UAG Suppressor Glutamine tRNA in Uninfected and Retrovirus-Infected Mammalian Cells

Abstract

Three codons, UAG, UAA, and UGA, in the genetic code are normally signals for termination of translation reaction and for the release of the completed polypeptide chain from its ultimate ribosome-bound tRNA. Recently, it has been reported that eubacterial and eukaryotic mRNAs contain a translatable nonsense codon and its readthrough by suppressor tRNA plays an important role in the synthesis of particular proteins which are necessary for specific cellular functions. For example, in-frame UGA nonsense codons have been found in the mouse glutathione peroxidase and Escherichia coli formate dehydrogenase genes (Chambers et al. 1986; Zinoni et al. 1986). The translational insertion of selenocystein at the UGA codon, which is the active site of the enzymes from those genes, is required for the expression of their enzyme activities. The internal nonsense codons have also been detected at the gag-pol junction of the retrovirus genomes, such as Moloney murine leukemia virus (Mo-MuLV) and Rous sarcoma virus (RSV) (Shinnick et al. 1981; Phillipson et al. 1978; Yoshinaka et al. 1985; Crawford and Goff 1985; Jacks and Varmus 1985). The readthrough of the nonsense codon produces a viral gag-pol precursor fusion protein which is later cleaved by proteases to yield the mature viral proteins, including protease. The translation of the internal nonsense codon of the retrovirus genomes so far reported is performed by nonsense suppression or frameshift suppression, resulting in the regulation of the level of gag and gag-pol readthrough proteins in the virus-infected cells, which is required for the vegetative virus proliferation.