All RNA viruses encode a polymerase that catalyzes both replication and transcription of the viral genome. For many RNA viruses, replication and transcription are two distinct processes with different initiation and termination mechanisms. Furthermore, the rates of mRNA and viral genomic RNA synthesis often peak at different times during infection. Our laboratory is interested in the catalytic mechanisms of viral RNA polymerases and in how RNA viruses exert regulation over RNA synthesis through polymerase interactions with viral and cellular factors.