ABSTRACT
A prokaryotic 70S ribosome comprises two subunits. The subunits are: a small subunit and a large subunit, each of which is a multicomponent complex of ribosomal RNAs(rRNAs) and ribosomal proteins. The 50S subunit has 23S and 5S rRNAs complexed with 34 polypeptides whereas the 30S subunit contains 16S rRNA and 21 polypeptides. The 3D structure of bacterial ribosomes shows that it is the rRNAs which fold and base pair with each other to form the overall structure of the ribosome, with the proteins located peripherally. A wide range of studies have built up a detailed picture of the fine structure of ribosomes, mapping the location of the various RNA and protein components and their interactions. The promoters of tRNA genes, which are also transcribed by RNA Pol III, contain control elements called the A box and B box located downstream of the transcriptional start site.
