AGROBACTERIUM-TUMEFACIENS VIRB7 AND VIRB9 FORM A DISULFIDE-LINKED PROTEIN COMPLEX

Agrobacterium tumefaciens VirB proteins are essential for gene transfe r from bacteria to plants, These proteins are postulated to form a tra nsport pore to allow transfer of the T-strand DNA intermediate, To stu dy the function of the VirB proteins in DNA transfer, we developed an expression system in A, tumefaciens. Analysis of one VirB protein, Vir B9, by Western blot assays showed that under nonreducing conditions Vi rB9, when expressed alone, migrates as a similar to 31-kDa band but th at it migrates as a similar to 36-kDa band when expressed with all oth er VirB proteins, The 36-kDa band is converted to the 31-kDa band by t he reducing agent 2-mercaptoethanol, Using strains that contain a dele tion in a defined virB gene and strains that express specific VirB pro teins, we demonstrate that the 36-kDa band is composed of VirB9 and Vi rB7 that are linked to each other by a disulfide bond. Mutational stud ies demonstrate that cysteine residues at positions 24 of VirB7 and 26 2 of VirB9 participate in the formation of this complex.