Evaluation of Nucleocapsid and Phosphoprotein P Functionality as Critical Factors During the Early Phase of Paramyxoviral Infection

In the beginning of a paramyxovirus infection after cell entry viral survival depends on efficient primary (1°) transcription and on the stability of only one input nucleocapsid. Here we examined the influence of the viral polymerase co-factor phosphoprotein P on the very early phase of an infection, i.e. before progeny nucleocapsids are synthesized. We used a novel set-up with Sendai virus (SeV) mutants incapable of genome replication: SeV-ΔP with the entire P ORF deleted, SeV-PΔ2-77 with the deletion of aa 2-77. These mutants allow maintaining the state of the very beginning of an infection when statistically one viral genome is present in the cell. This single genome serves as template for transcription. During SeV-ΔP infections only early 1° transcription takes place at low levels. However, when the truncated P protein is expressed in SeV-PΔ2-77 infections, 1° transcription levels rise significantly up to an 8-fold increased amount of viral mRNA. This shows that the P protein is able to support transcription and thereby mediates the transition from early to late 1° transcription. Importantly, nucleocapsids of both mutants could be shown to remain stable and functional for at least 5 days – even without de novo P protein synthesis. These results describe a novel function of the P protein: enhancing viral gene expression even before genome replication has started. Thus, the since long postulated supportive function of the P protein is not related to stabilization of the nucleocapsid but rather enhances the processivity of the viral polymerase during late 1° and secondary (2°) transcription and genome replication.