The architecture of the human Rad54-DNA complex provides evidence for protein translocation along DNA

Proper maintenance and duplication of the genome require accurate recombina tion between homologous DNA molecules. In eukaryotic cells, the Rad51 prote in mediates pairing between homologous DNA molecules. This reaction is assi sted by the Rad54 protein. To gain insight into how Rad54 functions, we stu died the interaction of the human Rad54 (hRad54) protein with double-strand ed DNA. We have recently shown that binding of hRad54 to DNA induces a chan ge in DNA topology. To determine whether this change was caused by a protei n-constrained change in twist, a protein-constrained change in writhe, or t he introduction of unconstrained plectonemic supercoils, we investigated th e hRad54-DNA complex by scanning force microscopy. The architecture of the observed complexes suggests that movement of the hRad54 protein complex alo ng the DNA helix generates unconstrained plectonemic supercoils. We discuss how hRad54-induced superhelical stress in the target DNA may function to f acilitate homologous DNA pairing by the hRad51 protein directly. In additio n, the induction of supercoiling by hRad54 could stimulate recombination in directly by displacing histones and/or other proteins packaging the DNA int o chromatin. This function of DNA translocating motors might be of general importance in chromatin metabolism.