MicroRNA biogenesis is the multistep process by which cells produce tiny regulatory RNA molecules, roughly 22 nucleotides long, that silence genes after they have been transcribed. The pathway begins in the nucleus with a long RNA transcript and ends in the cytoplasm with a single mature strand loaded onto a protein that guides it to its target. Each step involves dedicated molecular machinery, and disruption at any point can alter which genes get dialed up or down, with consequences ranging from failed embryonic development to cancer. The process is more regulated and more varied than early descriptions suggested.
From Gene to Primary Transcript
MicroRNA genes are scattered throughout the genome. Some sit between protein-coding genes, while others are embedded within introns of host genes. Regardless of location, most miRNA genes are transcribed by the same enzyme that copies protein-coding genes: RNA polymerase II. Early work showed that the initial transcripts, called primary miRNAs (pri-miRNAs), carry a cap on one end and a poly-A tail on the other, hallmarks of polymerase II activity. When researchers treated human cells with alpha-amanitin at a concentration that selectively blocks polymerase II, pri-miRNA levels dropped, and chromatin immunoprecipitation confirmed the enzyme sitting on a miRNA promoter.1Nature / EMBO Journal. MicroRNA genes are transcribed by RNA polymerase II This means miRNA production is subject to the same transcriptional controls as protein-coding genes: transcription factors, enhancers, epigenetic marks, and signaling pathways all influence how much pri-miRNA a cell makes at any given time.
Nuclear Cropping by the Microprocessor
Pri-miRNAs can be thousands of nucleotides long, but buried within them is a hairpin structure roughly 65 nucleotides in size. Extracting that hairpin is the job of a nuclear protein complex called the Microprocessor, which consists of the enzyme Drosha and its partner DGCR8. Drosha is an RNase III enzyme, meaning it cuts double-stranded RNA. DGCR8 contains two double-stranded RNA-binding domains and helps Drosha recognize and position itself on the correct hairpin.2PubMed Central. The Drosha-DGCR8 complex in primary microRNA processing The Microprocessor clips the pri-miRNA at a precise spot, releasing three fragments: a flanking piece from the 5′ side, the hairpin itself (now called a precursor miRNA, or pre-miRNA), and a flanking piece from the 3′ side.3Cell. The Primary MicroRNA Processing Complex Is Mediated by a Drosha-DGCR8 Complex The flanking fragments are degraded, while the pre-miRNA hairpin moves on to the next stage.
Getting Out of the Nucleus
The pre-miRNA hairpin needs to reach the cytoplasm, and it gets there via a dedicated shuttle protein called Exportin-5. This transport depends on a small signaling molecule, RanGTP, which acts as a molecular switch. When RanGTP is present, Exportin-5 grabs the pre-miRNA and carries it through nuclear pores into the cytoplasm.4PubMed Central. Exportin 5 is a RanGTP-dependent dsRNA-binding protein that mediates nuclear export of pre-miRNAs Depleting RanGTP or knocking down Exportin-5 stalls miRNA function, confirming that this is a bottleneck step.5PubMed Central. Exportin-5 mediates the nuclear export of pre-microRNAs and short hairpin RNAs Structural studies have visualized the export complex at near-atomic resolution, revealing how Exportin-5 cradles the double-stranded stem of the pre-miRNA and shields it from degradation during transit.6PubMed. A high-resolution structure of the pre-microRNA nuclear export machinery Once the complex reaches the cytoplasm, GTP is hydrolyzed to GDP, the grip loosens, and the pre-miRNA is released for the next round of processing.
Cytoplasmic Dicing
In the cytoplasm, a second RNase III enzyme called Dicer takes over. Dicer measures out roughly 22 nucleotides from the end of the pre-miRNA hairpin and cuts off the loop, converting the hairpin into a short double-stranded RNA duplex.7PubMed Central. A comprehensive analysis of precursor microRNA cleavage by human Dicer Dicer does not work alone. It associates with double-stranded RNA-binding proteins that help position the pre-miRNA and influence how accurately the cut is made.8PubMed Central. Modulation of MicroRNA Processing by Dicer via Its Associated dsRNA Binding Proteins The product of this cleavage is a small RNA duplex with two strands: one will become the mature miRNA, and the other, sometimes called the passenger strand, is usually degraded.
