title: Molecular basis for the ribosome functioning as an L-tryptophan sensor.
abstract: Elevated levels of the free amino acid L-tryptophan (L-Trp) trigger expression of the tryptophanase tnaCAB operon in E. coli. Activation depends on tryptophan-dependent ribosomal stalling during translation of the upstream TnaC peptide. Here, we present a cryoelectron microscopy (cryo-EM) reconstruction at 3.8 Å resolution of a ribosome stalled by the TnaC peptide. Unexpectedly, we observe two L-Trp molecules in the ribosomal exit tunnel coordinated within composite hydrophobic pockets formed by the nascent TnaC peptide and the tunnel wall. As a result, the peptidyl transferase center (PTC) adopts a distinct conformation that precludes productive accommodation of release factor 2 (RF2), thereby inducing translational stalling. Collectively, our results demonstrate how the translating ribosome can act as a small molecule sensor for gene regulation.
authors.affiliation: Gene Center and Center for integrated Protein Science Munich, Department of Biochemistry, Feodor-Lynen-Strasse 25, University of Munich, 81377 Munich, Germany.
authors.forename: Lukas
authors.initials: L
authors.lastname: Bischoff
authors.affiliation: Gene Center and Center for integrated Protein Science Munich, Department of Biochemistry, Feodor-Lynen-Strasse 25, University of Munich, 81377 Munich, Germany.
authors.forename: Otto
authors.initials: O
authors.lastname: Berninghausen
authors.affiliation: Gene Center and Center for integrated Protein Science Munich, Department of Biochemistry, Feodor-Lynen-Strasse 25, University of Munich, 81377 Munich, Germany. Electronic address: beckmann@lmb.uni-muenchen.de.
authors.forename: Roland
authors.initials: R
authors.lastname: Beckmann
mesh_terms: D000595:Amino Acid Sequence; D004926:Escherichia coli / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D029968:Escherichia coli Proteins / Q000737:chemistry / Q000378:metabolism; D015964:Gene Expression Regulation, Bacterial; D008969:Molecular Sequence Data; D009876:Operon; D010454:Peptide Termination Factors / Q000378:metabolism; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D012270:Ribosomes / Q000737:chemistry* / Q000378:metabolism; D014364:Tryptophan / Q000378:metabolism*
chemical_list: D029968:Escherichia coli Proteins; D010454:Peptide Termination Factors; C041460:peptide chain termination release factor 2; C081473:tnaC protein, E coli; D014364:Tryptophan