Nat. Struct. Mol. Biol. 14, 861–868 (2007); published online 5 August 2007; corrected after print 22 May 2008

In the version of this article initially published, the legend text for Figure 2c was incorrect, with the terms “clockwise” and “counterclockwise” swapped. The error has been corrected in the PDF and HTML versions of the article.