MicroRNAS ARE ANTISENSE RNAs THAT MODULATE GENE EXPRESSION
The development from embryo to adult of the worm C. elegans requires RNAi to turn off genes at appropriate times. In this case, RNAi is not triggered by intrusion of external sequences such as transgenes or viruses. During development, small noncoding RNA molecules known as microRNAs (miRNA) are transcribed from the worm’s own genome. These miRNAs regulate gene expression by blocking the translation of developmentally appropriate target mRNA. MicroRNAs, first identified in C. elegans, have now been identified in various plants and animals, including humans. RNAi induced by miRNAs is similar
to the mechanism described above. The mRNA targets are identified by antisense, that is, the miRNA has sequences that are complementary to the part of the target mRNA. Some miRNAs bind to the target mRNA and block the initiation of translation. In other cases, the miRNA binds to the 3′ UTR region of the mRNA.
MicroRNAs are transcribed as longer precursor molecules, pre-microRNAs, of approximately 70 nucleotides in length. In Drosophila, miRNAs are transcribed as polycistronic messages that are first cleaved by an endonuclease called Drosha. In plants, the pre-microRNAs can be even longer, up to 300 nucleotides. Dicer cleaves the plant pre-microRNA into segments of approximately 20 nucleotides. After cleavage of the precursor, the released miRNA forms a stem-loop structure by complementary base pairing. Dicer recognizes the stem-loop and cleaves the loop structure, thus separating the strands. The RISC complex then unravels the two strands. The miRNA found in animals such as C. elegans can tolerate a few mismatched base pairs within the binding domain. In animals, the antisense miRNA strand blocks translation of the target mRNA (Fig. 5.13), which is not degraded. In contrast, in plants, microRNA must have perfect matches and relies on RISC-mediated recognition and cleavage to degrade the target mRNA.