Polypeptide chain termination takes place when a termination (non-sense) codon enters the A-site on the 30S ribosomal subunit. Early studies suggested that the three bases of the termination codon (UAA, UAG, or UGA) provide all the information required for termination. However, it now appears that bases on either side of the termination codon influence the strength of the stop signal. For instance, UGA is a much stronger stop signal in E. coli when followed by a U than when followed by a C. The next two bases downstream as well as bases just upstream from the termination codon also influence stop signal strength but to a lesser extent.
Polypeptide chain termination in bacteria requires release factors (RFs). The existence of these release factors was first revealed in experiments performed by Mario Capecchi in 1967. He first prepared a ribosome●mRNA complex with a hexapeptide attached to tRNA at the P-site and a termination codon at the A-site and then used this complex to demonstrate that a soluble protein factor was required to release the hexapeptide. C. Thomas Caskey and coworkers devised a much simpler assay for RF activity the following year. Their assay was based on the observation that release factors stimulate an fMet-tRNA●AUG●ribosome complex to release fMet when incubated with a trinucleotide containing a termination codon. Using this simpler assay system, Caskey and coworkers demonstrated that bacterial extracts contain two different RFs. RF1 recognizes the termination codon UAG, whereas RF2 recognizes the termination codon UGA. In addition, both release factors recognize the termination codon UAA. Working independently in 1969, the Capecchi and Caskey laboratories discovered a third bacterial release factor, RF3, which stimulates the rate of peptide release when either RF1 or RF2 is also present but it does not act on its own.