1. Department of Biological Chemistry, University of Michigan, Ann Arbor, Michigan 48109–0606, USA.
2. Department of Plant and Microbial Biology, University of California Berkeley, Berkeley, CA 94720–3102, USA.
3. Department of Molecular Genetics and Microbiology, School of Medicine, State University of New York at Stony Brook, Stony Brook, New York 11794–5222, USA.
4. Department of Biology, Brookhaven National Laboratory, Upton, NY 11973–5000, USA.