In chapter 3 we examined a few aspects of translation. We learned that ribosomes are the cell’s protein factories and that transfer RNA plays a crucial adapter role, binding an amino acid at one end and an mRNA codon at the other. Chapters 17 and 18 expanded on the mechanisms of translation initiation, elongation, and termination, without dealing in depth with ribosomes and tRNA. In this chapter, let us continue discussion of translation with a closer look at these two essential agents. | Molecular Biology Fourth Edition Chapter 19 Ribosomes and Transfer RNA Lecture PowerPoint to accompany Robert F. Weaver Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Ribosomes E. coli ribosome is a two-part structure with a sedimentation coefficient of 70S Two subunits of this structure: 30S is the small subunit that decodes mRNA 50S subunit links amino acids together through peptide bonds 19- Fine Structure of 70S Ribosome T. thermophilus crystal structure of 70S ribosome in complex with mRNA analog and 3 tRNAs shows: Positions and tertiary structures of all 3 rRNA and most proteins can be determined Shapes and locations of tRNAs in A, P, and E sites are evident Binding sites for tRNAs in ribosome are composed of rRNA, not protein Anticodons of tRNAs in A and P sites approach each other closely enough to base-pair with adjacent codons bound to 30S subunit as mRNA kinks 45° 19- More Structural Detail Acceptor stems of tRNAs . | Molecular Biology Fourth Edition Chapter 19 Ribosomes and Transfer RNA Lecture PowerPoint to accompany Robert F. Weaver Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Ribosomes E. coli ribosome is a two-part structure with a sedimentation coefficient of 70S Two subunits of this structure: 30S is the small subunit that decodes mRNA 50S subunit links amino acids together through peptide bonds 19- Fine Structure of 70S Ribosome T. thermophilus crystal structure of 70S ribosome in complex with mRNA analog and 3 tRNAs shows: Positions and tertiary structures of all 3 rRNA and most proteins can be determined Shapes and locations of tRNAs in A, P, and E sites are evident Binding sites for tRNAs in ribosome are composed of rRNA, not protein Anticodons of tRNAs in A and P sites approach each other closely enough to base-pair with adjacent codons bound to 30S subunit as mRNA kinks 45° 19- More Structural Detail Acceptor stems of tRNAs in A and P sites also approach each other closely (5 Å) in the peptidyl transferase pocket of 50S subunit Consistent with need for 2 stems to interact during peptide bond formation 12 contacts are seen between subunits, most RNA-RNA interactions 19- E. coli Ribosome Structure Crystal structure of E. coli ribosome contains 2 structures differing from each other by rigid body motions of ribosome domains relative to each other Head of 30S particle rotates by 6 12 rotation compared to T. thermophilus ribosome Probably part of a ratchet action that occurs during translocation 19- Eukaryotic Ribosomes Eukaryotic cytoplasmic ribosomes are: Larger More complex Eukaryotic organellar ribosomes are smaller than prokaryotic ones 19- Ribosome Composition The E. coli 30 subunit contains 16S rRNA 21 proteins (S1 – S21) E. coli 50S subunit contains 5S rRNA 23S rRNA 34 proteins (L1 – L34) Eukaryotic cytoplasmic ribosomes are: Larger Contain more RNAs and proteins 19- Ribosome Assembly .