Strand Selection and RISC Assembly
Of the two strands in the duplex, only one is typically loaded into an Argonaute protein to form the RNA-induced silencing complex (RISC). This step is called strand selection, and it determines which strand becomes the functional guide.9PubMed Central. microRNA strand selection: Unwinding the rules The choice is not random. Thermodynamic asymmetry at the ends of the duplex plays a major role: the strand whose 5′ end sits at the less stably paired end of the duplex is preferentially retained. In humans, the RISC-loading complex containing Dicer, an RNA-binding protein called TRBP, and Argonaute 2 (Ago2) handles the dicing, strand selection, and loading in a coordinated fashion.10PubMed Central. In vitro reconstitution of the human RISC-loading complex
Strand preference is not always absolute. In fruit flies, different Argonaute proteins can favor opposite strands from the same duplex, effectively routing two distinct guide strands to two distinct silencing complexes.11Molecular Cell. Distinct Mechanisms for MicroRNA Strand Selection by Drosophila Argonautes This means a single precursor can potentially generate two functional miRNAs with different target sets, adding another layer of regulatory capacity.
Fine-Tuning Along the Way
The canonical pathway outlined above sounds orderly, but cells layer additional controls onto almost every step. Two modifications that reshape miRNA output are RNA editing and uridylation.
ADAR enzymes convert specific adenosine residues to inosine in double-stranded RNA. When this editing hits a pri-miRNA, it can block Drosha from recognizing and cutting it. Edited pri-miR-142, for example, is not processed by Drosha and is instead degraded by a ribonuclease called Tudor-SN. In mice lacking ADAR1 or ADAR2, mature miR-142 levels increased substantially because the editing brake was removed.12PubMed Central. Modulation of microRNA processing and expression through RNA editing by ADAR deaminases Editing thus provides a way to suppress specific miRNAs post-transcriptionally, independent of how actively the gene is being transcribed.
Uridylation, the addition of uracil residues to the 3′ end of a pre-miRNA, is carried out by terminal uridylyl transferases (TUTs). The outcome depends on context. Pre-miRNAs that naturally have a short 3′ overhang of just one nucleotide can receive a single uridine from TUT7, restoring the two-nucleotide overhang that Dicer prefers, which promotes efficient processing. But if the 3′ end is more severely trimmed, TUT7 adds a string of uridines instead, marking the molecule for destruction.13PubMed Central. TUT7 controls the fate of precursor microRNAs by using three different uridylation mechanisms The same enzyme, then, can act as either an activator or a destroyer depending on the structural state of its substrate.
Routes That Skip the Standard Steps
Not all miRNAs follow the canonical path through Drosha and Dicer. Several alternative biogenesis routes have been discovered, and the two best characterized skip one of those two enzymes entirely.
Mirtrons are short hairpin introns whose boundaries coincide perfectly with splice sites. When the intron is spliced out and the lariat is debranched, the resulting RNA folds into a hairpin that mimics a pre-miRNA, effectively bypassing the need for Drosha.14PubMed Central. The mirtron pathway generates microRNA-class regulatory RNAs in Drosophila These hairpins are then exported and processed by Dicer as usual.15PubMed Central. Mirtrons: microRNA biogenesis via splicing Mirtrons were first characterized in flies and worms but have since been identified in humans, where miRNAs such as miR-1227-3p, miR-1229-3p, and miR-1236-3p show splicing-dependent biogenesis.16PubMed Central. Splicing-dependent expression of microRNAs of mirtron origin in human digestive and excretory system cancer cells
A separate non-canonical route skips Dicer instead of Drosha. The best-known example is miR-451, which is processed normally by Drosha but then loaded directly into Argonaute 2 without Dicer cleavage. Ago2’s catalytic center cuts one strand of the hairpin to generate an intermediate, and exonucleases then trim the product to its final length.17PubMed Central. A dicer-independent miRNA biogenesis pathway that requires Ago catalysis This pathway has attracted interest partly because synthetic short hairpin RNAs designed as miR-451 mimics can exploit the same Ago2-dependent processing to achieve gene silencing without engaging Dicer.18Nucleic Acids Research. Dicer-independent processing of small RNA duplexes: mechanistic insights and applications
The LIN28–let-7 Switch in Development
One of the most dramatic examples of biogenesis regulation involves the let-7 family of miRNAs and the RNA-binding protein LIN28. In embryonic and stem cells, LIN28 binds to the loop region of let-7 precursors and blocks their processing, keeping let-7 levels low.19PubMed Central. LIN28 Selectively Modulates a Subclass of Let-7 MicroRNAs As cells differentiate, LIN28 expression drops, the brake lifts, and let-7 accumulates, suppressing genes associated with proliferation and pluripotency. Because let-7 itself can repress LIN28 expression, the two form a mutual inhibition loop. Modeling of this network shows it can behave as a bistable switch: cells exist in either a LIN28-high/let-7-low state or a LIN28-low/let-7-high state, with a sharp transition between them rather than a gradual slide.20PubMed Central. Bistable switch in let-7 miRNA biogenesis pathway involving Lin28 This switch-like behavior helps explain why cell fate transitions during development tend to be decisive rather than ambiguous.
