AbrB antirepressor AbbA is a competitive inhibitor of AbrB-phyC interaction

In Bacillus species, the interaction between the repressor AbrB and the antirepressor AbbA is vital for regulating gene expression. Our study reveals that AbrB binds DNA cooperatively the promoter of the phyC gene through multiple tetramers, forming a complex regulatory mechanism. AbbA disrupts AbrBs DNA binding by competing for its DNA-binding sites, as shown by surface plasmon resonance (SPR) and gel shift assays. Circular dichroism (CD) confirmed that AbbA does not bind directly to the phyC promoter but mimics DNA to interfere with AbrB. Small-angle X-ray scattering (SAXS) data suggest that AbbA resembles a deformed DNA double helix. Our results indicate that AbbA binds to AbrBs DNA-binding sites located with the N-terminal domain causing AbrB displacement. The interaction exhibits negative cooperativity, with both high- and low-affinity binding sites, as evidenced by Scatchard plots and kinetic studies. Our findings suggest that AbbA effectively mimics DNA to displace AbrB, activating transition-state genes. This research enhances our understanding of bacterial gene regulation and provides insights into the complex mechanisms controlling transcription in Bacillus species.