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Abstract
A transcriptional attenuation mechanism regulates expression of the bacterial tnaCAB operon. This mechanism requires ribosomal arrest induced by the regulatory nascent TnaC peptide in response to free L-tryptophan (L-Trp). In this study we demonstrate, using genetic and biochemical analyses, that in Escherichia coli, TnaC residue I19 and 23S rRNA nucleotide A2058 are essential for the ribosome’s ability to sense free L-Trp. We show that the mutational change A2058U in 23S rRNA reduces the concentration dependence of L-Trp-mediated tna operon induction, whereas the TnaC I19L change suppresses this phenotype, restoring the sensitivity of the translating A2058U mutant ribosome to free L-Trp. These findings suggest that interactions between TnaC residue I19 and 23S rRNA nucleotide A2058 contribute to the creation of a regulatory L-Trp binding site within the translating ribosome.
Highlights
Ribosomes are cellular molecular complexes whose primary function is carrying out protein synthesis in all organisms
The 23S rRNA nucleotides A2058 and A2059 are located at the surface of the exit tunnel, which makes them susceptible to interactions with nascent peptides and other ligands
In a search for other cellular proteins whose expression might depend on nascent peptide recognition in the tunnel, we used 2D-differential gel electrophoresis (2D-DIGE) to compare global expression of proteins in wild-type E. coli cells with those carrying the 23S rRNA A2058G mutant (‘Materials and Methods’)
Summary
Ribosomes are cellular molecular complexes whose primary function is carrying out protein synthesis in all organisms. Nascent polypeptides exit the ribosome through the peptide exit tunnel, a structure that begins at the PTC and spans the body of the large subunit. Several antibiotics (which interfere with protein synthesis) inhibit ribosome function by binding to either the PTC or the peptide exit tunnel. Nascent peptides can regulate the activities of the large subunit, modulating gene expression. These regulatory nascent peptides, termed ribosome arrest peptides (RAPs), induce translational arrest; the resulting arrested ribosomes control either transcription or translation of the downstream genes in the same operon [1,2,3]
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