Figure 4.

RNA synthesis activity of PB1 or PB2 double mutants in HeLa cells. Bar charts showing the level of viral mRNA (A), cRNA (B), or vRNA (C) synthesis by different RNA polymerase double mutants, compared with that of the wild-type (WT) polymerase or with PB2 absent (−PB2). RNA was isolated from HeLa cells transfected with plasmids for expression of viral RNP components. Using primers specific for viral mRNA, cRNA, or vRNA, the production of each RNA type could be separately assessed by quantitative PCR (see Materials and methods). In the absence of the PB2 subunit, enzyme activity is negligible. The results are mean and s.d. for three independent experiments.