Identification of a potential transcriptional regulator encoded by grass carp reovirus

Many transcription factors are encoded by DNA viruses and retroviruses due to their regulatory roles in gene expression in the host cell. However, no transcriptional regulator has been identified in any reovirus. Here, a non-structural protein, NS31, encoded by grass carp reovirus genomic segment S7 was characterized. The NS31 protein is predicted to contain a helix-turn-helix (HTH)-like domain and a C-terminal acidic α-helix motif. In yeast, a fusion protein composed of the Gal4-BD domain and NS31 (BD-NS31) was able to activate the expression of reporter genes (Gal1/MEL1 promoter) without the Gal4-AD domain. We also found that NS31 activated the reporter genes in a BD-dependent manner, and both the C- and N-termini contribute to the activation function of NS31. Furthermore, NS31 homologues from other aquareoviruses were also shown to possess a similar transcriptional activation function in yeast. Thus, the aquareovirus NS31 protein appears to act as a transcriptional regulatory protein, the first one identified in a member of the family Reoviridae.