Regulation of the vir genes of Agrobacterium tumefaciens plasmid pTiC58

The virulence (vir) region of pTiC58 was screened for promoter activities by using gene fusions to a promoterless lux operon in the broad-host-range vector pUCD615. Active vir fragments contained the strongly acetosyringone-inducible promoters of virB, virC, virD, and virE and the weakly inducible promoters of virA and virG. Identical induction patterns were obtained with freshly sliced carrot disks, suggesting that an inducer is released after plant tissue is wounded. Optimal conditions for vir gene induction were pH 5.7 for 50 microM acetosyringone or sinapic acid. The induction of virB and virE by acetosyringone was strictly dependent on intact virA and virG loci. An increase in the copy number of virG resulted in a proportional, acetosyringone-independent increase in vir gene expression, and a further increase occurred only if an inducing compound and virA were present.