Proteolytic processing of the primary translation products of cowpea mosaic virus RNAs

Cowpea mosaic virus (CPMV) is the type member of a group of plant viruses, the comoviruses, with a genome consisting of two single stranded RNA molecules separately encapsidated in icosahedral particles. A characteristic feature of the two genome RNAs is that they are both polyadenylated at their 3'-terminus and supplied with a small protein at their 5'end. The genetic information encoded in the virus RNAs is expressed by translation of each RNA into large-sized proteins referred to as polyproteins because these primary translation products are subsequently cleaved by specific proteolytic cleavages ("proteolytic processing") into a number of smaller-sized proteins, each with a specific function during virus multiplication. The research reported in this thesis deals with the identification of the proteolytic activities involved in this processing and their specificity.We have been able to demonstrate that the larger of the two virus RNAs, which contains the information necessary for virus RNA replication, also encodes two different proteolytic activities. One proteolytic activity is responsible for the cleavage of the overlapping polyproteins produced by the smaller of two virus RNAs and releases the two capsid proteins, encoded by this RNA (Chapter III and V), whereas the other proteolytic activity achieves the processing of the polyprotein produced by the larger RNA (Chapter VII). Besides this functional difference the two proteolytic activities recognise peptide bounds between different specific amino acid pairs (Chapter VI and VIII). The results of our studies have led to a detailed model for the processing of the proteins encoded on the two CM RNAs.The striking analogy between the plant comoviruses and the animal picornaviruses, like poliovirus and foot-and-mouth- disease virus, with regard to genome structure, replication, expression strategy and functional organisation of genes has prompted us to study the homology in amino acid sequences between corresponding proteins of the two groups of virus. It was found that some of the non-structural proteins of CM and the picornaviruses exhibit significant homology in amino acid sequence (Chapter VIII). These results suggest that animal picornaviruses and plant comoviruses have a common ancestor and throw a light on the evolution of RNA viruses