Evidence that a downstream pseudoknot is required for translational read-through of the Moloney murine leukemia virus gag stop codon

Approximately 5% of the ribosomes translating the gag gene of murine leukemia viruses read through the UAG terminator and translate the in-frame pol gene to produce the gag-pol fusion polyprotein, the sole source of the pol gene products. We show that a pseudoknot located eight nucleotides 3' of the UAG codon in the Moloney murine leukemia virus is required for read-through. This requirement is markedly different from that known to be involved in other cases of read-through but surprisingly similar to some stimulatory sequences known to promote ribosomal frameshifting.