Xenognosin sensing in virulence: is there a phenol receptor in Agrobacterium tumefaciens?

Background: The mechanisms of signal perception and transmission in the 'tw o-component' autokinase transmitters/response regulators are poorly underst ood, especially considering the vast number of such systems now known. Viru lence induction from the tumor-inducing (Ti) plasmid of Agrobacterium tumef aciens represents one of the best understood systems with regard to the che mistry of the activating signal, and yet the existing data does not support a receptor-mediated perception event for the xenognostic phenols. Results: Here we provide the first conclusive evidence that a specific rece ptor must be involved in xenognostic phenol perception, detail structural r equirements of the xenognosins necessary for perception by this receptor, a nd develop a genetic strategy that demonstrates critical components of the phenol recognition system are not encoded on the Ti plasmid. Conclusions: Although the basic elements of the two-component system requir ed for phenol-mediated induction of virulence gene expression are encoded o n the Ti plasmid, they are dependent on the chromosomal background for even the very first stage of signal perception. This discovery suggests a curio us evolutionary history, and also provides functional insight into the mech anisms of two-component signal detection and transmission in general.