There is accumulating evidence to suggest that palindromic AT-rich repeats (PATRRs) represent hot spots of double-strand breakage that lead to recurrent chromosomal translocations in humans. As a mechanism for such rearrangements, we proposed that the PATRR forms a cruciform structure that is the source of genomic instability. To test this hypothesis, we have investigated the tertiary structure of a cloned PATRR. We have observed that a plasmid containing this PATRR undergoes a conformational change, causing temperature-dependent mobility changes upon agarose gel electrophoresis. The mobility shift is observed in physiologic salt concentrations and is most prominent when the plasmid DNA is incubated at room temperature prior to electrophoresis. Analysis using two-dimensional gel electrophoresis indicates that the mobility shift results from the formation of a cruciform structure. S1 nuclease and T7 endonuclease both cut the plasmid into a linear form, also suggesting cruciform formation. Furthermore, anti-cruciform DNA antibody reduces the electrophoretic mobility of the PATRR-containing fragment. Finally, we have directly visualized cruciform extrusions from the plasmid DNA with the size expected of hairpin arms using atomic force microscopy. Our data imply that for human chromosomes, translocation susceptibility is mediated by PATRRs and likely results from their unstable conformation.
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