Signal-dependent DNA binding and functional domains of the quorum-sensing activator TraR as identified by repressor activity

TraR, a member of the LuxR family of quorum-sensing transcription factors, is responsible for the population density-dependent regulation of Ti plasmid conjugal transfer. The protein requires as coinducer an acyl-homoserine lactone signal molecule called AAI (Agrobacterium autoinducer) that is produced by the bacteria themselves. TraR only activates its target genes, making it difficult to determine whether interaction with AAI is required for binding DNA or for initiating transcription. To assess this, we converted TraR into a repressor by placing a copy of the tra box, an 18-bp inverted repeat believed to be the recognition site for this protein, over the -10 region of a promoter driving expression of lacZ, Repression of this promoter by TraR depended on AAI or, at higher concentrations, VAI, the closely related signal of Vibrio fischeri, C-terminal deletions as short as 2 aa and N-terminal deletions as short as 4 aa in TraR abolished both repressor and activator functions. The C-terminal mutants were strongly dominant over TraR, suggesting that they can form heteromultimers with the wild-type activator. Mutants of TraR with substitutions at Asp-10 and Gly-123 failed to activate a positively controlled reporter but continued to repress the chimeric promoter in an AAI-dependent manner. We conclude that TraR recognizes the tra box as its binding site, that binding of TraR to this site depends on AAI, and that the N-terminal half of the protein contains one or more domains that are required for activation but not for multimerization, for interaction with the acyl-homoserine lactone, or for DNA binding.