The soybean retroelement SIRE1 uses stop codon suppression to express its envelope-like protein

The soybean SIRE1 family of Ty1/copia retrotransposons encodes an envelope-like gene (env-like). We analysed the DNA sequences of nine SIRE1 insertions and observed that the gag/pol and env-like genes are in the same reading frame and separated by a single UAG stop codon. The six nucleotides immediately downstream of the stop codon conform to a degenerate nucleotide motif, CARYYA, which is sufficient to facilitate stop codon suppression in tobacco mosaic virus. In vivo stop codon suppression assays indicate that SIRE1 sequences confer leakiness to the UAG stop codon at an efficiency of 5%. These data suggest that SIRE1 retro-elements use translational suppression to express their envelope-like protein; this is in contrast with all characterized retroviruses, which express the envelope protein from a spliced genomic messenger RNA.