1. Centre for Chemical Biology, Department of Chemistry, Krebs Institute, University of Sheffield, Sheffield S3 7HF, U.K.
2. Skaggs Institute for Chemical Biology, The Scripps Research Institute, Department of Molecular Biology MB4, 10550 North Torrey Pines Road, La Jolla, CA 92037, U.S.A.
3. Life Sciences Division, Department of Molecular Biology, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, U.S.A.