10.2: Prokaryotic Ribosomes

The prokaryotic ribosomes contain 3 RNA strands and 52 protein subunits which can be divided into 1 RNA and 21 proteins in the small ribosomal subunit (aka the 30S subunit) and 2 RNA and 31 proteins in the large ribosomal subunit (50S subunit). The small subunit locates the start site and moves along the RNA. The large ribosomal subunit contains the aminoacyl transferase enzyme activity that connects amino acids to make a protein. Neither subunit is sufficient to carry out translation by itself. They must come together to form the full 70S ribosome for translation to occur. If you aren’t already familiar with the nomenclature, you’re probably thinking that it’s obvious why I went into biology rather than math. My competence at basic computation aside, there is a method to the madness. The “S” in 30S or 50S indicates Svedberg units, or a measurement of the sedimentation rate when the molecules in question are centrifuged under standard conditions. Because the rate of sedimentation depends on both the mass and the shape of a molecule, numbers do not always add up.

The genes for the prokaryotic rRNA molecules are arranged in an operon and thus come from a single transcript. Depending on the organism, there may be several such operons in the genome to ensure steady production of this crucial enzymatic complex. However, these RNAs are not translated, so instead of having multiple translation start codons to signal the beginning of each gene, the single transcript is cleaved post-transcriptionally by Ribonuclease III (RNase III) into 25S, 18S, and 5S segments, and these are then further trimmed by RNase III and RNase M into the final 23S, 16S, and 5S rRNAs found in the ribosomes. Although the 5S does not appreciably differ in sedimentation rate, it is in fact slightly “shaved” in post-transcriptional editing.