Identification and characterization of a second quorum-sensing system in Agrobacterium tumefaciens A6.

Quorum sensing (QS) is a widespread mechanism of bacterial communication in which individual cells produce and respond to small chemical signals. In Agrobacterium tumefaciens, an acylhomoserine lactone-dependent QS mechanism is known to regulate the replication and conjugation of the tumor-inducing (Ti) plasmid. Most of the QS regulatory proteins are encoded within the Ti plasmid. Among them, TraI is the LuxI-type enzyme synthesizing the QS signal N-3-oxooctanoyl-L-homoserine lactone (3OC8HSL), TraR is the LuxR-type transcriptional factor that recognizes 3OC8HSL, and TraM is an antiactivator that antagonizes TraR. Recently, we identified a TraM homolog encoded by the traM2 gene in the chromosomal background of A. tumefaciens A6. In this study, we further identified additional homologs (TraI2 and TraR2) of TraI and TraR in this strain. We showed that similar to TraI, TraI2 could predominantly synthesize the QS signal 3OC8HSL. We also showed that TraR2 could recognize 3OC8HSL and activate the tra box-containing promoters as efficiently as TraR. Further analysis showed that traM2, traI2, and traR2 are physically linked on a mobile genetic element that is not related to the Ti plasmid. These findings indicate that A. tumefaciens A6 carries a second QS system that may play a redundant role in the regulation of the replication and conjugation of the Ti plasmid.