Attenuation vs. repression:

Whole, for the trp operon, repression through the trp repressor determines whether transcription will occur or will not occur and attenuation then fine tunes transcription. The Attenuation occurs in at least six other operons which encode enzymes for amino acid biosynthetic pathways.  In some cases like as the trp operon, together repression and attenuation operate to regulate expression.  In comparison, for some other operons like as the his, thr and leu operons, transcription is regulated only through attenuation.