In vitro molecular genetics as a tool for determining the differential cleavage specificities of the poliovirus 3C proteinase

We describe a completely in vitro system for generating defined poliovirus proteinase mutations and subsequently assaying the phenotypic expression of such mutations. A complete cDNA copy of the entire poliovirus genome has been inserted into a bacteriophage T7 transcription vector. We have introduced proteinase and/or cleavage site mutations into this cDNA. Mutant RNA is transcribed from the altered cDNA template and is subsequently translated in vitro. Employing such a system, we provide direct evidence for the bimolecular cleavage events carried out by the 3C proteinase. We show that specific genetically-altered precursor polypeptides containing authentic Q-G cleavage sites will not act as substrates for 3C either in cis or in trans. We also provide evidence that almost the entire P3 region is required to generate 3C proteinase activity capable of cleaving the P1 precursor to capsid proteins. However, only the 3C portion of P3 is required to generate 3C proteinase activity capable of cleaving P2 and its processing products.