Regulation of the Transfer of TI Plasmids of Agrobacterium tumefaciens

Summary: Octopine-utilizing agrobacteria could be distinguished from non-utilizers on a solid medium containing octopine as the only added nitrogen source if a pH indicator such as bromothymolblue were present. On green bromothymolblue plates, octopine-utilizing strains formed orange-yellow colonies, while those of non-utilizers were translucent. Strains could be tested for utilization of other nitrogen sources such as nopaline by substituting these for octopine in the same medium. Using bromothymolblue plates, it was found that TI plasmids of octopine-utilizing strains of Agrobacterium tumefaciens were transferable in the presence of octopine, octopinic acid or lysopine, but not in the presence of the analogues nor-octopine, homo-octopine or desmethylhomo-octopine. Transfer in the presence of “inducer” was not detected when bacteria were mated at 37 °C instead of 29 °C, or when either methionine, cysteine or cystine was present in the mating medium. Mutant plasmids were obtained that were conjugative in the absence of an inducer; these were insensitive to methionine and cysteine but, like the wild-type plasmid, were thermosensitive for transfer. The TI plasmid of an octopine-utilizing strain was introduced into a strain that carried a deleted nopaline TI plasmid. These plasmids were found to be compatible. The properties of this transconjugant showed that the octopine oxidase does not accept nopaline as a substrate. Tumours induced on Kalanchoë daigremontiana by this bacterium were rough and contained octopine. Exclusion of phage S18 was found to be a marker specific for the TI(Kerr14) plasmid.