Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 173

Figure 2.21 shows the components of a bacterial ribosome. The complete ribosome, called the 70S ribosome in bacteria, consists of two subunits, the 30S subunit, which contains one molecule of 16S rRNA, and the 50S subunit, which contains one molecule each of 23S and 5S rRNA. Each subunit also contains ribosomal proteins; the 30S subunit contains approximately 21 different proteins (S1, S2, etc., where S indicates small subunit proteins), while the 50S subunit contains approximately 31 different proteins (L1, L2, etc., where L indicates large subunit proteins). Ribosomes from different bacterial species have very similar compositions, with small differences in the number of ribosomal proteins. Like the names for the different types of rRNA, the names of ribosomal subunits are derived from their sedimentation rates during ultracentrifugation. The 30S and 50S subunits normally exist separately in the cell and come together to form the complete 70S ribosome only when they are translating an mRNA. Note that sedimentation values are not additive; the complete ribosome is 70S in size, despite being composed of subunits that are 30S and 50S in size. This is because the two subunits fit together into a tight complex.

The two ribosomal subunits play very different roles in translation. To initiate translation, the 30S subunit first binds to the mRNA. Then, the 50S ribosome binds to the 30S subunit to make the 70S ribosome. From that point on, the 30S subunit mostly helps to select the correct aminoacyl-tRNA (aa-tRNA) for each codon while the 50S subunit does most of the work of forming the peptide bonds and translocating the tRNAs from one site on the ribosome to another (see below). The 70S ribosome moves along the mRNA, allowing tRNA anticodons to pair with the mRNA codons and translate the information from the nucleic acid chain into a polypeptide. After the polypeptide chain is completed, the ribosome separates again into the 30S and 50S subunits. The role of the subunits is discussed in more detail below.