RNase III proteins play vital roles in processing of several types of
RNA molecules and gene silencing. Recently, it has been discovered that some plant and animal viruses encode
RNase III-like
proteins as well. Genome sequencing of four virus species belonging to the Ascoviridae family has revealed sequence conservation of an
RNase III open reading frame among the viruses. These have not been explored in ascoviruses, and therefore their role in host-virus interaction is unknown. Here, we confirmed expression of Heliothis virescens ascovirus (HvAV-3e) open reading frame 27 (orf27) that encodes an
RNase III-like
protein after
infection and demonstrated dsRNA specific
endoribonuclease activity of the encoded
protein. Analysis of the expression patterns of orf27 in virus-infected insect cells and a bacterial expression system revealed autoregulation of this
protein over time. Moreover, HvAV-3e
RNase III was found essential for virus DNA replication and
infection using RNA interference (RNAi)-mediated gene silencing. In addition, using
green fluorescent protein gene as a marker, we provide evidence that
RNase III is involved in the suppression of gene silencing. To our knowledge, this is the first insect virus-encoded
RNase III described and shown to suppress host cell RNAi defense mechanism.