Association of double-stranded DNA fragments into multistranded DNA structures.

We have previously observed that double-stranded DNA fragments containing a tract of the tandemly repeated sequence poly(CA). poly(TG) can associate in vitro to form stable complexes of low electrophoretic mobility, which are recognized with high specificity by proteins HMG1 and HMG2. The formation of such complexes has since been observed to depend on interactions of DNA with polypropylene surfaces, with the suggestion that the formation of low mobility complexes might be the result of strand dissociation followed by misaligned reassociation of the repetitive sequences. The data presented here show that at high ionic strength the interactions of DNA with polypropylene are sufficiently strong for DNA to remain bound to the polypropylene surface, which suggests that DNA might also be involved in interactions with hydrophobic molecules in vivo. Under such conditions, low-mobility complexes are found only in the material adsorbed to the polypropylene surface, and all DNA fragments are able to form low-mobility structures, whether or not they contain repetitive sequences. Preventing the separation of strands by ligating hairpin loop oligonucleotides at both ends of the fragments does not prevent the formation of low-mobility complexes. Our results suggest two different pathways for the formation of complexes. In the first, dissociation is followed by misaligned reassociation of repetitive sequences, yielding duplexes with single-stranded end regions that associate to form multimeric complexes. In the second, repetitive as well as nonrepetitive DNA molecules bound to polypropylene adopt a conformation with locally unwound regions, which allows interactions between neighboring duplexes adsorbed on the surface, resulting in the formation of low-mobility complexes.