2-21 Ribosomes are composed of RNA and protein

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• Ribosomes are 62% RNA and 38% protein formed into two complexes in prokaryotes, the 50S ribosome subunit and the 30S ribosome subunit.
• Ribosome associated proteins are positively charged. This helps them to bind to the negatively charged mRNA.

In contrast to the case with mRNAs, the other RNAs involved in translation, tRNA and rRNA, have very distinct structures. Each rRNA folds into a known secondary structure and has a complex tertiary structure containing many short helical regions and long range base pair interactions. These structures are also maintained by interactions between the RNAs and protein.

The composition of ribosomes is 62 % RNA and 38 % protein by weight. Two complexes of RNA and protein make up the ribosome, the 30S subunit and the 50S subunit. (The S stands for Svedberg units, a measure of how fast something sediments in solution. For our purposes all you need to know is that larger molecules sediment faster and have larger Svedberg units. The unit is named after Theodor Svedberg who won the Nobel Prize in Chemistry in 1926 for his work on suspensions of large molecules and other compounds in solutions.) The 30S subunit is composed of 21 proteins and a single-stranded rRNA molecule of about 1,500 nucleotides, termed the 16S rRNA. The 50S subunit contains 31 proteins and two RNA species, a 5S rRNA of 150 nucleotides and a 23S rRNA of about 2,900 nucleotides. Several of the nucleotides on the 16S and 23S rRNAs have been modified by methylation and these modifications are probably critical to the function of the rRNAs since they always happen in regions conserved through evolution.

The ribosome-associated proteins are positively charged, with a high proportion of lysine and arginine residues. This facilitates complex formation between the acidic RNA and these basic proteins. The crystal structure of the entire 70S ribosome has been solved as shown in Figure 2-30, which is quite remarkable because solving such structures becomes more difficult as the size of the complex increases and the ribosome is huge. We are still trying to fully understand what that structure tells us about ribosome function. We also know that the ribosome is not static, but dynamic, changing shape during a translation cycle. It is also clear that this complex mix of protein and rRNAs can self-assemble when mixed in the right order, though the process of assembly in the cell is certainly more carefully controlled.

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