1. Contribution from the Center for Bio/Molecular Science and Engineering, Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, D.C. 20375, Nova Research Inc., Alexandria, Virginia, 22308, and Department of Molecular Biology, Center for Interactive Molecular Biosciences, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California, 92037