Transfer ribonucleic acid-mediated suppression of termination codons in Escherichia coli.

The universal genetic code includes three codons which signal polypeptide chain termination. These termination or nonsense codons are UAG (amber), UAA (ochre), and UGA (opal). Usually, Escherichia coli and other procaryotic cells do not contain transfer ribonucleic acids (tRNAs) which recognize these codons. However, such tRNAs can be created by suppressor mutations in tRNA genes, generating tRNAs with the ability to recognize termination codons. Most suppressor mutations are located in the anticodon triplet and alter it to CUA, UUA, or UCA, the characteristic anticodons in amber, ochre, and opal suppressor tRNAs, respectively. Whereas amber suppressor tRNAs only recognize amber codons, tRNAs with ochre anticodons (UUA) recognize both ochre and amber codons, and at least some tRNAs with opal anticodons (UCA) recognize UGG tryptophan codons in addition to opal codons. Because of the involvement of tRNA in this process, termination suppression (also called nonsense suppression) is part of the larger topic of "informational suppression" (77). Other mechanisms of tRNA-mediated informational suppression include missense suppression and frameshift suppression. These will not be discussed in this review. Research on missense suppression has been thoroughly