PROCESSING OF THE PLUM POX VIRUS POLYPROTEIN AT THE P3-6K(1) JUNCTIONIS NOT REQUIRED FOR VIRUS VIABILITY

Proteolytic processing of the potyvirus polyprotein is mainly performe d by the virus-encoded NIa protease, whose cleavage sites are characte rized by conserved heptapeptide sequences. Partial processing at the c leavage site present between the P3 and 6K(1) cistrons by the plum pox potyvirus (PPV) NIa protease has been previously shown to occur in vi tro. We have now studied the role of polyprotein processing at the P3- 6K(1) 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 processabilit y) are able to infect Nicotiana clevelandii plants, indicating that no rmal processing at the P3-6K(1) junction is not required for virus via bility. However, disease symptoms were not detected and virus accumula tion occurred after a second site mutation was introduced into the 6K( 1) cistron during replication. This additional change did not restore the in vitro processability of the mutant heptapeptide. Changes at oth er positions in the heptapeptide (that only slightly altered the in vi tro processability of this NIa site) were also engineered and it was f ound that these mutations affected the time course and severity of the symptom induction process. A possible regulatory effect on the functi on of the potyvirus P3 + 6K(1) protein by processing at the P3-6K(1) j unction is discussed in light of our present results with PPV.