1. Department of Molecular Biology, Faculty of Science, Nagoya University, Chikusa-ku, Nagoya 464-01, Japan.
2. Department of Genetics and Molecular Biology, Institute for Virus Research, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan.
3. Department of Biochemistry and Biophysics, University of North Carolina, Chapel Hill, NC 27599, USA.