TERMINI AND TELOMERES IN T-DNA TRANSFORMATION

A T-DNA vector for plant transformation has been constructed in which the cloning site is located 9 bp from the right-border (RB) end and 27 bp from the left-border (LB) end. In this vector cloned DNA homologou s to plant chromosomal sequences is Located at the T-DNA termini, and will thus be exposed by even limited exonucleolysis in planta. The amb idopsis ADH (alcohol dehydrogenase) locus was mobilized from Agrobacte rium, and integration into the recipient genome was studied. Despite t he terminal location of ADH homology in this vector, the T-DNA integra ted essentially at random in the Arabidopsis genome rather than at the endogenous ADH locus. T-DNA integration was blocked, however, when Ar abidopsis telomeric sequences were added to the construct at each end of the ADH homology. Thus the predominant mode by which incoming T-DNA is integrated into the continuity of chromosomal DNA involves free DN A ends, but, in contrast to modes of recombination such as gap repair, does not involve extensive terminal DNA sequence homology.