Generation and action of siRNAs and miRNAs

Summary

This script delves into the intricate world of small regulatory RNAs, focusing on microRNAs (miRNAs) and small interfering RNAs (siRNAs). It explains their origin, processing, and function in gene regulation. miRNAs originate from genes, undergo nuclear processing by Drosha and Dicer, and form the RISC complex to silence target mRNAs. siRNAs, derived from double-stranded RNA, follow a similar path but differ in their perfect complementarity and cleavage mechanism. Both types of small RNAs play crucial roles in controlling gene expression and are essential for understanding post-transcriptional regulation.

Takeaways

• 🌟 MicroRNAs (miRNAs) and small interfering RNAs (siRNAs) are two types of small regulatory RNAs that play crucial roles in gene regulation by affecting mRNA stability and translation.
• 🌿 miRNAs originate from actual genes found in the genomes of multicellular animals and plants, while siRNAs are derived from double-stranded RNA from various sources, including endogenous and exogenous origins.
• 📜 Primary miRNA transcripts, known as pri-miRNAs, are transcribed by RNA Polymerase II and fold into a stem-loop structure that is processed into a precursor miRNA (pre-miRNA) by the Drosha-DGCR8 microprocessor complex.
• 🚀 The pre-miRNA is exported to the cytoplasm where it undergoes further processing by Dicer, an RNase III enzyme, to generate a miRNA duplex with overhangs and monophosphates.
• 🔄 The miRNA duplex is loaded into the Argonaute protein, where one strand, the guide strand, is retained and the other, the passenger strand, is removed through a sorting process that is not fully understood.
• 🎯 The Argonaute-miRNA complex, known as the RISC (RNA-induced silencing complex), targets mRNAs with imperfect complementarity, particularly in the 3' untranslated regions (UTRs), guided by the 'seed' sequence of the miRNA.
• 🛡 miRNAs typically do not cleave their target mRNAs but instead recruit factors like TRBP (TNRC6 in humans) to repress translation and destabilize the mRNA, without relying on the slicer activity of Argonaute.
• 🧬 In contrast to miRNAs, siRNAs are fully complementary duplexes that, after being processed by Dicer, are loaded into Argonaute proteins where the passenger strand is cleaved by the endonuclease activity of the protein's PAZ domain.
• 🔍 siRNA-Argonaute complexes scan for and bind to fully complementary target RNAs, leading to the activation of the PAZ domain's cleavage activity and slicing of the target mRNA.
• ♻️ The products of slicing are thought to be immediately targeted for degradation by the exosome and other RNA decay pathways, thereby silencing the gene expression.
• 🔗 Both miRNAs and siRNAs associate with Argonaute proteins to mediate gene silencing, but they differ in their origin, processing pathway, targets, and mechanism of action.

Q & A

What are the two types of small regulatory RNAs mentioned in the script?

The two types of small regulatory RNAs mentioned are microRNAs (miRNAs) and small interfering RNAs (siRNAs).

What is the role of Argonaut proteins in relation to small RNAs?

Argonaut proteins are associated with both miRNAs and siRNAs and play a crucial role in mediating their regulatory functions, such as gene silencing.

How do miRNAs originate in the cell?

miRNAs originate from actual genes found in the genomes of multicellular animals and plants. They begin as primary transcripts or pri-miRNAs, which are transcribed by RNA polymerase II.

What is the function of the microprocessor complex in miRNA processing?

The microprocessor complex, consisting of Drosha and DGCR8, performs a cleavage reaction that removes the five prime and three prime extensions from the pri-miRNA, producing a precursor miRNA (pre-miRNA).

How is the pre-miRNA transported to the cytoplasm for further processing?

The pre-miRNA is recognized by Exportin 5, a nuclear export factor, which transports it to the cytoplasm for subsequent processing.

What enzyme is responsible for the second cleavage reaction of pre-miRNAs in the cytoplasm?

The second cleavage reaction, referred to as dicing, is catalyzed by Dicer, an RNase III enzyme.

What are the two strands of the miRNA duplex called after the second cleavage reaction?

The two strands of the miRNA duplex are called the guide strand and the passenger strand.

How does the miRNA guide strand determine which target RNAs will be silenced?

The miRNA guide strand, once loaded into the Argonaut protein, identifies target RNAs with imperfect complementarity, particularly in the 3' UTRs of mRNAs, through a region known as the seed sequence.

What is the difference between miRNAs and siRNAs in terms of their origin?

miRNAs are encoded in the genome as specific genes, while siRNAs are derived from double-stranded RNA that can come from various sources, including endogenous duplex RNA or exogenous sources like viral RNAs.

How does the mechanism of gene silencing differ between miRNAs and siRNAs?

miRNAs typically promote gene silencing through translational repression and mRNA destabilization without cleaving the target RNA, whereas siRNAs lead to the cleavage of fully complementary target RNAs, a process known as slicing.

What is the role of the TRBP protein in miRNA-mediated gene silencing?

The TRBP protein, which contains multiple glycine-tryptophan repeats, is thought to be involved in repressing translation and destabilizing mRNA, although the exact mechanisms are not fully understood.