Relationship between the limited and wide host range octopine-type Ti plasmids of Agrobacterium tumefaciens

Abstract

The relationship between the limited host range octopine Ti plasmids [of A. tumefaciens] and the wide host range octopine Ti plasmids pTiB6806 and pTiA6 was studied. The limited host range Ti plasmids shared extensive DNA homology; pTiAg63 and pTiAg162 were essentially completely homologous with pTiAg158 while pTiAg57 shared .apprx. 64% homology with pTiAg158. The limited host range octopine Ti plasmids only shared 6-15% homology with the wide host range octopine Ti plasmid pTiB6806. Thus, limited and wide host range octopine Ti plasmids comprise distinct families of plasmids. The DNA homology shared between the limited host range Ti plasmids and pTiB6806 was distributed over some 50% of pTiB6806, suggesting that both families of plasmids evolved from a common progenitor plasmid. The limited host range Ti plasmids showed relatively strong homology with pTiB6806 HpaI fragment 7, a region which codes for octopine use by the bacterium but showed only weak homology with pTiB6806 HpaI fragment 12, a region required for virulence. Homology between the limited host range octopine Ti plasmids and the common DNA sequences shown to have a central role in plant cell transformation was barely detectable when stringent hybridization conditions were used. A highly conserved version of the common DNA is evidently not required for crown gall tumorigenesis on all plant species.