Abstract
BackgroundThe orthoreoviruses are infectious agents that possess a genome comprised of 10 double-stranded RNA segments encased in two concentric protein capsids. Like virtually all RNA viruses, an RNA-dependent RNA polymerase (RdRp) enzyme is required for viral propagation. RdRp sequences have been determined for the prototype mammalian orthoreoviruses and for several other closely-related reoviruses, including aquareoviruses, but have not yet been reported for any avian orthoreoviruses.ResultsWe determined the L2 genome segment nucleotide sequences, which encode the RdRp proteins, of two different avian reoviruses, strains ARV138 and ARV176 in order to define conserved and variable regions within reovirus RdRp proteins and to better delineate structure/function of this important enzyme. The ARV138 L2 genome segment was 3829 base pairs long, whereas the ARV176 L2 segment was 3830 nucleotides long. Both segments were predicted to encode λB RdRp proteins 1259 amino acids in length. Alignments of these newly-determined ARV genome segments, and their corresponding proteins, were performed with all currently available homologous mammalian reovirus (MRV) and aquareovirus (AqRV) genome segment and protein sequences. There was ~55% amino acid identity between ARV λB and MRV λ3 proteins, making the RdRp protein the most highly conserved of currently known orthoreovirus proteins, and there was ~28% identity between ARV λB and homologous MRV and AqRV RdRp proteins. Predictive structure/function mapping of identical and conserved residues within the known MRV λ3 atomic structure indicated most identical amino acids and conservative substitutions were located near and within predicted catalytic domains and lining RdRp channels, whereas non-identical amino acids were generally located on the molecule's surfaces.ConclusionThe ARV λB and MRV λ3 proteins showed the highest ARV:MRV identity values (~55%) amongst all currently known ARV and MRV proteins. This implies significant evolutionary constraints are placed on dsRNA RdRp molecules, particularly in regions comprising the canonical polymerase motifs and residues thought to interact directly with template and nascent mRNA. This may point the way to improved design of anti-viral agents specifically targeting this enzyme.
Highlights
The orthoreoviruses are infectious agents that possess a genome comprised of 10 double-stranded RNA segments encased in two concentric protein capsids
The sequences of genes that encode the RNA-dependent RNA polymerase (RdRp) protein have been determined for a number of members of the Reoviridae family of viruses (Table 1)
BLAST searches indicated the avian reoviruses (ARVs) L2 genome segments were most similar to the mammalian reovirus (MRV) and aquareovirus (AqRV) L1 genome segments, which encode the RNA-dependent RNA polymerase [54,55]
Summary
The orthoreoviruses are infectious agents that possess a genome comprised of 10 double-stranded RNA segments encased in two concentric protein capsids. The ARVs are the prototypic members of syncytia-inducing, non-enveloped viruses within the Orthoreovirus genus. This genus is divided into 3 subgroups: non-syncytiainducing mammalian reovirus (MRV; subgroup 1; the prototype of the whole genus), avian reovirus and Nelson Bay virus (subgroup 2), and baboon reovirus (subgroup 3) [3]. Like MRV, ARV is a non-enveloped virus with 10 linear double-stranded RNA gene segments surrounded by a double concentric icosahedral capsid shell (inner shell [ called core] and outer shell) of 70–80 nm diameter [11,12]. The ARV genomic segments can be resolved into three size classes based on their electrophoretic mobilities, designated L (large), M (medium), and S (small) [11,12]. ARVs share many features with the prototypic MRVs, several notable differences exist including host range, pathogenicity, hemagglutination properties, and syncytium formation [11,12,16,17,18,19,20,21]
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