Eleven temperature-sensitive (ts) mutants of influenza A (fowl plague, Rostock) virus were analysed for in vitro RNA transcriptase activity in reactions primed by ApG or globin rnRNA at 31 °C or at 40.5 °C, the restrictive temperature for ts mutant growth. Only those ts mutants studied which were defective in RNA segment 1, coding for the virion P2 protein, were defective in RNA transcriptase activity when compared to wild-type virus. Mutants having a defect in the P2 protein had no significant

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... RNA transcriptase activity in reactions at 40.5 °C primed by globin mRNA. However, one mutant showed RNA transcriptase activity similar to wild-type virus at 40.5 °C when ApG (0.3 raM) was used as primer. The results suggest that influenza (fowl plague, Rostock) P2 protein is directly involved in the mRNA priming reaction, as well as in the RNA transcription reaction in vitro. • , J J J J J