Molecular basis for the ribosome functioning as an L-tryptophan sensor.

Area of Science:

• Molecular Biology
• Structural Biology
• Microbiology

Background:

• Gene expression in E. coli is regulated by environmental cues.
• The tryptophanase tnaCAB operon is activated by elevated L-tryptophan (L-Trp) levels.
• This activation is mediated by translational stalling of the TnaC peptide.

Purpose of the Study:

• To elucidate the structural mechanism of L-tryptophan-induced translational stalling.
• To visualize the interaction between the TnaC peptide, L-Trp, and the ribosome.

Main Methods:

• Cryoelectron microscopy (cryo-EM) was used to obtain a high-resolution (3.8 Å) reconstruction.
• The structure of a ribosome stalled by the TnaC peptide was determined.

Main Results:

• Two L-Trp molecules were observed within the ribosomal exit tunnel.
• L-Trp molecules are coordinated in hydrophobic pockets formed by the TnaC peptide and the ribosome.
• The stalled ribosome adopts a conformation that prevents the binding of release factor 2 (RF2).

Conclusions:

• The translating ribosome acts as a direct sensor for small molecules like L-Trp.
• Ribosomal stalling induced by small molecule binding is a novel mechanism for gene regulation.
• This structural insight provides a molecular basis for tryptophan-mediated control of the tnaCAB operon.