| description | Protein involved in RNA-mediated gene silencing. This term describes a number of related processes which use 21- to 25-nucleotide RNAs to repress the expression of specific target genes. These processes include the post-transcriptional regulation of mRNA by either RNA interference (RNAi) or endogenously encoded microRNAs (miRNAs) and the transcriptional regulation of mRNA by RNAi-mediated chromatin silencing. RNAi is generally triggered by the presence of dsRNA produced by bidirectional transcription or by the transcription of an inverted repeat or hairpin sequence. dsRNA is processed into small interfering RNAs (siRNAs) of around 21 nucleotide which are then incorporated into the RNA-induced silencing complex (RISC), which cleaves mRNAs with sequences fully complementary to the siRNA. miRNAs differ from siRNAs principally in their biogenesis. miRNA genes encode precursors with complex hairpin structures which are processed by endonucleolytic cleavage to form mature miRNAs. Like siRNAs, miRNAs function in RISC-like complexes. Animal miRNAs generally inhibit translation of target mRNAs following imperfect base pairing to the 3'-untranslated region (3'-UTR), while most plant miRNAs show nearly precise complementarity to coding regions of target mRNAs and trigger mRNA degradation similar to siRNAs. Finally, in RNAi-mediated chromatin silencing, siRNAs derived from repeat element transcripts are incorporated into the nuclear RNAi-induced initiator of transcriptional silencing complex (RITS), where they guide chromatin modifications such as histone methylation which lead to transcriptional silencing |
