GENETIC-VARIATION OF THE POLIOVIRUS GENOME WITH 2 VPG CODING UNITS

Amongst the picornaviruses, poliovirus encodes a single copy of the ge nome-linked protein, VPg, whereas foot-and-mouth disease virus uniquel y encodes three copies of VPg. We have previously shown that a genetic ally engineered poliovirus genome containing two tandemly arranged VPg s is quasi-infectious (qi) that, upon genome replication, inadvertentl y deleted one complete VPg sequence. Using two genetically marked vira l genomes with two VPg sequences, we now provide evidence that this de letion occurs via homologous recombination. The mechanism was abrogate d when the second VPg was engineered such that its nucleotide sequence differed from that of the first VPg sequence by 36%, Such genomes als o expressed a qi phenotype, but progeny viruses resulted from (i) rand om deletions yielding single VPg coding sequences of varying length la cking the QG cleavage site between the VPgs and (ii) mutations in the AKVQG cleavage sites between the VPgs at either the P-4, P-1 or P-1' position, These variants present a unique genetic system defining the cleavage signals recognized in 3C(pro)-catalyzed proteolysis, We prop ose a recognition event in the cis cleavages of the polyprotein P2-P3 region, and we present a hypothesis why the poliovirus genome does not tolerate two tandemly arranged VPg sequences.