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Mammalian Dicer is the gatekeeper into the essential gene-regulating miRNA pathway. What is committing mammalian Dicer to the miRNA pathway remains unknown. We report that Dicer's highly conserved DExD/H helicase domain is the key structural element supporting accurate miRNA biogenesis. While ATPase activity of the domain is non-essential, its loss is lethal in mice. It is required during canonical miRNA biogenesis for efficient recognition, high-fidelity cleavage, and strand selection. Structure of Dicer-miRNA precursor complexes showed that the DExD/H domain acquired helicase-unrelated function defining Dicer conformations, which affect substrate loading and facilitate pre-selection of miRNA precursors. Dicer lacking the DExD/H domain favors conformations enabling reduced substrate selectivity and supporting RNA interference, a different small RNA pathway. Therefore, Dicer's DExD/H domain ensures indispensable high-fidelity precursor processing of mammalian miRNAs, which constitutes a structural "mold" for adaptive miRNA evolution.

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