Proceedings OF THE National Academy of Sciences OF THE UNITED STATES OF AMERICA Officers BRUCE ALBERTS, President of the JACK HALPERN, Vice President Academy PETER H. RAVEN, Home Secretary JAMES B. WYNGAARDEN, Foreign Secretary MILDRED S. DRESSELHAUS, Treasurer Editorial Board LAWRENCE BOGORAD, Chairman of the JAMES E. DARNELL, JR. ERIC R. KANDEL HERBERT A. SIMON Proceedings IGOR B. DAWID PHILIP W. MAJERUS MAXINE F. SINGER RONALD L. GRAHAM DAVID D. SABATINI IRVING L. WEISSMAN GORDON G. HAMMES PAUL R. SCHIMMEL SHERMAN M. WEISSMAN Senior Contributing MICHAEL T. CLEGG, JAMES E. DARNELL, JR., RAYMOND L. ERIKSON, RONALD M. EVANS, Editors: ERIC R. KANDEL, RICHARD A. LERNER, DAVID J. L. LUCK, PHIL W. MAJERUS, ARNO G. MOTULSKY, DAVID D. SABATINI, PAUL R. SCHIMMEL, ALLAN C. SPRADLING, JAMES A. SPUDICH, CHARLES F. STEVENS, IRVING L. WEISSMAN, SHERMAN M. WEISSMAN Contributing Editors: MAX D. COOPER, HERMAN N. EISEN, WILLIAM E. PAUL Consulting Editors: JOSEPH L. 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PRINTED IN THE USA PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA (ISSN-0027-8424) is published biweekly by THE NATIONAL ACADEMY OF SCIENCES, 2101 Constitution Avenue, Washington, DC 20418. X 1994 by THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA. POSTMASTER: Send address changes to: PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 2101 Constitution Ave., Washington, DC 20418. Proc. Nail. Acad. Sci. USA Vol. 91, February 1994 CONDENSED INFORMATION FOR CONTRIBUTORS (See the first issue of the year for the complete Information for Contributors) The PROCEEDINGS OF THE NATIONAL ACADEMY OF SCI- computer programs will be made available to all qualified ENCES USA publishes articles describing original work in the investigators. The Editorial Board considers submission of sciences that are of interest to a wide audience. 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(iii) Publication in the PROCEEDINGS implies that Wisconsin Avenue, N.W., Suite 530, Washington, DC 20007 materials employed in the work such as cloned DNAs, (express and courier services); telephone (202) 625-4725; antibodies, bacterial, animal, or plant cells, viruses, and telefax (202) 625-4747. i AUTHOR INDEX, FEBRUAR Y 1, 1994 Aaronson, S. A. 989 Colanzi, A. 1114 Hammel, J. 903 Aaronson, S. A. 1074 Comeau, M. R. 1119 Hammond, C. 913 Abraham, J. A. 1069 Cooper, M. D. 1094 Hashimoto, K.-i. 1124 Adams, B. A. 1014 Corda, D. 1114 Hauser, C. A. 873 Adler, R. 974 Corey, J. L. 1188 Hayami, M. 1124 Aebersold, R. 1094 Cosman, D. 1119 Hegerl, R. 836 Agadir, A. 1178 Cowman, A. F. 1143 Heinemann, S. H. 1109 Alarid, E. T. 1074 Crapo, J. 1044 Helenius, A. 913 Albers, M. W. 984 Cunha, G. R. 1074 Henry, A. H. 969 Alberte, R. S. 1049 Cunningham, J. 1168 Herrick, G. 942 Allevato, G. 957 Dall'Acqua, W. 1089 Heurteaux, C. 878 Ames, B. N. 1183 Darwish, H. M. 900 Hirth, C. G. 1193 Arr6, M. 927 Davidson, N. 1188 Hoh, J. H. 836 Aruffo, A. 1099 De, A. 1084 Hollenbaugh, D. 1099 Astrom, A. 979 Debanne, D. 1148 Hood, L. 1094 Astrom, J.