RAPID DIVERGENCE OF AGROBACTERIUM-VITIS OCTOPINE-CUCUMOPINE TI-PLASMIDS FROM A RECENT COMMON ANCESTOR

The octopine/cucumopine (o/c) Ti plasmids of the grapevine-associated Agrobacterium vitis strains constitute a family of related DNA molecul es. Restriction maps were established of two limited-host-range o/c Ti plasmids, pTiAg57 and pTiAB3, and of the wide-host-range o/c Ti plasm id pTiHm1. Together with the previously obtained map of the wide-host- range o/c Ti plasmid pTiTm4, about 1000 kb were mapped with a resoluti on of 0.2 kb, allowing a detailed comparison of the various structures . One region of the o/c Ti plasmids is highly conserved and differs ma inly by the presence or absence of relatively small DNA fragments (0.9 -2.7 kb); the other region has been modified more extensively and carr ies large sequences specific for each Ti plasmid type. The sequence si milarity within large conserved regions shows that these plasmids have diverged recently and that their evolution was driven by large-scale genetic events rather than single nucleotide changes. These results ha ve important implications for studies on bacterial evolution.