The Sendai virus nonstructural C proteins specifically inhibit viral mRNA synthesis.

An in vitro transcription system for paramyxoviruses is described, in which polymerase-free templates are combined with cell extracts containing polymerase made in vivo via transfected plasmids. Both P and L are required for polymerase activity, and both must be coexpressed for optimum activity. mRNA synthesis here was found to be inversely proportional to the level of C expression, whereas defective interfering genome replication was largely unaffected by the level of C in the extract. The inhibition of transcription appeared to be due to the C' and C, but not the Y1 and Y2 proteins, and only occurred when C'/C was coexpressed with P and L. C'/C appears to intervene during polymerase formation, possibly by forming polymerase complexes which are inactive for transcription, but still competent for genome replication.