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. It was discovered by Mario Capecchi in 1967 that, instead, 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 leads to the dissociation of RF1/RF2 after peptide release
Likewise, eukaryotic translation termination 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
- Capecchi, M. R. (1967). Polypeptide chain termination in vitro: Isolation of a release factor. Proc. Natl. Acad. Sci. USA 58:1144-1151.
- Scolnick, E., R. Tompkins, T. Caskey, and M. Nirenberg (1968). Release factors differing in specificity for terminator codons, Proceedings of National Academy of Sciences USA 61:772.
- Weaver, Robert F. (2005). Molecular Biology, p.616-621. McGraw-Hill, New York, NY. ISBN 0-07-284611-9.
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