Correction to: Nature Structural & Molecular Biology https://doi.org/10.1038/nsmb.3063. Published online 3 August 2015.

In the version of this article initially published, a second affiliation for Osama Albarbarawi—Department of Chemistry, Faculty of Science, Taibah University, Al Madinah Al Munawwarah, Saudi Arabia—was omitted and should have been present in the article.