Several features make miRNAs especially useful regulators of gene expression. First, a single miRNA can regulate a whole set of different mRNAs, so long as the mRNAs carry a common short sequence in their UTRs. This situation is common in humans, where a single miRNA can control hundreds of different mRNAs. Second, regulation by miRNAs can be combinatorial. When the base-pairing between the miRNA and mRNA fails to trigger cleavage, additional miRNAs binding to the same mRNA lead to further reductions in its translation. As discussed earlier for transcription regulators, combinatorial control greatly expands the possibilities available to the cell by linking gene expression to a combination of different regulators rather than a single regulator. Third, an miRNA occupies relatively little
Figure 7–75 miRNA processing and mechanism of action. the precursor mirnA, through complementarity between one part of its sequence and another, forms a double-stranded structure. this rnA is cropped while still in the nucleus and then exported to the cytosol, where it is further cleaved by the Dicer enzyme to form the mirnA proper. Argonaute, in conjunction with other components of riSC, initially associates with both strands of the mirnA and then cleaves and discards one of them. the other strand guides riSC to specific mrnAs through base-pairing. if the rnA–rnA match is extensive, as is commonly seen in plants, Argonaute cleaves the target mrnA, causing its rapid degradation. in mammals, the mirnA–mrnA match often does not extend beyond a short seven-nucleotide “seed” region near the 5ʹ end of the mirnA. this less extensive base-pairing leads to inhibition of translation, mrnA destabilization, and transfer of the mrnA to p-bodies, where it is eventually degraded.
3˜ OH (end of RNA) active site, showing 2 Mg2+ atoms needed for “slicing” space in the genome when compared with a protein. Indeed, their small size is one reason that miRNAs were discovered only recently. Although we are only beginning to appreciate the full impact of miRNAs, it is clear that they represent an important part of the cell’s equipment for regulating the expression of genes. We discuss specific examples of miRNAs that have key roles in development in Chapter 21.
rnA interference is Also Used as a Cell Defense Mechanism
Many of the proteins that participate in the miRNA regulatory mechanisms just described also serve a second function as a defense mechanism: they orchestrate the degradation of foreign RNA molecules, specifically those that occur in double-stranded form. Many transposable elements and viruses produce double-stranded RNA, at least transiently, in their life cycles, and RNA interference helps to keep these potentially dangerous invaders in check. As we shall see, this form of RNAi also provides scientists with a powerful experimental technique to turn off the expression of individual genes.
The presence of double-stranded RNA in the cell triggers RNAi by attracting a protein complex containing Dicer, the same nuclease that processes miR-NAs (see Figure 7–75). This protein cleaves the double-stranded RNA into small fragments (approximately 23 nucleotide pairs) called small interfering RNAs (siRNAs). These double-stranded siRNAs are then bound by Argonaute and other components of RISC. As we saw above for miRNAs, one strand of the duplex RNA is then cleaved by Argonaute and discarded. The single-stranded siRNA molecule that remains directs RISC back to complementary RNA molecules produced by the virus or transposable element. Because the match is usually exact, Argonaute cleaves these molecules, leading to their rapid destruction.