DNA cruciforms facilitate in vitro strand transfer on nucleosomal templates.

A single, phased nucleosome assembled on a 240 bp DNA duplex molecule blocked Escherichia coli RecA protein-promoted strand transfer of the complementary strand of the duplex onto a homologous single-stranded circle. However, when a four-armed cruciform structure was coupled to either end of the duplex the barrier to strand transfer was overcome and joint molecules were efficiently formed. Micrococcal nuclease digestion indicated that the nucleosome was dissociated by the juxtaposition of the cruciform. We interpret these results to mean that cruciform structures can act over a distance to destabilize adjacent nucleosomes and suggest that, as a consequence, the chromatin structure surrounding a crossed strand recombination intermediate might be disrupted, enabling other recombination events to initiate or the process of branch migration to proceed.