1. School of Biosciences, The University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom1;
2. Institut Pasteur, Unitéde Physico-chimie des Macromolécules Biologiques, 75724 Paris Cedex 15, France2;
3. Department of Chemistry, University of California, Davis, California 956163; and
4. Department of Molecular Genetics, National Institute of Genetics, Mishima, Shizuoka 411, Japan4