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

Abstract

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.

Fig. 1

Activity of type B polymerase machinery with a type A promoter (non-coding region [NCR] derived from A/WSN segment). Polymerase activities are quantified by use of a SEAP assay of the supernatants of cells transfected with (A) B/Lee-, (B) B/Panama-, or (C) control A/WSN-derived plasmids. The SEAP units are expressed as absorbance values (optical density [OD₄₀₅]) and are reported as means ± standard deviation (n=3).

Fig. 2

Activity of type A/B mosaic polymerase machinery with a type A HA segment promoter. Combinations of type A and B components are shown at the bottom. Polymerase activities are quantified by use of a SEAP assay with B/Lee-, or B/Panama-derived components as described in the legend to Fig. 1. As controls, SEAP values for homologous type A (lane 1) and type B (lane 16) polymerase machinery are shown.

Fig. 3

Activity of type A/B mosaic polymerase machinery, containing type A PA and the other components from type B, with a type A HA segment promoters Combinations of type B components from B/Panama or B/Lee are shown at the bottom. Polymerase activities are quantified by use of a SEAP assay as described in the legend to Fig. 1. As controls, SEAP values for homologous type A (lane 1) and type B (lane 2 for B/Panama and lane 3 for B/Lee) polymerase machinery are shown.