Nucleotide sequence of the virulence gene virG of the Agrobacterium tumefaciens octopine Ti plasmid: significant homology between virG and the regulatory genes ompR, phoB and dye of E. coli.

The entire nucleotide sequence of the virG locus, from the octopine Ti plasmid of Agrobacterium tumefaciens strain 15955, has been determined. The virG gene is 801 nucleotides in length and has one open reading frame which encodes a protein of Mr 29,995. The virG gene is involved in the transcriptional activation of the Ti plasmid vir-loci, which occurs after induction by specific compounds present in plant exudate. Sequence analysis of the Agrobacterium virG protein showed significant homology with the Escherichia coli ompR, phoB and dye proteins, which are all positive regulatory genes for genes encoding envelope proteins. These results suggest that the virG gene encodes a positive regulatory protein which can activate vir gene expression.