Structural basis for the tryptophan sensitivity of TnaC-mediated ribosome stalling

Area of Science:

• Molecular Biology
• Structural Biology
• Biochemistry

Background:

• Ribosome stalling is a key regulatory mechanism in gene expression.
• The TnaC peptide in Escherichia coli controls tryptophanase operon expression.
• The precise molecular mechanism of L-tryptophan (L-Trp) sensing by the TnaC-ribosome complex is not fully understood.

Purpose of the Study:

• To elucidate the molecular mechanism of L-Trp recognition and ribosome response by the TnaC-ribosome complex.
• To characterize an L-Trp hypersensitive TnaC variant (R23F) to understand tryptophan sensing.

Main Methods:

• Combined biochemical and structural analyses were employed.
• Characterization of a TnaC variant (R23F) with enhanced L-Trp sensitivity.
• Investigated the interaction between the TnaC-ribosome complex, L-Trp, and release factor 2.

Main Results:

• The TnaC-ribosome complex captures a single L-Trp molecule, inducing termination arrest.
• Nascent TnaC peptide obstructs the active conformation of the GGQ loop in release factor 2 at the peptidyl transferase center.
• The R23F mutation does not alter the L-Trp binding site, indicating altered binding/cleavage rates influence sensitivity.

Conclusions:

• A nascent peptide can facilitate ribosome-mediated detection of small metabolites like L-Trp.
• The study reveals a novel mechanism of translational regulation involving metabolite sensing.
• Peptide sequence and metabolite binding kinetics dictate translational control sensitivity.