeae (Ng) isolates resistant to front-line antibiotics has focused efforts on understanding how alternative therapies, such as the expanded use of gentamicin (Gen), might counteract this global public health problem. Focusing on Gen as a viable alternative antibiotic for the treatment of gonorrheal infections, we previously used RNA-seq to determine if sub-lethal levels of Gen might impact gonococci on a transcriptional level and showed that expression of the putative HicA-HicB toxin-antitoxin (TA) system was increased in response to sub-lethal Gen. Importantly, loss of this TA system resulted in reduction of Ng biofilm formation in a strain specific manner. Focusing on this strain specificity, we found that the CpxR/CpxA two-component system (TCS) influences expression of the hicAB operon independently of HicB autoregulation. We now report that CpxR selectively binds to the hicAB operon to enhance expression of hicAB but does not interfere with binding of HicB to the promoter region. Furthermore, we show that single base pair differences in the intergenic region between hicA and hicB impact regulation by CpxR. Hence, the regulation of the HicAB TA in gonococcal strains is a highly coordinated response that can involve autoregulation by HicB and the CpxRA TCS. We propose that this dual regulatory scheme maximizes the ability of Ng to respond to Gen and hostile environmental conditions.