Termination of polypeptide

Content
Expression of Gene : Protein Synthesis 4. Translation in Prokaryotes and Eukaryotes
Formation of amino-acyl tRNA
Initiation of polypeptide 
Initiation in prokaryotes
Initiation in eukaryotes
Kozak's scanning hypothesis
Elongation of polypeptide
Binding of AA-tRNA at site 'A' of ribosome
Formation of peptide bond
Translocation of peptidyl tRNA from 'A' to 'P' site
Termination of polypeptide
Modification, folding and transport of released polypeptide
Translation in chloroplasts and mitochondria
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 and RF2. 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.