[1] LOUISIANA STATE UNIV,MED CTR,DEPT BIOCHEM & MOLEC BIOL,SHREVEPORT,LA 71130
[2] INST PASTEUR,UNITE REGULAT EXPRESS GENET,F-75724 PARIS 15,FRANCE
[3] INST PASTEUR,UNITE BIOCHIM MICROBIENNE,F-75724 PARIS 15,FRANCE
[4] CNRS,GDR 1029,F-75724 PARIS 15,FRANCE