Processing of the plum pox virus polyprotein at the P3-6K1 junction is not required for virus viability.

Proteolytic processing of the potyvirus polyprotein is mainly performed by the virus-encoded NIa protease, whose cleavage sites are characterized by conserved heptapeptide sequences. Partial processing at the cleavage site present between the P3 and 6K1 cistrons by the plum pox potyvirus (PPV) NIa protease has been previously shown to occur in vitro. We have now studied the role of polyprotein processing at the P3-6K1 junction in vivo, using a full-length PPV cDNA clone. PPV mutant transcripts containing a histidine for glutamine substitution in the cleavage site sequence (a change that abolishes in vitro processability) are able to infect Nicotiana clevelandii plants, indicating that normal processing at the P3-6K1 junction is not required for virus viability. However, disease symptoms were not detected and virus accumulation occurred after a second site mutation was introduced into the 6K1 cistron during replication. This additional change did not restore the in vitro processability of the mutant heptapeptide. Changes at other positions in the heptapeptide (that only slightly altered the in vitro processability of this NIa site) were also engineered and it was found that these mutations affected the time course and severity of the symptom induction process. A possible regulatory effect on the function of the potyvirus P3 + 6K1 protein by processing at the P3-6K1 junction is discussed in light of our present results with PPV.