Antagonism by the Type VI secretion system of is controlled by a TetR family regulator and released small molecule.

Antagonistic systems of bacteria are often tightly regulated. The human gut Bacteroidales harbor three distinct antagonistic Type VI secretion systems (T6SS), one of which is present only in , known as the GA3 T6SS. Although this is the best studied of the three T6SSs, little is known about how it is regulated. The gene upstream of the GA3 T6SS locus encodes a TetR family transcriptional regulator (TetR), which we show represses expression of the GA3 T6SS locus. The gene immediately upstream and divergently transcribed from , designated here as , encodes a product of the α-oxoamine synthase family of pyridoxal phosphate-dependent enzymes with structural homology to the CqsA autoinducer synthase of the CAI-1 quorum sensing system of . When is deleted, transcription of the GA3 T6SS locus is repressed in a TetR-dependent manner. Strains synthesizing Lgs produce a molecule present in the supernatant that likely serves as the TetR ligand, overcoming TetR transcriptional repression of the GA3 T6SS. We show that GA3 T6SS-specific immunity genes present on two acquired immunity defense islands are also regulated by Lgs coordinating expression of GA3 T6SS antagonism with protection from competitor's GA3 T6SS toxins. Production and firing of the GA3 T6SS and subsequent antagonism occurs in bacteria deleted for when grown with bacteria containing this gene or their supernatants. These data show that the GA3 T6SS is regulated by a small molecule acting through TetR allowing the bacteria to coordinate antagonistic and protective systems.