Rugby ball at the chromosome core.

The shape of the protein structure around which DNA wraps in plant and animal chromosomes resembles a rugby ball, report scientists at Johns Hopkins University in Baltimore, Md., in the May 3 SCIENCE. The shape they have just determined using X-ray crystallography differs dramatically from the previous model, proposed by British scientists in 1977. The protein structure, called the histone octamer, organizes DNA into supercoils to keep genes compact and untangled. It also may play a role in controlling which of a cell's many genes are expressed.

Hopkins scientists Rufus W. Burlingame, Evangelos N. Moudrianakis and their colleagues report the basic eight-part assemblage of histone proteins is a "prolate ellipsoid" with a tripartite organization. The central portion contains four subunits.

This sturcture is flanked by two sets of two subunits. The scientists speculate that the DNA helix, coiled like a spring, wraps around the histone octamer and holds the three sections together. They suggest that the degree to which these DNA supercoils tighten and loosen influences which genes are active in a cell. They have identified channels in the histone octamer through which small molecules may influence the shape of the complex and the interactions between protein and DNA.