RNase III Dicer produces small RNAs guiding sequence-specific regulations, with important biological roles in eukaryotes. Major Dicer-dependent mechanisms are RNA interference (RNAi) and microRNA (miRNA) pathways, which employ distinct types of small RNAs. Small interfering RNAs (siRNAs) for RNAi are produced by Dicer from long double-stranded RNA (dsRNA) as a pool of different small RNAs. In contrast, miRNAs have specific sequences because they are precisely cleaved out from small hairpin precursors. Some Dicer homologs efficiently generate both, siRNAs and miRNAs, while others are adapted for biogenesis of one small RNA type. Here, we review the wealth of recent structural analyses of animal and plant Dicers, which have revealed how different domains and their adaptations contribute to substrate recognition and cleavage in different organisms and pathways. These data imply that siRNA generation was Dicer's ancestral role and that miRNA biogenesis relies on derived features. While the key element of functional divergence is a RIG-I-like helicase domain, Dicer-mediated small RNA biogenesis also documents the impressive functional versatility of the dsRNA-binding domain.

• Keywords
• Dicer, dsRBD, helicase, miRNA, siRNA,
• MeSH
• RNA, Double-Stranded genetics   MeSH
• RNA, Small Interfering genetics   metabolism   MeSH
• MicroRNAs * genetics   metabolism   MeSH
• Ribonuclease III * genetics   MeSH
• RNA Interference MeSH
• Animals MeSH
• Check Tag
• Animals MeSH
• Publication type
• Journal Article MeSH
• Review MeSH
• Names of Substances
• RNA, Double-Stranded MeSH
• RNA, Small Interfering MeSH
• MicroRNAs * MeSH
• Ribonuclease III * MeSH

MicroRNA (miRNA) and RNA interference (RNAi) pathways rely on small RNAs produced by Dicer endonucleases. Mammalian Dicer primarily supports the essential gene-regulating miRNA pathway, but how it is specifically adapted to miRNA biogenesis is unknown. We show that the adaptation entails a unique structural role of Dicer's DExD/H helicase domain. Although mice tolerate loss of its putative ATPase function, the complete absence of the domain is lethal because it assures high-fidelity miRNA biogenesis. Structures of murine Dicer•-miRNA precursor complexes revealed that the DExD/H domain has a helicase-unrelated structural function. It locks Dicer in a closed state, which facilitates miRNA precursor selection. Transition to a cleavage-competent open state is stimulated by Dicer-binding protein TARBP2. Absence of the DExD/H domain or its mutations unlocks the closed state, reduces substrate selectivity, and activates RNAi. Thus, the DExD/H domain structurally contributes to mammalian miRNA biogenesis and underlies mechanistical partitioning of miRNA and RNAi pathways.

• Keywords
• DExD, Dicer, PKR, RNAi, TARBP2, cryo-EM, dsRBD, dsRNA, helicase, miRNA, mirtron,
• MeSH
• MicroRNAs * genetics   metabolism   MeSH
• Mice MeSH
• Ribonuclease III * metabolism   MeSH
• RNA Interference MeSH
• Mammals metabolism   MeSH
• Carrier Proteins metabolism   MeSH
• Animals MeSH
• Check Tag
• Mice MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Comment MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• MicroRNAs * MeSH
• Ribonuclease III * MeSH
• Carrier Proteins MeSH