1. Microbial Pathogenicity Research Group, Institute of Infections and Immunity, University Hospital, Queens Medical Centre, Nottingham NG7 2UH, U.K.
2. Institute of Child Health, University of Birmingham, Francis Road, Fiveways, Birmingham B16 8ET, U.K.
3. Department of Microbiology and Immunology, Leicester University, University Road, Leicester LE1 7RH, U.K.
4. Department of Protein Structure, N.I.M.R., The Ridgeway, Mill Hill, London NW7 1AA, U.K.