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RNA-induced transcriptional silencing

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Title: RNA-induced transcriptional silencing  
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RNA-induced transcriptional silencing

RNA-induced transcriptional silencing (RITS) is a form of RNA interference by which short RNA molecules - such as small interfering RNA (siRNA) - trigger the downregulation of transcription of a particular gene or genomic region. This is usually accomplished by posttranslational modification of histone tails (e.g. methylation of lysine 9 of histone H3) which target the genomic region for heterochromatin formation. The protein complex that binds to siRNAs and interacts with the methylated lysine 9 residue of histones H3 is the RITS complex.

RITS was discovered in the chromosome centromeres,[3] resulting in slow or stalled anaphase during cell division.[4]

The maintenance of heterochromatin regions by RITS complexes has been described as a self-reinforcing feedback loop, in which RITS complexes stably bind the methylated histones of a heterochromatin region and induce co-transcriptional degradation of any nascent messenger RNA (mRNA) transcripts, which are then used as RNA-dependent RNA polymerase substrates to replenish the complement of siRNA molecules.[5] Heterochromatin formation, but possibly not maintenance, is dependent on the ribonuclease protein dicer, which is used to generate the initial complement of siRNAs.[6]

The relevance of observations from fission yeast mating-type regions and centromeres to mammals is not clear, as some evidence suggests that heterochromatin maintenance in mammalian cells is independent of the components of the RNAi pathway.[7]

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