1. Contribution from the Steacie Institute for Molecular Sciences, National Research Council Canada, 100 Sussex Dr., Ottawa, Ontario K1A 0R6, Canada, The Institute for Physical and Chemical Research (RIKEN), Wako-shi, Saitama 351-01, Japan, and Institute for Biological Sciences, National Research Council Canada, 100 Sussex Dr., Ottawa, Ontario K1A 0R6, Canada