Measles virus (MeV) and all Paramyxoviridae members rely on a complex polymerase machinery to ensure viral transcription and replication. Their polymerase associates the phosphoprotein (P) and the L protein that is endowed with all necessary enzymatic activities. To be processive, the polymerase uses as template a nucleocapsid made of genomic RNA entirely wrapped into a continuous oligomer of the nucleoprotein (N). The polymerase enters the nucleocapsid at the 3'end of the genome where are located the promoters for transcription and replication. Transcription of the six genes occurs sequentially. This implies ending and re-initiating mRNA synthesis at each intergenic region (IGR). We explored here to which extent the binding of the X domain of P (XD) to the C-terminal region of the N protein (N TAIL) is involved in maintaining the P/L complex anchored to the nucleocapsid template during the sequential transcription. Amino acid substitutions introduced in the XD-binding site on N TAIL resulted in a wide range of binding affinities as determined by combining protein complementation assays in E. coli and human cells and isothermal titration calo-rimetry. Molecular dynamics simulations revealed that XD binding to N TAIL involves a complex network of hydrogen bonds, the disruption of which by two individual amino acid substitutions markedly reduced the binding affinity. Using a newly designed, highly sensitive dual-luciferase reporter minigenome assay, the efficiency of re-initiation through the five measles virus IGRs was found to correlate with N TAIL /XD K D. Correlatively, P transcript accumulation rate and F/N transcript ratios from recombinant viruses expressing N variants were also found to correlate with the N TAIL to XD binding strength. Altogether, our data support a key role for XD binding to N TAIL in maintaining proper anchor of the P/L complex thereby ensuring transcription re-initiation at each intergenic region. PLOS Pathogens |