DDX19B and infection: To confirm the interaction of the endogenous DDX19 protein with the viral polymerase during infection and to assess whether this interaction requires the presence of RNAs, DDX19 was immunoprecipitated from lysates of HEK-293T cells infected with a recombinant WSN virus expressing a Strep-tagged PB2 protein (WSN-PB2-Strep) in the presence or absence of RNAse (Fig. 5a).