HOST RECOGNITION BY THE VIRA, VIRG 2-COMPONENT REGULATORY PROTEINS OFAGROBACTERIUM-TUMEFACIENS

Agrobacterium tumefaciens contains about 25 vir genes localized on a 2 00-kb tumour-inducing (Ti) plasmid that direct a conjugation-like tran sfer of tumorigenic DNA from the bacterium to the nuclei of infected p lant cells. These genes are strongly and coordinately induced during i nfection in response to three different classes of stimuli which are t hought to be key chemical features of a typical wound site. These stim uli are (i) guaiacol and syringol derivatives such as acetosyringone, (ii) sugars such as glucose and glucuronic acid, and (iii) acidic pH. The sensing of these compounds is carried out by the VirA, VirG and Ch vE proteins. VirA is a four-domain histidine protein kinase, while Vir G is a transcriptional activator which is activated by VirA-mediated p hosphorylation. ChvE is a chromosomally encoded periplasmic sugar bind ing protein which is required for sensing sugars but dispensable for s ensing the other two stimuli. Here we will review the nature of these chemical stimuli, the structure and function of the three regulatory p roteins, their similarity to sensors found in human and animal pathoge ns, the factors influencing their pool size, and their role in the hos t range of different strains of A. tumefaciens.