Extension of DNA triple helix formation to a neighbouring (AT)n site.

We have used DNase I footprinting to examine the formation of intermolecular triple helices at a fragment containing the target sequence A11(AT)6.(AT)6T11, using oligonucleotides designed to form parallel T.AT and G.TA triplets. We find that, although (TG)6 does not form a complex with (AT)6.(AT)6, T11(TG)6 forms a stable structure producing a clear footprint which includes the (AT)6 portion of the target site. This complex is not formed in the presence of magnesium, but can be stabilised by either manganese or a triplex-binding ligand.