Tryptophan absent, repressor inactive, operon on.
   In  the  repressible  trp  operon,  5  genes  encoding  the enzymes to
   synthesize tryptophan are regulated by a promoter and an operator.
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   When  tryptophan  is  absent, the repressor (with its own promoter) is
   inactive  and  the operon is on. RNA polymerase attaches to the DNA at
   the promoter and transcribes the operon's genes.