Attenuator (genetics) - Mechanism in The trp Operon

Mechanism in The trp Operon

The proposed mechanism of how this mRNA secondary structure and the trp leader peptide could regulate transcription of the trp biosynthetic enzymes includes the following.

• RNAP initiates transcription of the trp promoter.
• RNAP pauses at about nucleotide 90 at a secondary structure (?the first one shown above?).
• Ribosomes engage this nascent mRNA and initiate translation of the leader peptide.
  • RNAP is then "released" from its pause and continues transcription.
• When RNAP reaches the region of the potential terminator, whether it continues or not is dependent on the position of the ribosome "trailing behind".
  • If the ribosome stalls at the tandem Trp codons, waiting for the appropriate tRNA, region 1 is sequestered within the ribosome and thus cannot base pair with region 2. This means that region 2 and 3 become based paired before region 4 can be transcribed. This forces region 4 when it is made to be single stranded, preventing the formation of the region 3/4 terminator structure. Transcription will then continue.
  • If the ribosome translates the leader peptide with no hesitation, it then covers a portion of region 2 preventing it from base pairing with region 3. Then when region 4 is transcribed, it forms a stem and loop with region 3 and transcription is terminated, generating a ca. 140 base transcript.
• This mechanism of control measures the amount of available, charged Trp-tRNA.

The location of ribosomes determines which alternate secondary structures form.