Bipartite Signal for Read-Through Suppression in Murine Leukemia Virus mRNA: an Eight-Nucleotide Purine-Rich Sequence Immediately Downstream of the gag Termination Codon Followed by an RNA Pseudoknot

The pol gene of murine leukemia virus and other mammalian type C retroviruses is expressed by read-through suppression of an in-frame UAG codon which separates the gag and pol coding regions. In this study, we have analyzed the sequence requirements for read-through suppression by placing different portions of wild-type and mutant viral sequences from the gag-pol junction between reporter genes and testing transcripts of these constructs for suppression in reticulocyte lysates. We find that the read-through signal is contained within the first 57 nucleotides on the 3 ' side of the UAG codon. Our results indicate that the identities of six conserved bases in the eight-nucleotide, purine-rich sequence immediately downstream of the UAG codon are critical for suppression, as is the existence of a pseudoknot structure spanning the next 49 nucleotides. Thus, read-through suppression depends on a complex, bipartite signal in the mRNA In murine leukemia virus (MuLV) and other mammalian type C retroviruses, the gag and pol coding regions are separated by an in-frame UAG termination codon (8, 13, 19, 28, 32). Read-through suppression of the UAG codon gives rise to a Gag-Pol fusion protein (17, 22, 24, 34), which is subsequently cleaved to give the Pol proteins, i.e., protease