RESUMO
Functional noncoding RNAs have distinct roles in epigenetic gene regulation. Large RNAs have been shown to control gene expression from a single locus (Tsix RNA), from chromosomal regions (Air RNA), and from entire chromosomes (roX and Xist RNAs). These RNAs regulate genes in cis; although the Drosophila roX RNAs can also function in trans. The chromatin modifications mediated by these RNAs can increase or decrease gene expression. These results suggest that the primary role of RNA molecules in epigenetic gene regulation is to restrict chromatin modifications to particular regions of the genome. However, given that RNA has been shown to be at the catalytic core of other ribonucleoprotein complexes, it is also possible that RNA also plays a role in modulating changes in chromatin structure.
Assuntos
Epigênese Genética/genética , RNA não Traduzido/genética , Animais , Regulação da Expressão Gênica , Inativação GênicaRESUMO
Early in the development of female mammals, one of the two X chromosomes is silenced in half of cells and the other X chromosome is silenced in the remaining half. The basis of this apparent randomness is not understood. We show that before X-inactivation, the two X chromosomes appear to exist in distinct states that correspond to their fates as the active and inactive X chromosomes. Xist and Tsix, noncoding RNAs that control X chromosome fates upon X-inactivation, also determine the states of the X chromosomes prior to X-inactivation. In wild-type ES cells, X chromosomes switch between states; among the progeny of a single cell, a given X chromosome exhibits each state with equal frequency. We propose a model in which the concerted switching of homologous X chromosomes between mutually exclusive future active and future inactive states provides the basis for the apparently random silencing of one X chromosome in female cells.
Assuntos
Inativação do Cromossomo X/fisiologia , Cromossomo X/genética , Animais , Células Cultivadas , Replicação do DNA , Feminino , Hibridização in Situ Fluorescente , Camundongos , Modelos Genéticos , RNA Longo não Codificante , RNA não Traduzido/fisiologia , Células-Tronco/citologia , Cromossomo X/metabolismoRESUMO
One X chromosome, selected at random, is silenced in each female mammalian cell. Xist encodes a noncoding RNA that influences the probability that the cis-linked X chromosome will be silenced. We found that the A-repeat, a highly conserved element within Xist, is required for the accumulation of spliced Xist RNA. In addition, the A-repeat is necessary for X-inactivation to occur randomly. In combination, our data suggest that normal Xist RNA processing is important in the regulation of random X-inactivation. We propose that modulation of Xist RNA processing may be part of the stochastic process that determines which X chromosome will be inactivated.
Assuntos
Proteínas Nucleares/metabolismo , Processamento Pós-Transcricional do RNA , RNA não Traduzido/genética , Proteínas de Ligação a RNA/metabolismo , Sequências Repetitivas de Ácido Nucleico/genética , Inativação do Cromossomo X/genética , Alelos , Animais , Sequência de Bases , Cromossomos de Mamíferos/metabolismo , Feminino , Células HeLa , Histonas/metabolismo , Humanos , Masculino , Camundongos , Modelos Biológicos , Dados de Sequência Molecular , Conformação de Ácido Nucleico , Ligação Proteica , Processamento de Proteína Pós-Traducional , RNA Longo não Codificante , RNA Mensageiro/genética , RNA Mensageiro/metabolismo , RNA não Traduzido/química , Deleção de Sequência/genética , Fatores de Processamento de Serina-ArgininaRESUMO
Substrate cleavage by the Neurospora Varkud satellite (VS) ribozyme involves a structural change in the stem-loop I substrate from an inactive to an active conformation. We have determined the NMR solution structure of a mutant stem-loop I that mimics the active conformation of the cleavage site internal loop. This structure shares many similarities, but also significant differences, with the previously determined structures of the inactive internal loop. The active internal loop displays different base-pairing interactions and forms a novel RNA fold composed exclusively of sheared G-A base pairs. From chemical-shift mapping we identified two Mg2+ binding sites in the active internal loop. One of the Mg2+ binding sites forms in the active but not the inactive conformation of the internal loop and is likely important for catalysis. Using the structure comparison program mc-search, we identified the active internal loop fold in other RNA structures. In Thermus thermophilus 16S rRNA, this RNA fold is directly involved in a long-range tertiary interaction. An analogous tertiary interaction may form between the active internal loop of the substrate and the catalytic domain of the VS ribozyme. The combination of NMR and bioinformatic approaches presented here has identified a novel RNA fold and provides insights into the structural basis of catalytic function in the Neurospora VS ribozyme.