We indicated that the PB1 and PA subunits of RNA polymerase and nucleoprotein (NP) can support replication of the influenza virus genome as well as transcription to yield uncapped poly(A)+-RNA (Y. Nakagawa, N. Kimura, T. Toyoda, K. Mizumoto, A. Ishihama, K. Oda, and S. Nakada, J. Virol. 69:728-733, 1995). To analyze the functions of the PB1 and PA subunits in replication and transcription, YP1N clones in which the PB1 and NP genes can be expressed in response to dexamethasone were established. cRNA was transcribed from model viral RNA (vRNA), but vRNA synthesis from model cRNA was not detected in YP1N clones. Furthermore, poly(A)+-RNA directed from model vRNA was synthesized in YP1N clones. These results indicated that PB1 and NP can support the syntheses of cRNA and poly(A)+- RNA and that the PA subunit, in addition to that of PB1 and to NP, is required for vRNA synthesis. In summary, the PB1 subunit is involved in the catalytic activities of nucleotide elongation, and the PA subunit may act as an allosteric modulator and cause a conformational change from a cRNA- to a vRNA-synthesizing form of the PB1 subunit.