In vitro polymerase activity of Thogoto virus: evidence for a unique cap-snatching mechanism in a tick-borne orthomyxovirus.

The tick-borne Thogoto virus (THOV) is the type species of a new genus in the family Orthomyxoviridae. Its genome comprises six segments of single-stranded, negative-sense RNA. Each segment possesses conserved regions of semicomplementary nucleotides at the 3' and 5' termini which strongly resemble those of influenza virus. An in vitro polymerase assay based on reconstituted THOV viral cores was developed, and activity was shown to rely on an interaction between the conserved 3'- and 5'-terminal sequences and to be primer dependent. Addition of globin mRNA primed transcription, catalyzing the addition of an extra nucleotide to the transcripts, corresponding to the 5'-terminal m7G cap residue. Priming with various cap analogs suggested that THOV transcription is initiated preferentially with m7GpppAm and involves base pairing. This is the first experimental evidence of endonuclease activity in THOV as part of a unique cap-snatching mechanism.