The present disclosure provides nucleic acids, expression vectors, host cells for producing recombinant viral RNA-dependent RNA polymerase (RdRp) polypeptides of viruses such as Ebola virus. The present disclosure also provides methods and substrates for assaying activity of a RdRp polypeptide or a RdRp complex. Also provided herein are inhibitors of RdRp polypeptides of viruses such as Ebola virus for use in treating or preventing viral infection.