microRNAs: tiny regulators with great potential.
Animal genomes contain an abundance of small genes that produce regulatory RNAs of about 22 nucleotides in length. These microRNAs are diverse in sequence and expression patterns, and are evolutionarily widespread, suggesting that they may participate in a wide range of genetic regulatory pathways. (+info)
MicroRNAs: hidden in the genome.
Genes for tiny RNAs have been found to be plentiful in the genomes of worms, flies, humans and probably all animals. Some of these microRNAs have been conserved through evolution, and many are expressed only at specific times or places. How they act is just beginning to be understood, but their importance to biology is likely to be great. (+info)
Two genetic circuits repress the Caenorhabditis elegans heterochronic gene lin-28 after translation initiation.
The heterochronic gene lin-28 of the nematode Caenorhabditis elegans controls the relative timing of diverse developmental events during the animal's larval stages. lin-28 is stage-specifically regulated by two genetic circuits: negatively by the 22-nt RNA lin-4 and positively by the heterochronic gene lin-14. Here, we show that lin-28 is repressed during normal development by a mechanism that acts on its mRNA after translation initiation. We provide evidence that lin-14 inhibits a negative regulation that is independent of the lin-4 RNA and involves the gene daf-12, which encodes a nuclear hormone receptor. The lin-4-independent repression does not affect the initiation of translation on the lin-28 mRNA, and like the lin-4-mediated repression, acts through the gene's 3'-untranslated region. In addition, we find that lin-4 is not sufficient to cause repression of lin-28 if the lin-4-independent circuit is inhibited. Therefore, the lin-4-independent circuit likely contributes substantially to the down-regulation of lin-28 that occurs during normal development. The role of lin-4 may be to initiate or potentiate the lin-4-independent circuit. We speculate that a parallel lin-4-independent regulatory mechanism regulates the expression of lin-14. (+info)
miRNPs: a novel class of ribonucleoproteins containing numerous microRNAs.
Gemin3 is a DEAD-box RNA helicase that binds to the Survival of Motor Neurons (SMN) protein and is a component of the SMN complex, which also comprises SMN, Gemin2, Gemin4, Gemin5, and Gemin6. Reduction in SMN protein results in Spinal muscular atrophy (SMA), a common neurodegenerative disease. The SMN complex has critical functions in the assembly/restructuring of diverse ribonucleoprotein (RNP) complexes. Here we report that Gemin3 and Gemin4 are also in a separate complex that contains eIF2C2, a member of the Argonaute protein family. This novel complex is a large approximately 15S RNP that contains numerous microRNAs (miRNAs). We describe 40 miRNAs, a few of which are identical to recently described human miRNAs, a class of small endogenous RNAs. The genomic sequences predict that miRNAs are likely to be derived from larger precursors that have the capacity to form stem-loop structures. (+info)
Identification of tissue-specific microRNAs from mouse.
MicroRNAs (miRNAs) are a new class of noncoding RNAs, which are encoded as short inverted repeats in the genomes of invertebrates and vertebrates. It is believed that miRNAs are modulators of target mRNA translation and stability, although most target mRNAs remain to be identified. Here we describe the identification of 34 novel miRNAs by tissue-specific cloning of approximately 21-nucleotide RNAs from mouse. Almost all identified miRNAs are conserved in the human genome and are also frequently found in nonmammalian vertebrate genomes, such as pufferfish. In heart, liver, or brain, it is found that a single, tissue-specifically expressed miRNA dominates the population of expressed miRNAs and suggests a role for these miRNAs in tissue specification or cell lineage decisions. Finally, a miRNA was identified that appears to be the fruitfly and mammalian ortholog of C. elegans lin-4 stRNA. (+info)
Both natural and designed micro RNAs can inhibit the expression of cognate mRNAs when expressed in human cells.
Animal cells have recently been shown to express a range of approximately 22 nucleotide noncoding RNAs termed micro RNAs (miRNAs). Here, we show that the human mir-30 miRNA can be excised from irrelevant, endogenously transcribed mRNAs encompassing the predicted 71 nucleotide mir-30 precursor. Expression of the mir-30 miRNA specifically blocked the translation in human cells of an mRNA containing artificial mir-30 target sites. Similarly, designed miRNAs were also excised from transcripts encompassing artificial miRNA precursors and could inhibit the expression of mRNAs containing a complementary target site. These data indicate that novel miRNAs can be readily produced in vivo and can be designed to specifically inactivate the expression of selected target genes in human cells. (+info)
MicroRNAs in plants.
MicroRNAs (miRNAs) are an extensive class of ~22-nucleotide noncoding RNAs thought to regulate gene expression in metazoans. We find that miRNAs are also present in plants, indicating that this class of noncoding RNA arose early in eukaryotic evolution. In this paper 16 Arabidopsis miRNAs are described, many of which have differential expression patterns in development. Eight are absolutely conserved in the rice genome. The plant miRNA loci potentially encode stem-loop precursors similar to those processed by Dicer (a ribonuclease III) in animals. Mutation of an Arabidopsis Dicer homolog, CARPEL FACTORY, prevents the accumulation of miRNAs, showing that similar mechanisms direct miRNA processing in plants and animals. The previously described roles of CARPEL FACTORY in the development of Arabidopsis embryos, leaves, and floral meristems suggest that the miRNAs could play regulatory roles in the development of plants as well as animals. (+info)
A microRNA in a multiple-turnover RNAi enzyme complex.
In animals, the double-stranded RNA-specific endonuclease Dicer produces two classes of functionally distinct, tiny RNAs: microRNAs (miRNAs) and small interfering RNAs (siRNAs). miRNAs regulate mRNA translation, whereas siRNAs direct RNA destruction via the RNA interference (RNAi) pathway. Here we show that, in human cell extracts, the miRNA let-7 naturally enters the RNAi pathway, which suggests that only the degree of complementarity between a miRNA and its RNA target determines its function. Human let-7 is a component of a previously identified, miRNA-containing ribonucleoprotein particle, which we show is an RNAi enzyme complex. Each let-7-containing complex directs multiple rounds of RNA cleavage, which explains the remarkable efficiency of the RNAi pathway in human cells. (+info)