Abstract
The vir region of Ti plasmids is responsible for the transfer of the T region from Agrobacteria to plant cells; previous experiments suggested that formation of independent T region DNA circles is one step in this process. To study this step in Escherichia coli, we developed a binary vector system. One plasmid (= substrate) contains correctly oriented right and left borders from octopine plasmid pTiAch5. A gene with a counterselectable function (galK) was cloned between these borders. The galK gene is under control of the tac promoter-operator and the lac repressor with the laci gene also in the selection cassette. This construction allows determination of substrate plasmid mutants which have lost the selectable galK function. The second component of the system is one of a set of compatible plasmids harbouring various cloned parts from the vir region of nopaline plasmid pTiC58. A 3.6-kbp segment of the vir region turned out to be necessary and sufficient for production of substrate plasmid mutants which represented the equivalent of the T region containing a complete left border. From this vir region fragment four discrete proteins were expressed in minicells. The coding regions were mapped to a part conserved in nopaline and octopine plasmids; in the latter it appears to correspond to virC/D.