1. Institute of Molecular Biology, The University of Hong Kong, Hong Kong.
2. Laboratory of Molecular Microbiology, National Institute of Allergy and Infectious Diseases, Bethesda, Maryland 20892.
3. Institute of Molecular Biology, The University of Hong Kong, Hong Kong and Laboratory of Molecular Microbiology, National Institute of Allergy and Infectious Diseases, Bethesda, Maryland 20892.