Regulation of Agrobacterium tumefaciens virulence gene expression: isolation of a mutation that restores virGD52E function.

Expression of Agrobacterium tumefaciens virulence (vir) genes is controlled by virA, virG, and a plant inducer. Isolation of two constitutive mutants of the transcriptional activator virG, virGN54D, and virGI106L, that support vir gene expression in a virA independent manner has previously been reported. Characterization of virGN54D by several groups showed considerable variation in its ability to activate vir gene transcription. In this study we demonstrate that these differences can be accounted for by plasmid copy number. We report the isolation of a third constitutive mutation, virGI77V, that partially restores transcription activation function of a nonfunctional virG mutant, virGD52E. The second regulator, VirA, in its extreme C-terminus, contains a domain that is homologous to the N-terminal domain of VirG. Deletion of this domain of VirA leads to a fully constitutive phenotype.