a, Cap-snatching from host pre-mRNA (red). The m 7G cap is bound by the cap-binding domain (orange, orientated as in the FluA structure) and the pre-mRNA cleaved 10–14 nucleotides downstream by the endonuclease (green). The single-stranded vRNA genome is bound by its 5′ (hook, pink) and 3′ (template, yellow) ends to the polymerase (blue, depicted as a cutaway section). b, Transcription initiation. The cap-binding domain rotates to the position observed in the FluB1 structure directing the capped primer into the PB1 active site, where it potentially makes limited base pairs with the extremity of the template. Template-directed NTP addition (white) extends the host sequences (red) with virally encoded sequences (cyan). Note that in b– d additional conformational changes in the polymerase are expected, but not depicted since they are currently unknown. c, Transcription elongation. Transcription elongation proceeds, eventually leading to the release of the cap from the cap-binding domain ( d) and the binding of host mRNP factors. d, Polyadenylation by stuttering. After most of the vRNA template has been translocated through the polymerase, only a tight turn connects it to the bound 5′-hook. The nucleotide sequence of this region is given at the bottom. This places the 5′ proximal oligo-U stretch in the PB1 active site allowing poly(A) tail synthesis by a stuttering mechanism in which the template is no longer translocated but the product strand is able to slip.