Ward Virulent strains of Agrobacterium tumefaciens initiate tumors on plants by transferring the T-DNA from its resident Ti plasmid into plant cells. A productive interaction between bacterium and plant cell requires the appropriate attachment of the bacterium to the plant cell, and expression of a series of virulence (vir) genes in the bacterium. While the functions of several vir proteins are known, the mechanism by which the T-DNA is transported across two sets of cell walls and membranes is not known. Recent data supports the hypothesis that the mechanism of DNA transfer is analogous to that of bacterial conjugation, suggesting that a DNA transfer apparatus, such as the conjugal pore encoded by the F plasmid of E. coli, is involved. The virB operon is the logical candidate to encode the proteins of such a pore. The possible role of virB proteins in the transfer of T- DNA from the bacterium to the plant cell will be investigated using in vitro mutagenesis and recombinant DNA techniques, immunocharacterization and immunolocalization of the proteins, and biochemical approaches to determine whether or not the proteins are involved in physical interactions with bacterial membranes or DNA. Currently, plant biotechnology is heavily dependent on gene transfer based on Agrobacterium. Some plants are readily transformable by this vector, while others are not. Thus, elucidating the mechanism of DNA transfer in this system will be of great importance to plant genetic engineering.
