EVOLUTIONARY RELATIONSHIPS IN THE ILARVIRUSES - NUCLEOTIDE-SEQUENCE OF PRUNUS NECROTIC RINGSPOT VIRUS-RNA-3

The complete nucleotide sequence of an isolate of prunus necrotic ring spot virus (PNRSV) RNA 3 has been determined. Elucidation of the amino acid sequence of the proteins encoded by the two large open reading f rames (ORFs) allowed us to carry out comparative and phylogenetic stud ies on the movement (MP) and coat (CP) proteins in the ilarvirus group . Amino acid sequence comparison of the MP revealed a highly conserved basic sequence motif with an amphipathic alpha-helical structure prec eding the conserved motif of the '30K superfamily' proposed by Mushegi an and Koonin [26] for MP's. Within this '30K' motif a strictly conser ved transmembrane domain is present in all ilarviruses sequenced so fa r. At the amino-terminal end, prune dwarf virus (PDV) has an extension not present in other ilarviruses but which is observed in all bromo- and cucumoviruses, suggesting a common ancestor or a recombinational e vent in the Bromoviridae family. Examination of the N-terminus of the CP's of all ilarviruses revealed a highly basic region, part of which resembles the Arg-rich motif that has been characterized in the RNA-bi nding protein family. This motif has also been found in the other memb ers of the Bromoviridae family, suggesting its involvement in a struct ural function. Furthermore this region is required for infectivity in ilarviruses. The similarities found in this Arg-rich motif are discuss ed in terms of this process known as genome activation. Finally, phylo genetic analysis of both the MP and CP proteins revealed a higher rela tionship of A1MV to PNRSV, apple mosaic virus (ApMV) and PDV than any other member of the ilarvirus group. In that sense, A1MV should be con sidered as a true ilarvirus instead of forming a distinct group of vir uses.