Release factor

A release factor is a protein that allows for the termination of translation by recognizing the termination codon or stop codon in a mRNA sequence. During translation of mRNA, most codons are recognized by "charged" tRNA molecules, called aminoacyl-tRNAs because they are adhered to specific amino acids corresponding to each tRNA's anticodon.

In the standard genetic code, there are three mRNA stop codons: UAG ("amber"), UAA ("ochre"), and UGA ("opal" or "umber").

Although these stop codons are triplets just like ordinary codons, they are not decoded by tRNAs. Instead, it was discovered by Mario Capecchi in 1967 that tRNAs do not ordinarily recognize stop codons at all, and that what he named "release factor" was not a tRNA molecule, but a protein. Later, it was demonstrated that different release factors recognize different stop codons.

Prokaryotic translation termination is mediated by three release factors: RF1 RF2 and RF3.
 * RF1 recognizes the termination codons UAA and UAG
 * RF2 recognizes UAA and UGA
 * RF3 is a GTP-binding protein that facilitates the binding of RF1 and RF2 to the ribosomal complex

Eukaryotic translation termination similarly involves two release factors: eRF1 and eRF3.
 * eRF1 recognizes all three termination codons
 * eRF3 is a ribosome-dependent GTPase that helps eRF1 release the completed polypeptide