Yards of DNA are packed into cells by wrapping the DNA around proteins called nucleosomes. But that tight packing makes it hard for the cell's machinery to get at the DNA code to read, copy or repair it. Now researchers at the University of California, Davis, have shown how two proteins form a molecular machine that shuffles the nucleosomes out of the way to expose the DNA double helix.

Postgraduate researchers Andrei Alexeev and Alexander Mazin, with Stephen Kowalczykowski, a professor of microbiology at UC Davis, studied a protein called Rad54 in brewer's yeast (Saccharomyces cerevisiae). Rad54 is known to bind to and change the shape of DNA strands.

Using a piece of DNA with artificial nucleosomes attached, the researchers found that Rad54 could not only move the nucleosomes along the strand but knock them off altogether.

When another protein, Rad51, was added, the process became much more efficient. Rad51 binds to single strands of DNA.

Together, Rad54 and Rad51 form a molecular machine that can carry a piece of DNA to the right place, push the nucleosomes out of the way, expose the DNA double helix and begin the process of stitching a new piece of DNA into place.

The work is published in the March issue of Nature Structural Biology.