19 September 1997
Vol 277, Issue 5333

About The Cover

-Exonuclease binds to an end of double-stranded DNA and degrades one of the strands in a highly processive manner. The structure of the exonuclease consists of a toroidal trimer (~94 angstroms in diameter) and is presumed to enclose its substrate in the manner illustrated. Its internal cavity is funnel-shaped, and the intact single-stranded DNA emerges from the narrow end of the funnel. See 1824. [Image: Rhett Kovall, Allen Orville, and Brian W. Matthews (figure prepared with Midasplus software)