The polypurine-polypyrimidine sequence requirements for the formation of sticky DNA were evaluated in Escherichia coli plasmid systems to determine the potential occurrence of this conformation throughout biological systems. A mirror repeat, dinucleotide tract of (GA·TC)37, which is ubiquitous in eukaryotes, formed sticky DNA, but shorter sequences of 10 or 20 repeats were inert. (GGA·TCC)n inserts (where n = 126, 159, and 222 bp) also formed sticky DNA. As shown previously, the control sequence (GAA·TTC)150 (450 bp) readily adopted the X-shaped sticky structure; however, this structure has never been found for the nonpathogenic (GAAGGA·TCCTTC)65 of the same approximate length (390 bp). A sequence that is replete with polypurine·polypyrimidine tracts that can form triplexes and slipped structures but lacks long repeating motifs (the 2.5-kbp intron 21 sequence from the polycystic kidney disease gene 1) was also inert. Interestingly, tracts of (GAA·TTC)n (where n = 176 or 80) readily formed sticky DNA with (GAAGGA·TCCTTC)65 cloned into the same plasmid when the pair of inserts was in the direct, but not in the indirect (inverted), orientation. The stabilities of the triple base (Watson-Crick and Hoogsteen) interactions in the DNA/DNA associated triplex region of the sticky conformations account for these observations. Our results have significant chemical and biological implications for the structure and function of this unusual DNA conformation in Friedreich's ataxia.