[1] NYU MED CTR, DEPT MICROBIOL, NEW YORK, NY 10016 USA
[2] CNRS, CTR RECH BIOCHIM & GENET CELLULAIRES, F-31077 TOULOUSE, FRANCE
[3] ROCHE INST MOLEC BIOL, DEPT CELL BIOL, NUTLEY, NJ 07110 USA
[4] COLL MED & DENT NEW JERSEY, NEW JERSEY MED SCH, DEPT MICROBIOL, NEWARK, NJ 07103 USA