Phillip Sharp (Massachusetts Institute of Technology, Cambridge, MA, USA)compared the expression profiles of microRNAs and their computationally predicted targets during T cell development, and identified at least one microRNA for each stage whose expression inversely correlated with its predicted-target levels. Moreover, the overexpression of miR-315 in the presumptive thorax of developing larvae caused dramatic developmental transformations associated with excess Wg activity. Consistent with a role for miR-315 in promoting Wg signaling in vivo,among the targets of miR-315 are the negative regulators of the Wg pathway, Axin and Notum. miR-315 was identified in a cell-based screen as a potentiator of Wingless (Wg) signaling. Another example of a microRNA-modulated signaling pathway in Drosophila was discussed by Eric Lai(Sloan-Kettering Institute, New York, NY, USA). Indeed, Carthew reported that exposing developing mir-7 mutant fly eyes to temperature cycles disrupts regular ommatidial patterning. Carthew proposed that microRNAs such as miR-7 function in such networks to confer robustness on developmental decisions and to provide genetic buffering under adverse physiological conditions. Richard Carthew (Northwestern University, Evanston, IL, USA) described how miR-7 functions in an intricate network of Notch and epidermal growth factor receptor (EGFR) signaling pathways to specify photoreceptors( Li and Carthew, 2005) and chordotonal sensory organ precursors in Drosophila. An existing rasiRNA guides the cleavage of a target RNA through the slicer activity of the associated Piwi subfamily of Argonaute protein to lead to the production of a second rasiRNA. These small RNAs bind the Piwi subfamily of Argonaute proteins -Piwi, Aubergine (Aub) and AGO3 - and target homologous transposons scattered around the genome for silencing. ( C) Via an unknown mechanism, repeat-associated siRNAs (rasiRNAs) in Drosophila are produced from a master locus composed of multiple transposons. Histone H3 lysine 9 (H3-K9) methylation and the siRNAs complexed with AGO act in a self-reinforcing loop to maintain heterochromatin at the loci. pombe and in plants are generated from long transcripts from repeated DNA or transposons via the activities of RNA-dependent RNA polymerases (RdRP) and Dicer. Related miRNAs(light and dark blue) often have shared predicted targets, but genetic studies reveal that members of a miRNA family may have distinct developmental functions. In animals, despite the fact that each miRNA is able to regulate scores of target transcripts, genetic studies show that one major target underlies the role of a miRNA in a developmental process. The miRNAs are bound by an effector protein Argonaute (AGO) and cause the cleavage or translational inhibition of target mRNAs. ![]() ![]() ( A) In animals, microRNA (MIR) genes (top) are transcribed by Pol II into microRNA (miRNA) precursors, which are processed by DROSHA and DICER (DICER1) into miRNAs. ![]() The biogenesis and function of endogenous small RNAs in animals, and in plants and fungi.
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