Termination of Polypeptide
Termination of the polypeptide chain is brought about by the presence of any one of the three termination codons, namely UAA, UAG and UGA (consult The Genetic Code
). These termination codons are recognized by one of the two release factors RF1
Of these release factors, RF1 recognizes UAA and UAG and RF2 recognizes UGA. They help the ribosome to recognize these triplets. The release factors seem to act on 'A' site, since suppressor tRNA capable of recognizing termination codons can compete with release factors by entry at 'A' site. A third release factor RF3
seems to stimulate the action of RF1 and RF2. For release reaction, the polypeptidyl tRNA must be present on 'P' site and the release factors help in splitting of the carboxyl group between the polypeptide and the last tRNA carrying this chain. Polypeptide is thus released and the ribosome dissociates into two subunits with the help of IF-3.
In eukaryotic system only one release factor is known i.e. eRFI.
GTP seems to be necessary for binding of this factor to ribosome. GTP is cleaved after the termination step has occurred which may be a prerequisite for the release of eRFI from the ribosome.