PRESIDENT OF Cellular and Developmental Barry Ganetzky Thomas J. Silhavy THE ACADEMY Biology Iva S. Greenwald Thomas E. Wellems Jeffrey C. Hall Ralph J. Cicerone C. David Allis Philip C. Hanawalt Donald D. Brown Curtis G. Callan, Jr. EDITOR-IN-CHIEF Mary-Claire King Eric H. Davidson Anthony Leggett Jasper Rine Brigid L. M. Hogan Paul C. Martin Eric N. Olson Reed B. Wickner ASSOCIATE EDITORS Michael Rosbash Geology Physiology and David Chandler David D. Sabatini Pharmacology Alan Fersht Thure E. Cerling Gertrud M. Schüpbach W. G. Ernst Richard W. Aldrich Jack Halpern Susan G. Amara Cellular and Molecular Andrew H. Knoll Dolores R. Piperno David Julius Neuroscience Geophysics Solomon H. Snyder Arthur Karlin B. L. Turner II David E. Clapham Mark H. Thiemens Ramón Latorre Peter K. Vogt Pietro V. De Camilli Human Environmental Plant Biology Susan R. Wessler Richard L. Huganir Sciences L. L. Iversen Ruth S. DeFries SPECIAL FEATURE EDITOR Yuh-Nung Jan Susan Hanson Anthony R. Cashmore Michael F. Goodchild Eve Marder Maarten J. Chrispeels Jeremy Nathans Immunology Enrico Coen EDITORIAL BOARD Charles F. Stevens Peter Cresswell Joseph R. Ecker Douglas T. Fearon Animal, Nutritional, and Thomas C. Südhof Robert Haselkorn Joseph S. Takahashi Tak Wah Mak June B. Nasrallah Applied Microbial Sciences Philippa Marrack David L. Denlinger Chemistry William E. Paul Plant, Soil, and R. Michael Roberts Stephen J. Benkovic Ralph M. Steinman Microbial Sciences Linda J. Saif Harry B. Gray Tadatsugu Taniguchi James C. Carrington Ryuzo Yanagimachi Michael L. Klein Arthur Weiss Vicki L. Chandler Raphael D. Levine Brian J. Staskawicz Anthropology Mathematics Tobin J. Marks Richard G. Klein Richard V. Kadison Psychological and Cognitive Jerrold Meinwald C. Owen Lovejoy Robion C. Kirby Sciences James F. O’Connell David A. Tirrell Nicholas J. Turro Medical Genetics, Hematology, Wilson S. Geisler Applied Mathematical Sciences and Oncology J. Anthony Movshon Computer and Peter J. Bickel Dennis A. Carson Dale Purves David L. Donoho Information Sciences C. Thomas Caskey Edward E. Smith Stephen E. Fienberg Butler W. Lampson Joseph L. Goldstein William H. Press Social and Political Sciences Applied Physical Sciences Mark T. Groudine Alejandro Portes Francisco de la Cruz Economic Sciences Tony Hunter Brian Skyrms Jerry P. Gollub Roy Radner Philip W. Majerus Kenneth W. Wachter Tom C. Lubensky Jose A. Scheinkman Inder M. Verma Owen N. Witte Sustainability Science David A. Weitz Engineering Sciences Barry R. Bloom Astronomy Alexis T. Bell Medical Physiology William C. Clark Neta A. Bahcall Mark E. Davis and Metabolism Partha Sarathi Dasgupta Mildred S. Dresselhaus Robert J. Lefkowitz Robert W. Kates Biochemistry Evelyn L. Hu David J. Mangelsdorf Michael R. Botchan Pamela A. Matson William R. Schowalter Salvador Moncada Elinor Ostrom Jennifer A. Doudna David W. Russell Edward D. Korn Environmental Sciences Stephen Polasky Microbial Biology Stephen C. Kowalczykowski and Ecology Hans Joachim Schellnhuber John M. Coffin John Kuriyan Robert E. Dickinson Systems Biology R. John Collier Michael A. Marletta Paul G. Falkowski Marc W. Kirschner Emil C. Gotschlich Kiyoshi Mizuuchi David M. Karl Wing Hung Wong E. Peter Greenberg Charles M. Radding Robert May Diane E. Griffin Systems Neuroscience Venkatraman Ramakrishnan William W. Murdoch Peter M. Howley Tom A. Rapoport David W. Schindler Thomas D. Albright Ralph R. Isberg Robert Desimone James A. Wells Evolutionary Biology Elliott D. Kieff Fred H. Gage William T. Wickner Francisco J. Ayala Robert A. Lamb Charles D. Gilbert May R. Berenbaum and Peter Palese Marcus E. Raichle Sean B. Carroll Thomas E. Shenk Leslie G. Ungerleider David Baker W. Ford Doolittle Adriaan Bax Douglas J. Futuyma Daniel L. Hartl Axel T. Brunger Submission to PNAS William A. Eaton Richard E. Lenski Authors (members and nonmembers) may submit their manuscripts Angela M. Gronenborn Masatoshi Nei Barry H. Honig Gene E. Robinson directly to PNAS at www.pnascentral.org. Authors must recommend three Robert Langer Genetics appropriate Editorial Board members, three NAS members who are ’ fi fi fi Michael Levitt Kathryn V. Anderson expert in the paper s scienti c area, and ve quali ed referees. No Gregory A. Petsko Thomas W. Cline Academy member sponsor is required. John W. Sedat Stanley N. Cohen PNAS (ISSN-0027-8424) is published weekly in print by PNAS STAFF the National Academy of Sciences. Publisher Production Manager Correspondence: PNAS, 700 11th Street, NW, Suite Kenneth R. Fulton Dana M. Compton 450, Washington, DC 20001 USA. E-mail: [email protected]. Executive Editor Production Staff Copyright: Volumes 90–105, copyright © 1993–2008 by Diane M. Sullenberger Kelly A. Gerrity the National Academy of Sciences of the United States of Sophie Mohin Managing Editor America, all rights reserved. Volumes 1–89 and 106–108, Kelly A. Newton Daniel