A streamlined ABC extruder-repressor module drives multi-bacteriocin resistance in streptococci

ABSTRACT Bacteria inhabiting competitive microbial environments must rapidly detect and neutralize antimicrobial peptides (AMPs) produced by rivals. The activation of defense pathways relies on dedicated sensors and complex regulatory cascades. Here, we uncover a streamlined, membrane-embedded mechanism in Gram-positive bacteria that directly links detection to transcriptional control. We show that a YtrA-family transcriptional repressor is regulated through an unconventional direct physical interaction with its cognate ABC transporter. In Streptococcus salivarius, the MbrAB efflux pump sequesters the YtrA-like repressor MbrR through a competitive binding mechanism. Upon bacteriocin sensing, MbrR shifts from promoter-proximal DNA repression sites to membrane-associated sequestration via interaction with MbrA, thereby activating the bacteriocin defense system. ATP binding by MbrA facilitates MbrR recruitment, whereas ATP hydrolysis promotes its release, providing a dynamic, flux-responsive feedback loop finely tuned to environmental threat. Structural modeling, synteny, and conservation analyses reveal that this transporter-mediated sequestration mechanism is highly conserved across Gram-positive bacteria, suggesting a widespread and efficient strategy for AMP resistance that bypasses the need for classical sensor kinases. These findings expand the known repertoire of bacterial sensing and resistance systems and provide new insight into how Gram-positive bacteria swiftly adapt to interbacterial antagonism. IMPORTANCE Bacteria in crowded microbial communities face constant chemical warfare, yet how they sense and counteract antimicrobial peptides (AMPs) remains incompletely understood. Here we uncover a minimalist sensing-response module in streptococci in which YtrA-family transcriptional repressors are directly regulated by their partner ABC transporters. In Streptococcus salivarius, the MbrAB transporter detects incoming bacteriocins and physically sequesters the MbrR repressor, triggering rapid induction of a multi-gene defense program. ATP binding drives MbrR capture, while hydrolysis resets the system, providing a fast, energy-coupled switch that bypasses canonical two-component signaling. Comparative genomics shows that this transporter-repressor circuit is highly conserved across Bacillota, pointing to a broadly distributed and efficient strategy for AMP resistance. These findings reveal a direct physical link between membrane transport and transcriptional control, redefining how Gram-positive bacteria can sense and respond to microbial threats. ONE SENTENCE SUMMARY We identify a conserved transporter-repressor module in streptococci that directly couples bacteriocin detection to transcriptional activation, revealing a minimalist and rapid mechanism for antimicrobial peptide resistance. Competing Interest Statement The authors have declared no competing interest.