Viruses have often evolved overlapping reading frames in order to maximise their coding capacity. Until recently, the segmented double-stranded (ds) RNA genome of viruses of the Orbivirus genus was thought to be monocistronic but the identification of the bluetongue virus (BTV) NS4 protein changed this assumption. A small open reading frame (ORF) in segment 10, overlapping the NS3 ORF in the +1 position that is maintained in more than 300 strains of the 26 different BTV serotypes and in more of 200 strains of the phylogenetically related African horse sickness (AHSV). In BTV, this ORF (named S10-ORF2 in this study) encodes a putative protein of 50-59 amino acid residues in length and appears to be under a stron... More
Viruses have often evolved overlapping reading frames in order to maximise their coding capacity. Until recently, the segmented double-stranded (ds) RNA genome of viruses of the Orbivirus genus was thought to be monocistronic but the identification of the bluetongue virus (BTV) NS4 protein changed this assumption. A small open reading frame (ORF) in segment 10, overlapping the NS3 ORF in the +1 position that is maintained in more than 300 strains of the 26 different BTV serotypes and in more of 200 strains of the phylogenetically related African horse sickness (AHSV). In BTV, this ORF (named S10-ORF2 in this study) encodes a putative protein of 50-59 amino acid residues in length and appears to be under a strong positive selection. HA- or GFP-tagged versions of S10-ORF2 expressed from transfected plasmids localised within the nucleoli of transfected cells unless a putative nucleolar localisation signal was mutated S10-ORF2 inhibited gene expression, but not RNA translation, in transient transfection reporter assays. In both mammalian and insect cells, BTV S10-ORF2 deletion mutants (BTV8ΔS10-ORF2) displayed similar replication kinetics to wild type virus. In vivo, S10-ORF2 deletion mutants were pathogenic in mouse models of disease. Although further evidence is required for S10-ORF2 expression during infection, the data presented provide an initial characterisation of this open reading frame.
