AGROBACTERIUM-TUMEFACIENS TRANSFERS SINGLE-STRANDED TRANSFERRED DNA (T-DNA) INTO THE PLANT-CELL NUCLEUS

Transferred DNA (T-DNA) is transferred as a single-stranded derivative from Agrobacterium to the plant cell nucleus. This conclusion is draw n from experiments exploiting the different properties of single- and double-stranded DNA to perform extrachromosomal homologous recombinati on in plant cells. After transfer from Agrobacterium to plant cells, T -DNA molecules recombined much more efficiently if the homologous sequ ences were of opposite polarity than if they were of the same polarity . This observation reflects the properties of single stranded DNA; sin gle-stranded DNA molecules of opposite polarity can anneal directly, w hereas single-stranded DNA molecules of the same polarity first have t o become double stranded to anneal. Judging from the relative amounts of single- to double-stranded T-DNA derivatives undergoing recombinati on, we infer that the T-DNA derivatives enter the plant nucleus in the ir single-stranded form.