The Making of a Picornavirus Genome

Picornavirus genomes have a unique structure, and they developed mechanisms of gene expression different from those of their prokaryotic counterparts. Genetic analyses have played a crucial role in deciphering the genome function of picornaviruses. One of the hallmarks in RNA virus research was the discovery of genetic recombination, an accomplishment received with great skepticism for several years. Cell-free synthesis, which duplicates essential steps of viral proliferation in the living cell, opened new strategies for studying individual steps of picornavirus replication in the absence of cell membrane barriers. In poliovirus (PV), the cloverleaf (CL) is followed by a short spacer of 24 nt that does not appear to engage in any base-pairing with adjacent nucleotides, as deduced from a detailed study with coxsackievirus B3 (CVB3). In cardioviruses and aphthoviruses, the genome segment preceding the internal ribosome entry site (IRES) differs completely from that of the enteroviruses. There is no CL; instead, a succession of elaborate RNA structures is followed by a long stretch of poly(C) that in encephalomyocarditis virus (EMCV) can exceed 600 nt (and plays a role in mouse pathogenesis). The most interesting trait of the poly proteins is that they contain the information for cis-cleavages, either as self-processing oligopeptide sequences (in aphtho- and cardioviruses) and/or as proteinases that are able to clip the polypeptide chain at their own N termini. Genetic complementation has been firmly established in picornavirus replication.