Molecular Strategies in the Interaction Between Agrobacterium and its Hosts

Abstract

Agrobacterium induces plant tumors by inserting a fragment (T-DNA) of its tumor inducing (Ti) plasmid into the plant chromosome. Some understanding of the functioning of genes required for this process is being elucidated. The vir genes are activated by two classes of plant signal molecules, plant phenolic compounds and a variety of plant sugars. These molecules interact with the VirA protein which then activates the VirG protein, which in turn transcriptionally activates all of the vir genes following binding to their promoter regions. The VirA and VirG proteins, members of a two component regulatory system undergo the phosphorylation and phosphate transfer demonstrated by other members of the group. The virD operon is concerned with early stages in the processing of T-DNA. The VirD 1 protein has topoisomerase activity for the Ti plasmid. The virD2 locus codes for a site specific endonuclease which nicks at the right and left borders of the T-DNA. The VirC 1 protein interacts with the overdrive sequence and in conjunction with the VirD 1 and VirD2 proteins promotes T-strand formation and the efficiency of transfer of the T-DNA. The virE operon codes for a single stranded DNA binding protein which associates with the T-DNA. The virB operon has now been sequenced and many of its protein products have been shown to be associated with the cytoplasmic membrane. Presumably these protein components provide the pore through the bacterial envelope for the exit of the T-DNA. The virB11 gene product has ATP’ase activity and autophosphorylates. Its DNA sequence is similar to that of a gene in B. subtilis required for development of competence. The VirB 10 protein is oligomeric and anchored in the inner membrane. Open reading frames 9, 10 and 11 all code for gene products which are essential for tumor formation.