1. Department of Biochemistry and Molecular Biology, F. Edward Hébert School of Medicine, Uniformed Services University of the Health Sciences, Bethesda, MD 20814
2. Microbiology Section, Department of Medicine, Imperial College London, London SW7 2AZ, England, UK
3. Department of Cellular and Molecular Biology, Centro de Investigaciones Biológicas, Consejo Superior de Investigaciones Científicas, 28040 Madrid, Spain