The internal proteins, nucleocaspid protein (NP), phosphoprotein (P) and large polymerase protein (L) of Newcastle disease virus (NDV), play an important role in transcription and replication of the viral genome. However, their role in NDV pathogenesis has not been explored. In this study, the importance of internal proteins in NDV virulence was evaluated through a chimeric approach using an established reverse genetics technique. The L gene between an avirulent NDV strain LaSota and a moderately virulent NDV strain Beaudette C (BC) was exchanged, recombinant chimeric viruses were recovered and studied for their pathogenicity in the natural host, chicken. The results obtained from in vivo studies indicated that the L gene of NDV modulate role in NDV virulence in chickens.

The NP and P genes of NDV were exchanged between BC and LaSota individually as well as in combination; chimeric viruses were recovered, indicating that heterologous NP and P genes were functional. In vitro replication of chimeric NP and P recombinant viruses in DF-1 cells indicated that the exchange of NP or P gene in NDV did not affect the replication of the chimeric viruses. The in vivo studies in chickens showed that the change in pathogenicity of these chimeric viruses was minimal and homotypic interaction between NP and P proteins is necessary for optimum pathogenicity of the virus.