trar

TraR is a transcriptional activator protein found in Agrobacterium tumefaciens and related species carrying the Ti plasmid. It is a member of the LuxR family of quorum-sensing regulators and sits at the center of the Ti plasmid’s autoinducer signaling system. TraR activity depends on binding to the autoinducer 3-oxo-octanoyl-homoserine lactone (3OC8-HSL) produced by the cognate synthase TraI. When 3OC8-HSL accumulates to a threshold level, TraR binds the ligand, dimerizes, and associates with promoter sequences known as tra boxes to activate transcription of tra genes, including traI itself. This creates a positive feedback loop that elevates AHL production and promotes plasmid conjugation among bacterial cells.

TraR function is modulated by TraM, an anti-activator encoded on the Ti plasmid that inhibits TraR-mediated transcription at lower AHL concentrations, thereby enforcing quorum-sensing control tied to cell density.

Through its regulation of tra genes, the TraR/TraI/TraM system governs horizontal transfer of the Ti plasmid, which can influence the spread of plasmid-borne traits within bacterial communities. The TraR pathway is a widely studied model of LuxR-type quorum sensing and illustrates how AHL-dependent regulators coordinate gene expression in Gram-negative bacteria.