H. Salsbury copyright as a collective work only; author(s) retains Kat Rodenhizer copyright to individual articles. Requests for Permission: Recruiting Editor Audrey Springer See www.pnas.org/misc/rightperm.shtml for details. David Stopak Address requests to reproduce material published in Marketing Manager Volumes 1–89 to the original author(s); e-mail Editorial Staff Mark Chesnek other requests to [email protected], James B. Allison fax 1-202-334-2739, or PNAS Permissions Editor, Marketing Staff 700 11th Street, NW, Suite 450, Washington, DC Jenna Al-Malawi 20001 USA. Please cite the exact material to be Josiah W. Armour Jennifer Bradley reprinted and state specifically where it will be used. Teresa V. Callahan Photocopies: PNAS is registered with the Copyright Victoria Chihos Media Staff Clearance Center (CCC), 222 Rosewood Drive, Ryan Conley Sola Ayeni-Biu Danvers, MA 01923 USA, fax 1-978-750-4470, or Ali C. Guio Ann Griswold www.copyright.com. Authorization to photocopy Amanda Hopkins Todd Johnston items for the internal or personal use of specific clients Andrew Huber Jenny Ruth Morber is granted by the National Academy of Sciences Benjamin Jipson Prashant Nair provided that the proper fee is paid directly Meredith C. Jones Ashley Truxon to CCC. Microforms: Contact UMI at Etta Kavanagh www.umi.com or P.O. Box 1346, Ann Arbor, Jacob Kendall-Taylor Senior Finance Officer MI 48106-1346 USA. Jeffrey King Janice Guzman Advertising: Mark Chesnek, 700 11th Street, NW, Suite Kay McLaughlin 450, Washington, DC 20001 USA. Phone 1-202-334-2696, Kathryn Murphy Business Staff fax 1-202-334-1346, e-mail [email protected]. Tom Myers Metina Booze Jennifer Nelson Corey P. Colwill Subscriptions: Send payments to PNAS, c/o AIP, P.O. Lesli H. Martin Box 503284, St. Louis, MO 63150-3284 USA. For Matthew Parks subscription help, e-mail [email protected], phone May B. Piotrowski 1-516-576-2270, or visit www.pnas.org/subscriptions. 2011 Dane Secor Rates: US, individual–$375 (first class delivery available at Mitchell Sipus a surcharge of $375); non-US, individual–$585; US/non-US, institutional online only, $1,305–7,215. Subscriptions are entered on a calendar-year basis. Exclusive Agent for Subscribers in Japan: USACO Authorship Corporation, 17-12 Higashi-Azabu, 2-Chome, Minato-ku, Authorship should be limited to those who have contributed substantially to the Tokyo 106-0044, Japan. Phone 81 3 3505 3256, fax 81 3 fi 3505 6282, e-mail [email protected]. Change of work. Authors must indicate their speci c contributions to the published work; Address: PNAS, c/o AIP, Suite 1NO1, 2 Huntington this information will be published as a footnote to the paper. The corresponding Quadrangle, Melville, NY 11747 USA. Phone author must have obtained permission from all authors for the submission of 1-800-344-6902, fax 1-516-349-9704, e-mail [email protected]. each version of the paper and for any change in authorship. Please send notification 6 weeks in advance and list the Conflict of Interest old and new addresses. Claims: Requests for replacement All authors, members, referees, and editors must disclose any association that poses a copies will not be honored more than 60 days after the conflict of interest in connection with the manuscript. Authors must acknowledge all issue date for domestic subscribers and not more than 90 funding sources supporting the work. See www.pnas.org/misc/coi.shtml for details. days after the issue date for foreign subscribers. Claims will not be honored for more than two issues per calendar Supporting Information year for the same subscriber. To make a claim, e-mail Authors may use Supporting Information to enhance their papers in PNAS by [email protected] or call 1-800-344-6902. Single Copies: $40 providing additional substantive material for online posting, but the print version per issue in the US, $50 non-US To order, e-mail of the paper must stand on its own merits. [email protected] or call 1-800-344-6902. Canadian GST: Cover Images Registration Number R-133130880. Authors are encouraged to submit scientifically interesting and visually arresting Postmaster: Send address changes to PNAS, c/o AIP, images for the cover. Suite 1NO1, 2 Huntington Quadrangle, Melville, NY 11747 USA. Periodicals postage paid at Washington, DC, Information for Authors and additional mailing offices. Please see the complete Information for Authors, available online at www.pnas.org. PNAS is available online at www.pnas.org. PNAS Online PRINTED IN THE USA PNAS articles are published daily online before print at www.pnas.org in PNAS Early Edition. Printed on acid-free paper effective Volume 84, The articles in PNAS report original research by independent authors and do not Issue 1, and on 80% post-consumer recycled necessarily represent the views of the National Academies. paper effective Volume 107, Issue 1. Printed with soy ink.