When the Machinery Breaks Down in Cancer
Because miRNAs collectively regulate a large fraction of the transcriptome, defects in the biogenesis machinery can have sweeping effects on gene expression. Global reductions in miRNA levels, caused by genetic or epigenetic changes in biogenesis components, promote tumor formation.21PubMed Central. MicroRNA biogenesis pathways in cancer This is not limited to a single gene going wrong. Mutations in Drosha, DGCR8, Dicer, Exportin-5, Argonaute proteins, and other pathway components have all been implicated in different cancer types.22PubMed Central. Dysregulation of microRNA biogenesis machinery in cancer
A pan-cancer analysis of over 10,000 tumor-normal sample pairs across 33 cancer types cataloged mutations in 29 miRNA biogenesis genes. After removing hypermutated samples, the study found at least one mutation in about 21% of tumors. DICER1 was among the most frequently mutated genes, altered in roughly 3% of samples across all cancer types. Mutation frequency varied widely, from 45% of samples in skin cutaneous melanoma down to 0% in kidney chromophobe.23Nucleic Acids Research. A pan-cancer atlas of somatic mutations in miRNA biogenesis genes The practical takeaway is that miRNA biogenesis defects are not limited to rare cancer subtypes; they are a widespread feature of tumor biology.
Targeting miRNA Biogenesis with Small Molecules
If flawed biogenesis machinery contributes to disease, deliberately manipulating that machinery could be therapeutic. Researchers have begun exploring small molecules that can either inhibit specific miRNAs or disrupt the protein interactions that produce them. One compound selectively impeded the biogenesis of miR-544 in a hypoxic breast cancer model, boosting mTOR expression and sensitizing tumor cells to chemotherapy both in cell culture and in animals.24PubMed Central. Small Molecule Inhibition of miR-544 Biogenesis Disrupts Adaptive Responses to Hypoxia by Modulating ATM-mTOR Signaling Another approach targets the TRBP-Dicer interaction. A molecule called CIB-3b was found to physically bind TRBP and disrupt its partnership with Dicer, broadly altering miRNA profiles in hepatocellular carcinoma cells and suppressing tumor growth and metastasis.25PubMed. Discovery of a Novel Small-Molecule Inhibitor Disrupting TRBP-Dicer Interaction against Hepatocellular Carcinoma via the Modulation of microRNA Biogenesis The field is still early-stage, with most compounds in preclinical development, but the diversity of druggable nodes along the biogenesis pathway makes it an active area of medicinal chemistry.26PubMed Central. Targeting MicroRNAs with Small Molecules
How Targets Can Destroy Their Own Regulators
For a long time, mature miRNAs loaded into Argonaute were considered stable molecules with half-lives measured in days. That picture has shifted. Certain RNA targets with extensive complementarity to a miRNA can trigger that miRNA’s destruction, a process called target-directed miRNA degradation (TDMD).27PubMed Central. Target-directed microRNA degradation: Mechanisms, significance, and functional implications The mechanism works through the ubiquitin-proteasome system. When a miRNA engages a highly complementary target, a ubiquitin ligase complex built around the protein ZSWIM8 is recruited to the Argonaute protein. ZSWIM8 tags Argonaute for destruction by proteasomes, and once the protein shell is gone, the miRNA is exposed and degraded by cellular nucleases.28PubMed Central. A ubiquitin ligase mediates target-directed microRNA decay independently of tailing and trimming Loss-of-function studies have shown that ZSWIM8 sets the half-lives of most short-lived miRNAs in mammals, flies, and worms, meaning this is not a quirky exception but a conserved and widespread regulatory mechanism.29PubMed Central. The ZSWIM8 ubiquitin ligase mediates target-directed microRNA degradation Viruses exploit related principles, deploying transcripts or proteins that interfere with miRNA biogenesis or stability to reshape the host cell’s gene expression landscape in their favor.30PubMed Central. Regulation of microRNA biogenesis and turnover by animals and their viruses
