Molecular characterization of plum pox potyvirus.

In recent years, our knowledge of the molecular biology of plum pox potyvirus (PPV) and of potyviruses in general has greatly increased. The genome of PPV consists of a unique single-strand RNA molecule with messenger polarity. A terminal protein (VPg) and a poly-(A) tail are present at the 5’and 3’ends, respectively, of the genomic RNA. Four complete and other partial nucleotide sequences of PPV isolates have been reported. According to the levels of homology three different PPV strains can be defined. PPV RNA is translated into a unique polyprotein starting at the second AUG codon of the large open-reading frame that covers most of the genome. The polyprotein is proteolytically processed by three virus encoded proteases (P1Pro, HCPro and NIaPro). While P1Pro and HCPro cut at their own carboxyl termini, NIaPro recognizes seven cleavage sites characterized by conserved heptapeptides. These sites differ in their susceptibility to cis and trans processing and in their reaction profiles. Immunoelectron microscopy has shown NIa and NIb proteins to form nuclear inclusions, but also dense aggregates in the cytoplasm, and CI protein to make typical pinwheel cytoplasmic inclusions. On the basis of sequence comparisons, NIb protein has been proposed to be the viral RNA replicase. CI protein has been purified from infected leaves and from Escherichia coli harbouring a plasmid that encodes it. It has nucleic acid-stimulated NTPase and RNA helicase activities. Virus infection has been achieved by inoculation with a PPV full-length cDNA clone and with transcripts synthesized using another one as template. These clones can be used to apply genetic engineering techniques to the in vitro manipulation of the PPV genome.