Sequence analysis of the parsnip yellow fleck virus polyprotein: evidence of affinities with picornaviruses.

The 9.9 kb monopartite ssRNA genome of parsnip yellow fleck virus (PYFV) encodes a polyprotein from which the functional proteins are assumed to arise by proteolytic cleavage. The 22.5K, 26K and 31K particle proteins were mapped in the polyprotein by determining their N-terminal amino acid sequences, and were found to begin at amino acid positions 395, 589 and 811, respectively. There could be polypeptide(s) of up to 43K on the N-terminal side of the particle protein sequences. A region within the 26K particle protein has sequence similarity to the VP3 particle protein of picornaviruses. Three other regions in the PYFV polyprotein have sequence similarity to regions thought to have RNA polymerase, NTP-binding and protease functions in the polyproteins of picornaviruses, comoviruses and nepoviruses. Despite these similarities in sequence and in genome organization to viruses in the picorna-like supergroup, PYFV is distinct from all other plant and animal viruses described. This justifies placing it in a separate plant virus genus for which the name 'sequivirus' has been proposed.