Limited compatibility between the RNA polymerase components of influenza virus type A and B.

Reassortants between type A and B influenza viruses have not been detected in nature, although both viruses co-circulate in human populations. One explanation for this may be functional incompatibility of RNA transcription and replication between type A and B viruses. To test this possibility, we constructed type A/B mosaic polymerase machinery, containing PB2, PB1, PA and nucleoprotein from each of the two virus types, and assessed their polymerase activities with a type A promoter in a reporter assay. Type B polymerase machinery containing homologous components was functional with the type A promoter albeit to various extents depending on the segments from which the regions downstream of the promoter sequence were derived, indicating functional compatibility between the type A promoter and B polymerase machinery. However, all of the A/B mosaic polymerase machinery, except that containing PA from a type A and the others from a type B virus strain, did not function with the type A promoter, indicating limited compatibility among polymerase components of both types. Taken together, these data suggest that incompatibility among components of the polymerase machinery for RNA transcription and replication alone is not responsible for the lack of heterotypic reassortants.