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.