Characteristics of the nopaline catabolic plasmid in Agrobacterium strains K84 and K1026 used for biological control of crown gall disease

Wild-type Agrobacterium radiobacter strain 84 and its Tra- derivative K1026, used for biological control of crown gall disease, each contain the plasmid pAtK84b. It confers incompatibility to tumor-inducing (Ti) plasmids of pathogenic A. tumefaciens, thus preventing transfer of Ti plasmids into K84 and K1026, and the consequent development of pathogens resistant to the specific antibiotic, agrocin 84 produced by K84 and K1026. pAtK84b also resembles one group of Ti plasmids in its capacity for directing nopaline catabolism. A study of the DNA homology among pAtK84b, pTiC58, and pTiAch5 was carried out. pAtK84b was transferred by conjugation to a plasmidless recipient and, after isolation, was hybridized with Ti plasmid DNA. Areas of DNA homology were located on published maps of pTiC58 and pTiAch5, a restriction enzyme map of pAtK84b was constructed, and areas of homology with DNA of known genetic function were located on the map. Strong and extensive (over 50%) homology was found between pAtK84b and pTiC58 (nopaline catabolic, Noc), but much less between pAtK84b and pTiAch5 (octopine catabolic). There was no detectable homology between pAtK84b and the oncogenic T-DNA and virulence (Vir) regions of either Ti plasmid. The size of pAtK84b was 173 kb and the orientation of regions of identified gene function (Noc, incompatability/origin of replication, and conjugal transfer) on pTiC58 was matched by the locations of homologous areas on pAtK84b. It is concluded that pAtK84b may be a deletion product of a pTiC58-type plasmid which has been disarmed in the oncogenic T-DNA and Vir regions.