Involvement of Agrobacterium tumefaciens Galacturonate Tripartite ATP-Independent Periplasmic (TRAP) Transporter GaaPQM in Virulence Gene Expression

Monosaccharides capable of serving as nutrients for the soil bacteriumAgrobacterium tumefaciens are also inducers of thevir regulon present in the tumor-inducing (Ti) plasmid of this plant pathogen. One such monosaccharide is galacturonate, the predominant monomer of pectin found in plant cell walls. This ligand is recognized by the periplasmic sugar binding protein ChvE, which interacts with the VirA histidine kinase that controlsvir gene expression. Although ChvE is also a member of the ChvE-MmsAB ABC transporter involved in the utilization of many neutral sugars, it is not involved in galacturonate utilization. In this study, a putative tripartite ATP-independent periplasmic (TRAP) transporter, GaaPQM, is shown to be essential for the utilization of galacturonic acid; we show that residue R169 in the predicted sugar binding site of the GaaP is required for activity. The gene upstream ofgaaPQM (gaaR ) encodes a member of the GntR family of regulators. GaaR is shown to repress the expression ofgaaPQM , and the repression is relieved in the presence of the substrate for GaaPQM. Moreover, GaaR is shown to bind putative promoter regions in the sequences required for galacturonic acid utilization. Finally,A. tumefaciens strains carrying a deletion ofgaaPQM are more sensitive to galacturonate as an inducer ofvir gene expression, while the overexpression ofgaaPQM results in strains being less sensitive to thisvir inducer. This supports a model in which transporter activity is crucial in ensuring thatvir gene expression occurs only at sites of high ligand concentration, such as those at a plant wound site.