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The DNA motif 5'-AAGGGAGAAXGGGTATAGGGYAAGAGGGAA-3' (named XY32) is an H-palindrome and has been shown to undergo a superhelix-induced, pH-dependent structural transition to H-form (pyrimidine purineo pyrimidine triplex) DNA when X = Y = A (AA32) or X = Y = G (GG32), but when X = A and Y = G (AG32) or X = G and Y = A (GA32), the transition is much more difficult [Mirkin, S. (1987) Nature (London) 330, 495-497]. Furthermore, AA32, GG32, and GA32 triplexes have the proper sequence structure to potentially form pyrimidineopurineopurine (*H-form) triplexes, but AG32 does not [Beal, P. A. &amp; Dervan, P. B. (1992) Nucleic Acids Res. 20, 2773-2776]. Using an in vivo plasmid-plasmid recombination assay system in cultured human cells, we have found that AA32, GA32, and GG32 stimulate homologous recombination between plasmids 3- to 5-fold when both recombination substrates contain these triplex-forming sequences, whereas AG32, which differs from the others by only 1 or 2 bp, does not significantly affect the frequency of recombination. Double-strand breaks, which destroy supercoiling, nullify the stimulation. Therefore, stimulation of homologous recombination between plasmids containing these sequences correlates with their triplex-forming potential. Crosses in which the triplex-forming sequence is inserted into only one substrate exhibit an intermediate stimulation, suggesting that the inserts are acting alone as intramolecular triplexes.

Antiparallel, intramolecular triplex DNA stimulates homologous recombination in human cells.