Regulation of Agrobacterium tumefaciens virulence gene expression

Abstract

Expression of Agrobacterium tumefaciens virulence (vir) genes is positively regulated by virA, virG and a plant phenolic inducer(s). VirA and virG are members of the bacterial two-component regulatory systems. VirA functions as the sensor and VirG is the transcriptional activator. A cis-acting regulatory element, the vir box, serves as the VirG binding site and is essential for vir gene expression. The vir box is 14 residues in length, has a dyad symmetry and has the consensus sequence dPuPyTNCAATTGNAAPy. Expression of different vir genes requires the participation of one or more vir boxes. Inducer independent mutations in virA that support virB expression mapped to four regions of the VirA protein: the first transmembrane domain, near the active site histidine, a glycine-rich region that probably constitutes the ATP-binding domain and the carboxy-terminal domain that is homologous to the amino-terminal receiver domain of virG. Inducer and virA independent mutations in virG mapped to the receiver domain. The virA and virG mutations led to an 11 to 1350 fold increase in the basal level of virB expression. All constitutive mutations require low pH for maximal vir gene expression. The phenotype of virG and some of its derivatives are suppressed by virA in the absence of an inducer.