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Formation of DNA triple helices incorporating blocks of G.GC and T.AT triplets using short acridine-linked oligonucleotides.

We have used DNase I footprinting to assess triple helix formation at target sites containing the sequences A6G6.C6T6 and G6A6.T6C6. These sequences can be recognized by the acridine-linked oligopyrimidines Acr-T5C5 and Acr-C5T5 respectively at low pH, using well-characterised T.AT and C+.GC triplets. At pH 7.5 A6G6.C6T6 is specifically bound by Acr-G5T5, utilising G.GC and T.AT triplets in which the third strand runs antiparallel to the purine strand of the duplex. This interaction requires the presence of magnesium ions. No interaction was detected with Acr-T5G5, an oligonucleotide designed to form parallel G.GC and T.AT triplets. In contrast neither Acr-T5G5 nor Acr-G5T5 produced DNase I footprints with the target sequence G6A6.T6C6. These results suggest that, in an antiparallel R.RY triple helix, the T.AT triplet is weaker than the G.GC triplet. We find no evidence for the formation of structures containing parallel G.GC triplets.