When  tryptophan  is abundant, the
   ribosome  proceeds  normally  through  the  UGG  codons  in the leader
   peptide (LP).

   As  a  result,  a  terminator  hairpin  forms,  and  mRNA synthesis is
   terminated  near  the  end  of  the  attenuator  (A);  this  is called
   attenuation.