1. U.S. Department of Agriculture, Agricultural Research Service, Beltsville, MD 20705.
2. Protein and Nucleic Acid Laboratory, Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742.
3. Applied Biosystems, Inc., Foster City, CA 94404.
4. National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, MD 20892.
5. Howard Hughes Medical Institute, Department of Molecular Biology and Genetics, The Johns Hopkins University School of Medicine, Baltimore, MD 21205.