| Streszczenie: | <p>Influenza A virus encodes a viral RNA-dependent RNA polymerase (FluPol<sub>A</sub>), which is responsible for transcribing and replicating the negative-sense viral RNA (vRNA) genome. FluPol<sub>A</sub> transcribes vRNA using a host capped mRNA primer, and replicates it by synthesising a positive-sense complementary RNA (cRNA) intermediate which is copied back into vRNA. To carry out these functions, FluPol<sub>A</sub> interacts with vRNA and cRNA using conserved promoter elements at the 5′ and 3′ termini. Recent structural studies have identified a new surface binding site for the 3′ vRNA and cRNA promoters on FluPol<sub>A</sub>, referred to as the Mode B site. However, the role of this binding site in FluPol<sub>A</sub> function is unknown. In this study we used a combination of cell-based and biochemical assays to show that the Mode B site is important for both viral genome transcription and replication. Furthermore, we show that the Mode B site is not needed for initiating transcription <em>in vitro</em> but is required to synthesise a full-length product. This is consistent with a model in which the 3′ terminus of the vRNA template binds in the Mode B site during elongation. Our data provide the first functional insights into the role of the Mode B site on FluPol<sub>A</sub>, which advances our understanding of FluPol<sub>A</sub> function and influenza A virus replication.</p>
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