Ribosomes

The ribosome is the structure on which translation occurs. Figure 27.13 shows that the prokaryotic ribosome consists of two large subunits. Each subunit, in turn, consists of multiple proteins and RNA. The 70S, 50S, and 30S designations refer to centrifugation sedimentation coefficients, which reflect the size of each entity, but are not strictly additive.

The 50S subunit is called the large subunit. It contains 31 separate single-copy proteins and one, designated as L7/L12, which is present in four copies. Proteins in the large subunit are numbered and begin with the letter 'L' to indicate they are from the large subunit. The 50S subunit also contains 2 ribosomal RNA (rRNA) sequences called 23S (2904 residues in length) and 5S (120 residues in length). Again, the 'S' is a relative size measure based on centrifugation properties.

The 30S subunit is called the small subunit. It contains 21 proteins and a single rRNA sequence, called 16S (1542 residues in length). Proteins in the small subunit are numbered and begin with the letter 'S' to indicate they are from the small subunit. One protein is found in both the large and small subunits. It is called L26 in the large subunit and S20 in the small subunit.

The amino acid sequence of each ribosomal protein is known. There are no significant homologies among the proteins of a ribosome, but sequences of corresponding proteins in different organisms show considerable evolutionary conservatism.

The 16S rRNA in the small subunit contains many regions of self-complementarity, which are capable of forming double-helical segments (Figure 27.15). The potentially double-stranded regions are highly conserved among a wide variety of otherwise unrelated organisms. The 16S rRNA folds into a three dimensional structure in the ribosome and is bound by multiple ribosomal proteins.

See also: Internal Ribosomal Structure, Translation, Initiation of Translation, Elongation of Translation, Termination of Translation, Antibiotic Inhibition of Translation, Genetic Code, Codons

INTERNET LINKS:

> 1. Ribosome Database Project

> 2. rRNA Database

> 3. 16S rRNA Secondary Structures

> 4. Ribosomal RNA Mutation Database