Abstract
Yeast L-A double-stranded RNA virus furnishes its transcript with a 5' cap structure by a novel cap-snatching mechanism in which m(7)Gp from a host mRNA cap structure is transferred to the 5'-diphosphate terminus of the viral transcript. His-154 of the coat protein Gag forms an m(7)Gp adduct, and the H154R mutation abolishes both m(7)Gp adduct formation and cap snatching. Here we show that L-BC, another totivirus closely related to L-A, also synthesizes 5'-diphosphorylated transcripts and transfers m(7)Gp from mRNA to the 5' termini of the transcripts. L-BC Gag also covalently binds to the cap structure and the mutation H156R, which corresponds to H154R of L-A Gag, abolishes cap adduct formation. Cap snatching of the L-BC virus is very similar to that of L-A; N7 methylation of the mRNA cap is essential for cap donor activity, and only 5'-diphosphorylated RNA is used as cap acceptor. L-BC cap snatching is also activated by viral transcription. Furthermore, both viruses require Mg(2+) and Mn(2+) for cap snatching. These cations are not only required for transcription activation but also directly involved in the cap transfer process. These findings support our previous proposal that the cap-snatching mechanism of the L-A virus is shared by fungal totiviruses closely related to L-A. Interestingly, L-A and L-BC viruses accept either viral transcript as cap acceptor in vitro. Because L-A and L-BC viruses cohabit in many yeast strains, it raises the possibility that their cohabitation in the same host may be beneficial for their mutual cap acquisition.
Highlights
L-A totivirus furnishes its transcript by a novel cap-snatching mechanism
L-A virus can form a cap structure on an ongoing L-A transcript and on an externally added L-A transcript [27]
These results suggest that M1 and L-BC preferentially install the cap structure on their ongoing transcript by cap snatching in cis in the cell
Summary
L-A totivirus furnishes its transcript by a novel cap-snatching mechanism. Results: L-BC virus transfers m7Gp from mRNA to the diphosphorylated 5Ј end of its transcript to form a cap structure. Yeast L-A double-stranded RNA virus furnishes its transcript with a 5 cap structure by a novel cap-snatching mechanism in which m7Gp from a host mRNA cap structure is transferred to the 5-diphosphate terminus of the viral transcript. These cations are required for transcription activation and directly involved in the cap transfer process These findings support our previous proposal that the cap-snatching mechanism of the L-A virus is shared by fungal totiviruses closely related to L-A. Fungal totiviruses (including L-BC) closely related to L-A share His-154 and other amino acid residues of L-A Gag important for cap binding, at comparative positions in their coat proteins (Fig. 1B) We proposed that these viruses acquire a cap structure on their transcripts by a mechanism similar to that of L-A [11]. Because these viruses cohabit in many yeast strains, it raises the possibility that their cohabitation is beneficial for their mutual cap acquisition
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