Studies on the structure of cointegrates between octopine and nopaline Ti-plasmids and their tumour-inducing properties.

Stable cointegrates between incRh-1 octopine (Ach5) and nopaline (C58) Ti-plasmids, present in ten independently isolated Agrobacterium tumefaciens strains, showed identical restriction endonuclease patterns. Each cointegration event had taken place in the common sequence between the T-regions of both Ti-plasmids. This illustrates a high preference for this region when used in the formation of cointegrates. Four crown gall tissues, obtained after transformation of Nicotiana tabacum cells by one of the mutants, were analysed by using Southern blot analysis for their T-DNA structure. The borders of T-DNA frequently appeared to differ from T-DNA borders previously detected in tumour tissues that had been induced by Agrobacterium strain C58 or Ach5. Therefore, it was concluded that possibly a less stringent mechanism exists for the integration into plant DNA of T-DNA, derived from a composite (octopine/nopaline) T-region than for integration of T-DNA from a normal (octopine or nopaline) T-region.