TRICHOVIRUS, A NEW GENUS OF PLANT-VIRUSES

The genus Trichovirus embraces five viral species (two definitive and three tentative) with similar biological, morphological, physicochemic al, and ultrastructural properties. Vital replication is likely to occ ur in the cytoplasm, where virions accumulate in more or less loose bu ndles or paracrystalline aggregates. The genome is a 3' polyadenylated , positive-sense, single stranded RNA of 7.5-8.7 kb in size. In defini tive species (apple chlorotic leaf spot and potato T viruses), the gen ome is constructed of three slightly overlapping open reading frames c oding for replication-related proteins (ORF 1), a putative movement pr otein (ORF 2), and the coat protein (ORF 3), respectively. ORFs 2 and 3 are probably expressed through subgenomic RNAs. Grapevine virus A (G VA) and grapevine virus B (GVB), two tentative species, may express an extra small open reading frame at the 3' terminus, encoding, in the c ase of GVB, a polypeptide with homologies with the small RNA-binding p rotein of carlaviruses. The taxonomic relevance of this difference in genome organization remains to be ascertained.