1. Astbury Centre for Structural Molecular Biology, University of Leeds, Leeds LS2 9JT, U.K.
2. Department of Bacteriology and Immunology, Veterinary and Agrochemical Research Centre, VAR-CODA-CERVA, Groeselenberg 99, B-1180 Ukkel, Belgium
3. Aston Academy of Life Sciences, Aston University, Aston Triangle, Birmingham B4 7ET, U.K.
4. Department of Biological Sciences, University of Warwick, Gibbet Hill Road, Coventry CV47AL, U.K.