1. Department of Biochemistry; E. K. Shriver Center; 200 Trapelo Road Waltham MA 02254 U.S.A.
2. Department of Microbiology-Surgery; Columbia University and St. Luke's-Roosevelt Hospital Center; New York NY 10025 U.S.A.
3. Department of Microbiology-Immunology and Surgery; Duke University Medical Center; Durham NC 27710 U.S.A.