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Agrobacterium tumefaciens wild-type strains have a unique quorum-sensing (QS)-dependent Ti plasmid conjugative transfer phenotype in which QS signaling is activated by corresponding conjugative opine inducers. Strain K588, with a nopaline-type chromosomal background harboring an octopine-type Ti plasmid, however, is a spontaneous mutant displaying a constitutive phenotype in QS. In this study, we show that a single amino acid mutation (L54P) in the QS antiactivator TraM encoded by the traM gene of Ti plasmid is responsible for the constitutive phenotype of strain K588. Introduction of the L54P point mutation to the TraM of wild-type strain A6 by allelic replacement, however, failed to generate the expected constitutive phenotype in this octopine-type strain. Intriguingly, the QS-constitutive phenotype appeared when the pTiA6 carrying the mutated traM was placed in the chromosomal background of the nopaline-type strain C58C1RS, suggesting an unknown inhibitory factor(s) encoded by the chromosomal background of strain A6 but not by C58C1RS. Low-stringency Southern blotting analysis showed that strain A6, but not strain C58 and its derivatives, contains a second traM homologue. The homologue, designated traM2, has 64% and 65% identities with traM at the DNA and peptide levels, respectively. Similar to TraM, TraM2 is a potent antiactivator that functions by blocking TraR, the QS activator, from specific binding to the tra gene promoters. Deletion of traM2 in strain A6 harboring the mutated traM confers a constitutive QS phenotype. The results demonstrate that the QS system in strain A6 is subjected to the dual control of TraM and TraM2.

Dual Control of Quorum Sensing by Two TraM-Type Antiactivators in Agrobacterium tumefaciens Octopine Strain A6