Key Concepts of Section 5.4

• Bacterial, archaeal, and eukaryotic ribosomes—the large ribonucleoprotein complexes on which translation occurs—consist of a small and a large subunit (see Table 5-3). Each subunit contains numerous different proteins and one major rRNA molecule (small or large). The large subunit also contains one accessory 5S rRNA in bacteria and archaea and two accessory rRNAs in eukaryotes (5S and 5.8S).

• Analogous rRNAs from many different species fold into quite similar three-dimensional structures containing numerous stem-loops and binding sites for proteins, mRNA, and tRNAs. Much smaller ribosomal proteins are associated with the periphery of the rRNAs.

• Of the two methionine tRNAs found in all cells, only one (tRNA_i^Met) functions in initiation of translation.

• Each stage of translation—initiation, chain elongation, and termination—requires specific protein factors, including GTP-binding proteins that hydrolyze their bound GTP to GDP when a step has been completed successfully.

• During initiation, the ribosomal subunits assemble near the translation start site in an mRNA molecule on which a tRNA carrying the amino-terminal methionine (Met-tRNA_i^Met) has base-paired with the start codon (see Figure 5-24).

• Chain elongation entails a repetitive four-step cycle: loose binding of an incoming aminoacyl-tRNA to the A site on the ribosome; tight binding of the correct aminoacyl-tRNA to the A site accompanied by release of the previously used tRNA from the E site; transfer of the growing polypeptide chain to the incoming amino acid catalyzed by large rRNA; and translocation of the ribosome to the next codon, thereby moving the peptidyl-tRNA in the A site to the P site and the now unacylated tRNA in the P site to the E site (see Figure 5-25).

• In each cycle of chain elongation, the ribosome undergoes two conformational changes monitored by GTP-binding proteins. The first of these proteins (EF1α) permits tight binding of the incoming aminoacyl-tRNA to the A site and ejection of a tRNA from the E site, and the second (EF2) monitors translocation.

• Termination of translation is carried out by two types of termination factors: those that recognize stop codons and those that promote hydrolysis of peptidyl-tRNA (see Figure 5-26). Once again, correct recognition of a stop codon is monitored by a GTPase (eRF3).

• The efficiency of protein synthesis is increased by the simultaneous translation of a single mRNA by multiple ribosomes, forming a polyribosome or polysome. In eukaryotic cells, protein-mediated interactions bring the two ends of a polyribosome close together, thereby promoting the rapid recycling of ribosomal subunits, which further increases the efficiency of protein synthesis (see Figure 5-27b).