1. Department of Molecular Biology and Biochemistry, Rutgers University, Piscataway, New Jersey 08854-8082 USA, Laboratoire d’Ingénierie des Systèmes Macromoléculaires, Institut de Biologie Structurale et Microbiologie, CNRS, 31 Chemin Joseph Aiguier, 13402 Marseilles Cedex 20, France, and Department of Molecular Biology and Biotechnology, University of Sheffield, Sheffield S10 2TN, United Kingdom