[1] MAX DELBRUCK CTR MOL MED,D-13122 BERLIN,GERMANY
[2] UNIV ULM,D-89081 ULM,GERMANY
[3] UNIV CALIF SAN FRANCISCO,HOWARD HUGHES MED INST,SAN FRANCISCO,CA 94143
[4] UNIV CALIF SAN FRANCISCO,DEPT MICROBIOL,SAN FRANCISCO,CA 94143
[5] UNIV CALIF SAN FRANCISCO,DEPT BIOCHEM,SAN FRANCISCO,CA 94143