1. Department of Medical Microbiology and Immunology, College of Medicine, Texas A&M University, College Station, Texas 778431;
2. Department of Microbiology and Immunology, University of Tennessee, Memphis, Tennessee 381632;
3. The Sanger Centre, The Wellcome Trust Genome Campus, Hixton, Cambridge,3 and
4. Chemical and Life Sciences Laboratory, University of Illinois, Urbana, Illinois 618014; and
5. Department of Biochemistry, Imperial College, London SW7 2AZ,5United Kingdom