The Extended Loops of Ribosomal Proteins L4 and L22 are Not Essential for Ribosome Function, Cell Survival, or Peptide‐Mediated Translational Arrest

Abstract

Ribosomes are the universal organelles responsible for polymerizing amino acids into proteins. Ribosome‐mediated peptide synthesis takes place within the peptidyl‐transferase center of the large subunit of the ribosome. Once synthesized, nascent peptides traverse the ribosome exit tunnel before reaching the extraribosomal environment. Several nascent peptides have been shown to interact with the exit tunnel to stall translation elongation at specific sites within their peptide chain. The exit tunnel is primarily lined by rRNA, but the extended loops of ribosomal proteins L4 and L22 contribute to the lining, forming the narrowest portion of the tunnel. This constriction has also been shown to be important for the action of and resistance to macrolide‐lincomycin‐streptogamin (MLS) antibiotics. Several mutations localized near the constriction of the tunnel have been shown to interfere with nascent peptide‐mediated pausing. Previous studies of MLS antibiotics led to the proposal that the extended loops of L4 and L22 provide a gating mechanism by which the ribosome exit tunnel is narrowed or widened. The ability of these two proteins to adjust tunnel width has been suggested to play important roles in both antibiotic resistance and in regulating the passage of newly formed peptides through the ribosome exit tunnel. Through mutation and the use of translational reporters we have obtained results which show that, contrary to the proposed model, the extended loops of these two proteins are not required for ribosomal function, cell survival, or several cases of peptide‐mediated translational arrest. We also present work showing that different classes of peptides known to stall translation respond differentially to mutations in L4 and L22.