Cap Snatching in Yeast L-BC Double-stranded RNA Totivirus

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. Background: L-A totivirus furnishes its transcript by a novel cap-snatching mechanism. Results: L-BC virus transfers m 7 Gp from mRNA to the diphosphorylated 5′ end of its transcript to form a cap structure. Conclusion: L-BC totivirus also can perform cap snatching. Significance: The novel cap-snatching mechanism is shared among fungal totiviruses.