Structural studies of the RNA pseudoknot required for readthrough of the gag-termination codon of murine leukemia virus.

Retroviruses, such as murine leukemia virus (MuLV), whose gag and pol genes are in the same reading frame but separated by a UAG stop codon, require that 5-10 % of ribosomes decode the UAG as an amino acid and continue translation to synthesize the Gag-Pol fusion polyprotein. A specific pseudoknot located eight nucleotides 3' of the UAG is required for this redefinition of the UAG stop codon. The structural probing and mutagenic analyses presented here provide evidence that loop I of the pseudoknot is one nucleotide, stem II has seven base-pairs, and the nucleotides 3' of stem II are important for function. Stem II is more resistant to single-strand-specific probes than stem I. Sequences upstream of the UAG codon allow formation of two competing structures, a stem-loop and the pseudoknot.