Composition of prokaryotic ribosome

An E. coli ribosome is a ribonucleoprotein assembly with a mass of about 2500 kDa. Ribosomes are abundant (10% of total protein and 80% RNA). A ribosome (70S) can be dissociated into large subunit (50S) and a small subunit (30S). The formation of 70S initiation complex is the rate-limiting step in protein synthesis. These subunits can be further split into their consituent proteins and RNAs.

The 30S subunit contains 21 different proteins and a 16S RNA molecule. 16S subunit pairs with purine-rich region called Shine-Dalgano sequence. The number of base pairs linking mRNA and 16S rRNA ranges from 3 to 9. In addition, AUG codon pairs with initiator tRNA.

The 50S subunit conains 34 different proteins and two RNA molecules, a 23S and a 5S species. 23S subunit is important in the formation of peptide bond.

S = Svedberg unit (measure sedimentation rate in centrifuge and thus size).