[1] LAWRENCE BERKELEY LAB, DIV STRUCT BIOL, BERKELEY, CA 94720 USA
[2] COLUMBIA UNIV, DEPT GENET & DEV, NEW YORK, NY 10032 USA
[3] UNIV CALIF BERKELEY, DEPT MOLEC & CELL BIOL, BERKELEY, CA 94720 USA
[4] INST NATL AGRON PARIS GRIGNON, CTR BIOTECHNOL AGROIND, GENET MICROORGANISMES LAB, F-78850 THIVERVAL GRIGNON, FRANCE