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MicroRNAs
MicroRNA (miRNA) are short, regulatory RNA transcribed from introns—non-coding regions of a gene—or intergenic regions—stretches of DNA present betwee…
MicroRNA, miRNA, is a type of small, regulatory RNA that does not code for protein. Instead it regulates gene expression by inhibiting the translation of messenger RNA, mRNA, into protein.
MicroRNAs are around 22 nucleotides long and are produced through the cleavage of a double-stranded RNA precursor molecule with a hairpin turn. The two strands separate, and one becomes the mature miRNA, which forms a complex with a group of proteins called RISC, RNA-induced silencing complex.
This complex binds to mRNA through complimentary base pairing between the miRNA and specific sequences in the mRNA, typically in the 3-prime untranslated region. This pairing is usually not perfect, but it is enough to inhibit translation, either through cleavage and destruction of the mRNA or by interfering with the translation process itself.
Either way, gene expression is silenced, which is an important type of post-transcriptional regulation. In fact, dysregulation of miRNA is correlated with potentially deadly diseases, such as cancer and heart disease.
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Q1: What is microRNA and how does it regulate gene expression?
MicroRNA (miRNA) is a small regulatory RNA approximately 22 nucleotides long that does not code for protein. Instead, it regulates gene expression by inhibiting messenger RNA (mRNA) translation into protein. MiRNA forms a complex with proteins called RISC (RNA-induced silencing complex), which binds to specific mRNA sequences through complementary base pairing, typically in the 3-prime untranslated region, silencing gene expression through mRNA cleavage or translation interference.
Q2: How is mature microRNA produced from its precursor?
Mature miRNA is produced through sequential enzymatic processing. Primary miRNA (pri-miRNA) forms a stem-loop structure and is shortened by the Drosha enzyme into hairpin-shaped pre-miRNA within the nucleus. After methylation to prevent degradation, pre-miRNA is exported to the cytoplasm where the Dicer enzyme cleaves it into a 21-24 nucleotide miRNA duplex, then releases a single strand of mature miRNA ready for loading into RISC.
Q3: What determines whether miRNA causes mRNA degradation or translation inhibition?
The extent of complementary base-pairing between miRNA and the 3-prime untranslated region of target mRNA determines the silencing mechanism. Extensive or near-perfect complementarity causes irreversible mRNA degradation. Limited base-pairing inhibits translation, which is reversible since stable mRNA can resume translation after repressors are eliminated.
Q4: Why is dysregulation of microRNA associated with cancer?
Loss of let-7 miRNA is observed in lung, liver, breast, prostate, and ovarian cancers. Let-7 normally inhibits oncogenes—genes that promote cell growth, survival, and proliferation. When let-7 is lost, these oncogenes remain active, promoting tumor formation. Dysregulation of miRNA is correlated with potentially deadly diseases including cancer and heart disease.
Q5: Where are microRNAs transcribed from in the genome?
MicroRNAs are transcribed from introns, which are non-coding regions within genes, or from intergenic regions, which are stretches of DNA located between genes. These genomic sources are processed through multiple enzymatic steps to produce biologically active mature miRNA that can regulate target mRNA molecules.
Q6: How does the RISC complex function in gene silencing?
The RNA-induced silencing complex (RISC) is a protein complex that loads mature miRNA and guides it to complementary regions of target mRNA. Once bound through base-pairing interactions, RISC either facilitates mRNA degradation or blocks translation, depending on the degree of sequence complementarity. This mechanism represents an important type of post-transcriptional regulation in gene expression transcription splicing and translation.
Q7: What happens to pre-miRNA after it is exported from the nucleus?
After pre-miRNA is methylated at its ends to prevent degradation and exported from the nucleus into the cytoplasm, the Dicer enzyme cleaves it into a 21-24 nucleotide miRNA duplex. Dicer then separates one strand from the duplex, releasing the single-stranded mature miRNA that is subsequently loaded into the RISC complex for target mRNA recognition.