A novel eukaryotic RdRP-dependent small RNA pathway represses antiviral immunity by controlling an ERK pathway component in the black-legged tick

Abstract

Small regulatory RNAs (sRNAs) are involved in anti-viral defense and gene regulation. Although RNA-dependent RNA Polymerases (RdRPs) are known to produce sRNA in nematodes, plants and fungi, whether they play roles in sRNA biogenesis in other animals remains controversial. In this study, we study sRNAs in the ISE6 cell line, which is derived from the black-legged tick, an important vector of human and animal pathogens. We identify abundant classes of ~22nt sRNAs that require specific combinations of RdRPs and sRNA effector proteins (Argonautes or AGOs). RdRP-dependent sRNAs are mainly derived from sense and antisense strands of RNA polymerase III-transcribed genes and repetitive elements. Unlike C. elegans sRNA pathways, 5′-tri-phosphorylated sRNAs are not detected, suggesting that the tick pathways are distinct from the pathways known in worms. Knockdown of one of the RdRPs unexpectedly results in downregulation of a subset of viral transcripts, in contrast to their upregulation by AGO knockdown. Furthermore, we show that knockdown of AGO/RdRP causes misregulation of protein-coding genes including RNAi-related genes, suggesting feedback regulation. Luciferase assays demonstrate that one of the RdRP-regulated genes, the MEK1 ortholog IscDsor1 is regulated through its 3′UTR, where a putative sRNA target site resides. These results provide evidence that arachnid RdRPs are important sRNA biogenesis factors, and the discovery of novel pathways underscores the importance of characterizing sRNA biogenesis in various organisms to understand virus-vector interactions and to exploit RNAi for pest control.