Dicer functions transcriptionally and posttranscriptionally in a multilayer antiviral defense

Abstract

In antiviral RNA interference (RNAi), Dicer plays a primary role in processing double-stranded RNA (dsRNA) molecules into small-interfering RNAs (siRNAs) that guide Argonaute effectors to posttranscriptional suppression of target viral genes. Here, we show a distinct role for Dicer in the siRNA-independent transcriptional induction of certain host genes upon viral infection in a filamentous fungus. Previous studies have shown that the two key players, dicer-like 2 (dcl2) and argonaute-like 2 (agl2), of antiviral RNAi in a phytopathogenic ascomycete,Cryphonectria parasitica, are highly transcriptionally induced upon infection with certain RNA mycoviruses, including the positive-stranded RNA hypovirus mutant lacking the RNAi suppressor (Cryphonectriahypovirus 1-Δp69, CHV1-Δp69). This induction is regulated by the Spt–Ada–Gcn5 acetyltransferase (SAGA) complex, a well-known transcriptional coactivator. The present study shows that diverse host genes, in addition todcl2andagl2, were up-regulated more than 10-fold by SAGA upon infection with CHV1-Δp69. Interestingly, DCL2, but not AGL2, was essential for SAGA-mediated global gene up-regulation. Moreover, deletion of certain virus-induced genes enhanced a CHV1-Δp69 symptom (growth rate) but not its accumulation. Constitutive, modest levels ofdcl2expression drastically reduced viral siRNA accumulation but were sufficient for full-scale up-regulation of host genes, suggesting that high induction ofdcl2and siRNA production are not essential for the transcriptional up-regulation function of DCL2. These data clearly demonstrate the dual functionality of DCL2: as a dsRNA-specific nuclease in posttranscriptional antiviral RNA silencing and as a key player in SAGA-mediated host gene induction, which independently represses viral replication and alleviates virus-induced symptom expression.

Establishment of Neurospora crassa as a model organism for fungal virology

“…Similarly, Nuss and colleagues showed transcriptional induction of two key RNAi genes, dcl2 and agl2, by dsRNA expression and virus infection in C. parasitica 37,51 . This transcriptional regulation requires DCL2 and SAGA (a universal transcriptional coactivator) 38,39 . Comparison of RNAi regulation between the two fungi reveals interesting conservation and differences.…”

“…Comparison of RNAi regulation between the two fungi reveals interesting conservation and differences. It is likely that dsRNA, regardless of viral or host origin, can trigger transcriptional induction, but not its small RNAs, suggesting that the dicing activity of Dicers is not required for the induction 39 . In contrast, an interesting difference was observed in the degree of induction: dcl2 of C. parasitica was induced more highly than that in N. crassa, i.e.,~40-fold vs.~8-fold, whereas the induction of rrp-3 and its ortholog rdr4 are comparably induced in the two fungi, i.e., 20-30-fold.…”

“…Of four RDRs, two Dicer, and four Argonaute genes, one Dicer ( dcl2 ) and one Argonaute ( agl2 ) gene play major roles in antiviral RNAi 36 , 37 . Of note is that the key genes are transcriptionally upregulated upon virus infection, for which a general transcriptional activator SAGA ( S pt– A da– G cn5 acetyltransferase) complex and DCL2 (positive-feedback player) are essential 38 , 39 .…”

The ORF2 protein of Fusarium graminearum virus 1 suppresses the transcription ofFgDICER2andFgAGO1to limit host antiviral defences

“…it significantly represses the transcriptional induction of dcl2 and agl2 transcripts in C. parasitica (Sun et al , ). Another study recently demonstrated that p29 might block the SAGA‐mediated up‐regulation of host genes (Andika et al , ). The mechanism of silencing suppression seems to be quite different for the FgV1 pORF2 in F. graminearum than for CHV1 p29 in C. parasitica .…”

“…Andika et al () recently demonstrated that the Spt–Ada–Gcn5 acetyltransferase (SAGA) complex is involved in the transcriptional activation of agl2 and dcl2 for RNA‐interference (RNAi) activation in C. parasitica . DCL2 might have critical roles for SAGA‐mediated host gene up‐regulation as well as for cleavage of virus‐derived dsRNA molecules into virus‐derived siRNA (Andika et al , ). Although the S10 protein encoded by Rosellinia necatrix mycoreovirus 3 (RnMyRV3) has the ability to suppress RNA silencing, it might not repress the up‐regulation of the RNA silencing‐related genes in Rosellinia necatrix but may instead repress another RNA‐silencing step (Yaegashi et al , , ).…”