EPR Sizes Up Histones

Using an electron paramagnetic resonance spin-labeling technique, researchers in the U.K. have ma9naged to take measurements of histone proteins (J. Am. Chem. Soc., DOI: 10.1021/ja807918f). Complexes of histones form the many spools around which eukaryotic cells wind their DNA for packing and control purposes. David G. Norman of the University of Dundee, in Scotland, and coworkers measured distances spanning the spools by using an EPR technique known as PELDOR, or pulsed electron double resonance. The method permits measurement of distances between nitroxide spin-label pairs installed at various places on the histone spool. Ranging from 59 to 70 Å, the distances Norman’s team measured are among the longest PELDOR measurements ever made on a biological system. Crystallography has yielded static pictures of histone spools with and without DNA, but PELDOR measurements could be used to reveal how DNA-threaded histone spools are assembled and accessed by other cell components, Norman says. More generally, the work demonstrates that PELDOR is a “very powerful technique to characterize structure and functional changes in biomacromolecular complexes,” says Olav Schiemann, an EPR expert at the University of St. Andrews, in Scotland.