Comparative genomics of nidoviruses: towards understanding the biology and evolution of the largest RNA viruses

The order Nidovirales, including families Coronaviridae and Arteriviridae, is a monophyletic group of highly divergent (+)ssRNA viruses that infect vertebrate and invertebrate hosts; they share conserved genome organization and replication mechanisms. The genome sequence is the only information available about many newly discovered nidoviruses whose number is fast increasing driven by technology advancements. This development makes comparative genomics, an approach that already has been used extensively in nidovirology, increasingly important. In this thesis, diverse methods of comparative genomics were used to address scientific questions about composition and evolution of the nidovirus genome and proteome, and their connection to the biology of nidoviruses. Three studies were conducted in collaboration with experimental researchers, and ranged from the analysis of the highly divergent polyprotein N-terminus in arteriviruses, to identification of the fifth universally conserved domain of nidoviruses, and to characterization of a nidovirus with the largest known RNA genome. The latter study prompted the development of a bioinformatics tool facilitating functional annotation of large multidomain polyproteins. The thesis illustrates how a notion of nidovirus-specific conservation has been steadily refined as a result of recent discoveries.