Molecular cloning, DNA sequence analysis, and expression of cDNA sequence of RNA genomic segment 6 (S6) that encodes a viral outer capsid protein of threadfin aquareovirus (TFV).

The genome segment 6 (S6) of threadfin reovirus (TFV) was cloned and sequenced. The entire S6 nucleotide sequence is 2056 bp long with an open reading frame that encodes a protein of 653 amino acids. Sequence analysis of the TFV S6 genome revealed that the 5'-terminal sequence, GTTTTA and the 3'-terminal sequence, ATTCATC of the plus strand is common to other genome segments of TFV. The pentanucleotide, TCATC, at the 3'-terminal of the plus strand was also conserved in other reported isolates of Aquareovirus such as chum salmon reovirus (CSV), striped bass reovirus (SBR), grass carp reovirus (GCRV) and golden shiner reovirus (GSV) as well as to the 10 genome segments of mammalian reovirus (MRV). Blast results indicated that the TFV S6 gene segment sequence had high identity towards the CSV S6 gene sequence, which codes for the CSV outer coat protein. This implied that the TFV S6 gene segment codes for an outer capsid protein (OCP) of the virus. Amino acid sequence analysis of this TFV OCP sequence revealed the presence of a putative conserved asparagine-proline (Asn-Pro) protease cleavage site, which was found in all reported isolates of Aquareovirus as well as in the MRV mu1 protein. N-terminal sequencing of the corresponding S6 native protein obtained from purified TFV particles verified the presence of this cleavage site. Phylogenetic analysis of the TFV S6 protein revealed that TFV was closely related to CSV, from Aquareovirus species, ARV-A. Cloning of the TFV S6 gene sequence into an Escherichia coli expression host produced a recombinant protein that corresponded to the predicated size of the OCP of TFV. Immunization of mice using this recombinant outer capsid protein (rOCP) revealed that the protein was able to elicit an antibody response, thus indicating that the rOCP of TFV was immunogenic.