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Identification of amino acid residues of the pheromone‐binding domain of the transcription factor TraR that are required for positive control

Genes required for replication and for conjugal transfer of the Agrobacterium tumefaciens Ti plasmid are regulated by the quorum sensing transcription factor TraR, whose N‐terminal domain binds to the pheromone 3‐oxo‐octanoylhomoserine lactone (OOHL) and whose C‐terminal domain binds to specific DNA sequences called tra boxes. Here, we constructed 117 mutants, altering 103 surface‐exposed amino acid residues of the TraR N‐terminal domain. Each mutant was tested for activation of the traI promoter, where TraR binds to a site centred 45 nucleotides upstream of the transcription start site, and of the traM promoter, where TraR binds a site centred 66 nucleotides upstream. Alteration of 18 residues blocked activity at the traI promoter. Of these, alteration at three positions impaired TraR abundance or DNA binding, leaving 15 residues that are specifically needed for positive control. Of these 15 residues, nine also blocked or reduced activity at the traM promoter, while six had no effect. Amino acid residues required for activation of both promoters probably contact the C‐terminal domain of the RNA polymerase α subunit, while residues required only for traI promoter activation may contact another RNA polymerase component.