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Direct targeted degradation of transposon RNAs by the non-canonical RNAi pathway of the fungus Mucor lusitanicus

ABSTRACT: Mucor lusitanicus has emerged as a model organism for studying RNAi in early-diverging fungi. This fungus exhibits intricate RNAi pathways that play crucial roles in regulating gene expression, de-stroying invasive exogenous genetic material, and controlling the movement of transposable ele-ments (TEs) to ensure genome stability. One of the most fascinating RNAi pathways of this fungus is the non-canonical RNAi pathway (NCRIP), which is independent of Dicer and Argonaute proteins and uses the atypical RNase III R3B2 to degrade specific target mRNAs, playing an essential role in genome stability and virulence. Despite accumulating data suggesting that this pathway is a deg-radation mechanism, there has been no conclusive evidence. Here, we conducted a comparative transcriptomic analysis of mRNA and small RNAs regulated by r3b2, identifying 35 direct NCRIP targets. Most of these direct NCRIP targets correspond to TEs, highlighting the significant role of this RNAi pathway in TE control. Detailed functional analysis of the NCRIP targets confirms the crucial role on r3b2 in regulating gene expression of protein-coding genes and controlling TEs other than centromeric GremLINE1 transposons, emphasizing the important role of r3b2 in genome stability. Interestingly, the RNAs of the NCRIP targets harbor a unique motif consisting of CAG repeats which are known to form hairpin structures which are targeted by RNA interference. Additionally, the generation of transformants expressing mRNAs containing the luciferase reporter gene along direct NCRIP targets reveals that this RNAi pathway is a true degradation mechanism for specific mRNAs. These results are expected to contribute to the understanding of the regulation of the NCRIP pathway through the analysis of its direct targets identified here.

ORGANISM(S): Mucor lusitanicus

PROVIDER: GSE288970 | GEO | 2025/03/28

REPOSITORIES: GEO

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Project description:Regulation of mRNA stability is a critical step in the control of gene expression. The nonsense mediated mRNA decay (NMD) pathway is one important mRNA stability pathway and has been shown to function in regulating a significant portion of the transcriptome. Perhaps the most significant outstanding question surrounding NMD regulation of endogenous gene expression is the identification of transcripts directly targeted by the NMD pathway versus secondary response genes, whose expression levels are dependent on the activity of direct targets. Here, we present for the first time in an intact animal, the experimental identification of direct targets of NMD. We find that most genes (74%) that are upregulated in NMD mutants appear not to be direct targets of NMD. More surprisingly, we find that the vast majority (97%) of candidate direct targets are not found to be increased in expression in a NMD mutant background. These results imply negative feedback renormalizes the levels of most direct targets of NMD, suggesting they would have been missed in previous analyses. We find that our candidate direct target genes have disproportionately long 3’ UTRs compared to non-targeted control genes. Long 3’ UTRs have been shown to target mRNAs for NMD in experimental systems, and our results imply this is the case in vivo. Our results will allow for the understanding of the function of the NMD pathway in endogenous gene regulation, during normal development, as well as in pathological states. Starting with a Nonsense Mediated Decay(NMD) mutant, in which expression levels of both primary and secondary NMD targets should be increased, we introduce a wild-type copy of the mutated factor and evaluated the transcriptome profile change overtime. Following reintroduction, expression levels of transcripts that are direct targets of NMD should decrease rapidly. Conversely, the levels of indirect targets will remain high until direct targets return to wild-type levels, which will occur later in the timecourse. This allows us to identify direct NMD targets.

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Direct targeted degradation of transposon RNAs by the non-canonical RNAi pathway of the fungus Mucor lusitanicus

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