[1] UNIV CALIF BERKELEY,DEPT MOLEC BIOL,BERKELEY,CA 94720
[2] UNIV CALIF BERKELEY,DEPT MOLEC & CELL BIOL,BERKELEY,CA 94720
[3] UNIV MILAN,DIPARTIMENTO GENET & BIOL MICRORGANISMI,I-20133 MILAN,ITALY
[4] WASHINGTON STATE UNIV,DEPT MICROBIOL,PULLMAN,WA 99164
[5] WASHINGTON STATE UNIV,PROGRAM GENET & CELL BIOL,PULLMAN,WA 99164
[6] UNIV IOWA,SCH MED,DEPT MICROBIOL,IOWA CITY,IA 52242