Pseudo-polyprotein translated from the full-length ORF1 of capillovirus is important for pathogenicity, but a truncated ORF1 protein without variable and CP regions is sufficient for replication

The first open-reading frame (ORF) of the genus Capillovirus encodes an apparently chimeric polyprotein containing conserved regions for replicase (Rep) and coat protein (CP), while other viruses in the family Flexiviridae have separate ORFs encoding these proteins. To investigate the role of the full-length ORF1 polyprotein of capillovirus, we generated truncation mutants of ORF1 of apple stem grooving virus by inserting a termination codon into the variable region located between the putative Rep- and CP-coding regions. These mutants were capable of systemic infection, although their pathogenicity was attenuated. In vitro translation of ORF1 produced both the full-length polyprotein and the smaller Rep protein. The results of in vivo reporter assays suggested that the mechanism of this early termination is a ribosomal −1 frame-shift occurring downstream from the conserved Rep domains. The mechanism of capillovirus gene expression and the very close evolutionary relationship between the genera Capillovirus and Trichovirus are discussed.