Fundamentals of Molecular Biology - V.I. Rezyapkin 2009

RNA, RNA Processing
rRNA. Ribosomes

Ribosomes are subcellular structures that serve as the site of Protein Synthesis. They are large ribonucleoprotein complexes with a Molecular Weight of 2.5–4.2 MDa, composed of Proteins and rRNA. The number of ribosomes is about 10⁴ in E. coli Cells and 10⁶ in eukaryotes.

Prokaryotic and eukaryotic ribosomes differ in size. In eukaryotes, they are represented by 80S particles, while in prokaryotes, they are 70S (Fig. 5.4). Ribosomes consist of a small and a large subunit: prokaryotic subunits are 30S and 50S, respectively, whereas eukaryotic ones are 40S and 60S.

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Fig. 5.4. Model of E. coli ribosomes

Prokaryotic ribosomes contain Three types of rRNA and 55–60 ribosomal proteins, whereas eukaryotic ones contain 4 rRNA species and 70–85 ribosomal proteins (Fig. 5.5). rRNAs play a crucial role in the Structure and biosynthetic function of ribosomes. Specifically, rRNA catalyzes the peptidyl transferase reaction, which drives peptide bond formation during Protein Synthesis on Ribosomes. Ribosomal Proteins are essential for maintaining the proper conformation of rRNA.

Fig. 5.5. Ribosome composition

rRNA is concentrated in the core of the particles, whereas ribosomal proteins occupy a peripheral position (Fig. 5.6).

Fig. 5.6. Outlines of ribosomal subunits and isolated rRNAs in a compact conformation

Approximately 2/3 of the rRNA molecule forms double-stranded regions (Fig. 5.7).

Fig. 5.7. rRNA

Each ribosomal protein specifically interacts with a distinct region of rRNA. Mixing 16S rRNA with the Proteins of the small subunit leads to its spontaneous self-assembly. Similarly, the large subunit can be reconstructed in vitro from its constituent rRNAs and proteins.