2. Learning Objectives

2.4. Attenuation of the trp Operon

  • The leader sequence (L) of the trp Operon lies before the trpE gene at its 5′ end . This sequence about 160 bp is size and controls the expression of the operon through a process called attentuation.
  • This sequence has four domains (1-4). Domain 3 (nucleotides 108-121) of the mRNA can base pair with either domain 2 (nucleotides 74-94) or domain 4 (nucleotides 126-134).
  •  At high levels of tryptophan in the medium :  domain 3 pairs with domain 4, a stem and loop structure forms on the mRNA and transcription stops.
  • At low levels of tryptophan in the medium :     domain 3 pairs with domain 2, then the stem and loop structure does not form and transcription continues through the operon, and all of the enzymes required for tryptophan biosynthesis are produced.
  • Attenuators: Domain 4 is called the attenuator because its presence is required to reduce (attenuate) mRNA transcription in the presence of high levels of tryptophan.
  • Domain 1 is also an important component of the attenuation process. The section of the leader sequence encodes a 14 amino acid peptide that has two tryptophan residues.

trp Operon Transcription Under High Levels of Tryptophan

  • When the cellular levels of tryptophan are high, the levels of the tryptophan tRNA are also high.
  • Immediately after transcription, the mRNA moves quickly through the ribosome complex translating the leader peptide .
  • Translation is quick because of the high levels of tryptophan tRNA.
  • Domain 2 becomes associated with the ribosome complex.
  • As result the domain 3 binds with domain 4, and
  • transcription is attenuated because of the stem and loop formation.

trp Operon Transcription Under Low Levels of Tryptophan

  • Under low cellular levels of tryptophan, the translation of the peptide on domain 1 is slow.
  • Domain 2 does not become associated with the ribosome.
  • Rather domain 2 associates with domain 3.
  • This structure permits the continued transcription of the operon.
  • Then the trpE-A genes are translated, and the biosynthesis of tryptophan occurs.