Aureusvirus, a novel genus in the family Tombusviridae

Abstract

Aureusvirus is a new genus of plant viruses typified by pothos latent virus (PoLV) and comprising cucumber leaf spot virus (CLSV), previously classified as definitive species in the genus Carmovirus. Aureus viruses are soil-borne viruses readily transmitted by sap inoculation to a moderate range of hosts. Natural transmission of CLSV is by the chytrid fungus Olpidium bornovanus, whereas PoLV infects the host without the apparent intervention of a vector. Aureusviruses have isometric particles with size (c. 30 nm) and structure similar to those of the family Tombusviridae, to which the genus belongs. The genome consists of a molecule of single-stranded, positive-sense RNA c. 4.4 kb in size comprising five ORFs. The structural organization (i.e. number and order of genes) is virtually identical to that of members of the genus Tombusvirus. However, the aureusvirus genome has a smaller size and shows distinct differences in the amino acid sequence of some of the ORFs. ORF 1 encodes a 25 kDa product and terminates with a leaky amber codon the readthrough of which results in a 84 kDa protein (ORF 2) with the conserved motifs of RNA dependent RNA polymerase. ORF 3 encodes the coat protein (40-41 kDa), ORF 4 the movement protein (27 kDa), and ORF 5 a 14-17 kDa product responsible for symptom severity. Virions accumulate in great quantity in the cytoplasm, often forming crystalline aggregates, and in bubble-like evaginations of the tonoplast protruding into the vacuole. Replication is likely to occur in the cytoplasm with a stategy based on direct expression of the 5' proximal ORF and expression of downstream ORFs through subgenomic RNAs.