Suppression of UAA and UGA termination codons in mutant murine leukemia viruses.

Genomes of mammalian type C retroviruses contain a UAG termination codon between the gag and pol coding regions. The pol region is expressed in the form of a gag-pol fusion protein following readthrough suppression of the UAG codon. We have used oligonucleotide-directed mutagenesis to change the UAG in Moloney murine leukemia virus to UAA or UGA. These alternate termination codons were also suppressed, both in infected cells and in reticulocyte lysates. Thus, the signal or context inducing suppression of UAG in wild-type Moloney murine leukemia virus is also effective with UAA and UGA. Further, mammalian cells and cell extracts contain tRNAs capable of translating UAA and UGA as amino acids. To our knowledge, this is the first example of natural suppression of UAA in higher eucaryotes.