THE PRESENCE AND CHARACTERIZATION OF A VIRF GENE ON AGROBACTERIUM-VITIS TI PLASMIDS

Octopine and nopaline strains of Agrobacterium tumefaciens differ in t heir ability to induce tumors on Nicotiana glauca, The presence of a v irF locus on the octopine Ti plasmid makes N. glauca a host plant for these strains, indicating that the VirF protein is a host-range determ inant. Here we show the presence of a virF locus not only on the Agrob acterium vitis octopine/cucumopine plasmids pTiAg57 and pTiTm4, but al so on the nopaline Ti plasmids pTiAT1, pTiAT66(a), and pTiAT66(b). On the octopine Ti plasmids from A. tumefaciens the virF gene is located between the virE locus and the left border of the T-region. In contras t, the virF gene on Ti plasmids of A. vitis is located at the very lef t end of the vir-region near the virA locus. The virF gene of pTiAg57 has been sequenced and codes for a protein of 202 amino acids with a m olecular mass of 22,280 Da, Comparison showed that the virF gene from A. vitis strain Ag57 is almost identical to that from A. tumefaciens o ctopine strains. The transcription of the pTiAg57 virF is inducible by the plant phenolic compound acetosyringone through the presence of a vir-box consensus sequence in its promoter region. The VirF protein fr om pTiAg57 can complement octopine A. tumefaciens strains deleted for virF as shown by tumor formation on N. glauca.