RNA interference as a cellular defense mechanism against the DNA virus baculovirus

In insects, the RNA interference (RNAi) pathway plays a major role in antiviral responses, as shown against many RNA viruses. The response includes the cleavage of double-stranded RNA genome or intermediates, produced during replication, into viral short interfering RNAs (v-siRNAs). Using deep sequencing, we found that a large number of small reads of ∼20 nucleotides from Helicoverpa armigera larvae infected with Helicoverpa armigera single nucleopolyhedrovirus (HaSNPV) were mapped to certain open reading frames in the viral genome (hot spots) that are mostly structural and auxiliary late genes. After excluding the possibility of these small RNAs being microRNAs, it was determined that Dicer-2, the main enzyme implicated in the RNAi response in insects, is involved in the generation of v-siRNAs. In Dicer-2- but not Dicer-1-silenced cells, higher transcript levels of the hot spot genes were detected, and as a consequence the virus replicated more efficiently. The results suggest that the viral transcripts are degraded by the RNAi response of the host. This may, however, be to the advantage of the virus by preventing overreplication of the virus, which may otherwise lead to the premature death of the host cells.