miRNAs That Leave the Cell
Mature miRNAs do not always stay in the cell that made them. Cells package select miRNAs into small extracellular vesicles and secrete them into the surrounding fluid, where they can be taken up by distant cells. The loading process is not random. Specific short sequences within the miRNA, called EXOmotifs (such as GGAG), interact with RNA-binding proteins like hnRNPA2B1, which has been modified by SUMOylation, to direct certain miRNAs into vesicles for export.31PubMed Central. miRNA packaging into small extracellular vesicles and implications in pain Other RNA-binding proteins contribute too. In cancer cells, the La protein was shown to be necessary for efficiently packaging miR-122 into vesicles; depleting La from the cytoplasm severely reduced miR-122 loading.32PubMed Central. Distinct mechanisms of microRNA sorting into cancer cell-derived extracellular vesicle subtypes This intercellular trafficking adds a communication dimension to miRNA biology: a miRNA produced in one tissue can end up regulating gene expression somewhere else entirely.
Unexpected Destinations Inside the Cell
Even within a single cell, miRNAs do not simply float around in the cytoplasm waiting to find targets. Some are specifically routed to mitochondria. In HeLa cells, a set of 13 nuclear-encoded miRNAs was reproducibly enriched in the mitochondrial fraction compared with the cytoplasm, and Argonaute 2 was confirmed inside mitochondria by both immunoblotting and confocal microscopy.33PubMed Central. Nuclear outsourcing of RNA interference components to human mitochondria Functional consequences have been documented in heart cells, where miR-181c enters mitochondria and regulates the translation of a mitochondrially encoded protein, cytochrome c oxidase subunit 1. Overexpressing miR-181c decreased that protein, remodeled the electron transport chain, and increased reactive oxygen species production.34PubMed Central. Nuclear MicroRNA Regulates the Mitochondrial Genome in the Heart The biogenesis machinery itself has mitochondrial connections: the enzyme ELAC2, which processes transfer RNAs, operates in both the nucleus and mitochondria, and its activity is linked to balanced production of miRNAs and other regulatory RNAs.35PubMed Central. Concerted regulation of mitochondrial and nuclear non-coding RNAs by a dual-targeted RNase Z
How Plants and Animals Diverged
miRNAs exist in both plants and animals, but their biogenesis pathways differ in telling ways. In animals, Drosha acts in the nucleus and Dicer in the cytoplasm, with Exportin-5 ferrying the intermediate between the two compartments. In plants, the equivalent processing steps are handled largely within the nucleus by Dicer-like proteins, and the mature miRNA is exported rather than the precursor. Plants also tend to have multiple Dicer-like enzymes with specialized roles, while most animals get by with a single Dicer.36PubMed Central. Vive la différence: biogenesis and evolution of microRNAs in plants and animals Evolutionary analyses suggest that the core biogenesis proteins were co-opted from larger, more diverse protein families. Human biogenesis proteins have direct counterparts in fish and, for some components, in more distant relatives. But lineage-specific expansions occurred repeatedly: plants and invertebrates independently expanded their Dicer families, while vertebrates expanded their Argonaute and RNA-binding protein repertoires.37PubMed Central. The evolution of core proteins involved in microRNA biogenesis These expansions hint that organisms have repeatedly found it advantageous to evolve new ways of producing and deploying small regulatory RNAs, even when the basic machinery was already in place. Environmental stresses can also reshape miRNA output by altering expression or activity of core pathway components, a phenomenon especially well studied in plants.38PubMed. Keep calm and carry on: miRNA biogenesis under stress