Intrahelical pseudoknots and interhelical associations mediated by mispaired human minisatellite DNA sequences in...

The human minisatellite arrays, 33.6 and 33.15, consist of tandem reiterations of a 37-nucleotide (nt) and a 16-nt repeat unit sequence, respectively, both of which contain a majority of purine bases on one strand. Knot-like tertiary structures, which mapped to the cloned arrays, were observed by electron microscopy (EM) in homoduplex molecules produced by denaturation and reannealing in vitro. They result from a primary hybridization between misaligned repeat units of the array, forming a slipped-strand structure with staggered single-stranded DNA loops, followed by a secondary hybridization between repeat units in the two loops. Depending on the relative alignment of the loops when they hybridize, a particular form of intrahelical pseudoknot is produced. Theta-shaped, figure-of-eight, and bow-shaped structures were the most common conformational isomers observed in homoduplexes flattened into two dimensions during EM preparation. At the site of a bow-shaped structure, a conformation-dependent bend of approximately 60 degrees between the flanking DNA segments is induced; the other conformations generally do not deflect the line of the main DNA axis. Paired loops, similar to the bow-shaped structure, were apically situated in some supercoiled plasmids containing the 33.6 array. Both plasmids formed intermolecular associations, consisting of two (or more) homoduplex molecules held together at or immediately adjacent to a nexus which mapped to the minisatellite sequences. These associations might arise either by interhelical hybridization between arrays or by knot-like structures interfering with branch migration of chi-form Holliday junctions.