Molecular Biology

Chapter 19 Outline

Ribosome structure:

∑ Electron microscopic studies were initially used to define the shapes of the 30S and 50S ribosomal subunits; Structural features of the 30S subunit (F19.2); Structural features of the 50S subunit (F19.4); Structure of the 70S ribosome (F19.5).

∑ X-ray crystallography has refined ribosome structural features and defined where contact is made between the subunits (F19.7); The P, A and E tRNA binding sites have been identified within the small and large subunits (F19.8; F19.9); A cavity between the 30S and 50S subunits can accommodate three tRNAs, the anticodon end of the tRNAs interact with the mRNA in the30S subunit, and the acceptor stem interacts with the 50S subunit where the peptidyl transferase resides.

∑ The 30S subunit contains 16S rRNA and 21 ≥S≤ proteins; The 50S subunit contains 23S and 5S rRNAs and 34 ≥L≤ proteins.

∑ When the constituent 30S rRNAs and proteins were added together, functional ribosomes could be generated when combined with 50S subunits ≠ thus demonstrating that the 30S subunit is self-assembling; The order of assembly for the 30S subunit has been determined (F19.12); The 50S subunit can also self assemble, but the assembly order is not known.

∑ The 16S rRNA folds into specific secondary and tertiary structures due to extensive intramolecular base pairing (F19.16, F19.17).

∑ Proteins are thought to be too far from the peptidyl transferase active site to be directly involved in the transition state; A conserved A residue (A2486) in the 23S rRNA is likely to serve as a catalyst for the peptidyl transferase reaction (F19.26); The peptide exits through a tunnel within the 50S subunit (F19.27).

∑ Evidence for polysomes in eukaryotes (F19.28) and prokaryotes (F19.29).

tRNA structure and function:

∑ tRNAs were discovered as small RNA that were coupled to amino acids and lost these amino acids during the process of translation (F19.30).

∑ tRNAs are depicted as cloverleaf structures with a D loop, anticodon, variable loop, T loop and acceptor stem from 5π to 3π (F19.31); tRNAs fold into a 3-dimensional structure with the acceptor and anticodon at either end (F19.33, F19.34).

∑ The ribosome recognizes the tRNA rather than the amino acid (F19.37), indicating that tRNAs must be accurately matched with their cognate amino acid; Aminoacyl-tRNA synthetases match the tRNA to the appropriate amino acid; The acceptor stem and the anticodon of tRNAs are the primary sites for recognition by aminoacyl-tRNA synthetases (F19.39); Amino acids are converted to aminoacyl adenylate forms before becoming aminoacyl-tRNAs; Formation of aminoacyl adenylate occurs at the activation site, which rejects amino acids that are too large (F19.41); If the aminoacyl adenylate is too small for the aminoacyl ≠tRNA synthetase, they will be hydrolyzed in the editing site and never adde to the tRNA (F19.41); Thus, only amino acids that pass through the activation site (as aminoacyl adenylate) and are rejected from the editing site are transferred to the cognate tRNA to make aminoacyl-tRNA.