Generation and action of siRNAs and miRNAs

two families are small regulatory RNAs called micro RNAs and small interfering RNAs or si RNAs regulate the stability and translation of mRNAs in eukaryotes both classes small RNA associate with a family of proteins known as Argonauts but differ in their origin processing pathway targets a mechanism of action we will first focus on micro RNAs and then point out the similarities within differences from SI RNAs micro rna's derive from actual genes found in genomes across the lineages of multicellular animals and plants these RNAs begin as primary transcripts or pry micro RNAs and they usually transcribed by RNA polymerase to the prime micro RNA transcript folds into a stem loop structure that generally has some unpaired nucleotides and single-stranded extensions are found at both the 5-prime and 3-prime ends this structure serves as the initial substrate for processing by Jascha a nuclear rnas 3 enzyme joshua interacts with a specialized RNA binding protein called d GC r 8 in humans to form what is termed the micro processor complex this complex performs a cleavage reaction that removes the 5-prime and 3-prime extension and liberates a 60 to 70 nucleotide transcript known as a pre micro RNA the pre micro RNA generated by the microprocessor complex is recognized by a nuclear export factor called export in v which transports the pre micro RNA to the cytoplasm for subsequent processing in the cytoplasm a second endonucleolytic cleavage reaction generally referred to as dicing is catalyzed by Dicer another RNAs 3 enzyme Dicer is found in complex with a double-stranded RNA binding protein called the tar RNA binding protein 2 or TRB P 2 the product of this second cleavage reaction is called the mir mir star duplex it carries five prime mono phosphate and three prime overhangs of two nucleotides which is not fully complimentary the two strands of this duplex RNA are called the guide and passenger strands once the mere mere start duplex has been generated it is loaded into the Argonaut protein that is dedicated to micro RNA mediated silencing reactions if the duplex is loaded in an appropriate orientation one of the RNA strands called the meer guide strand is retained while the other strand the mir star passenger strand is selectively removed the guide strand will ultimately be involved in determining which target RNAs will be silenced this process of retaining one strand removing the other is called sorting for imperfect Mir Mir star duplexes it is not certain how the passenger strand is identified and selectively removed though the action of a helicase may well be helpful Argonaut proteins charged with their guide micro RNAs are referred to as the MI RISC complex the complex is now ready to bind its target RNA and promote gene silencing Argonauts bound to micro RNAs typically identify sequences with imperfect complementarity in the three prime utrs of mRNA s the most important pairing region for the micro RNA is referred to as the seed sequence which generally encompasses nucleotide positions two to eight of the micro RNA species the absence of extended duplex formation allows the three prime end of the micro RNA j-- remain securely bound to the pals domain of the Argonaut protein as a consequence the Pirie domain of the Argonaut protein is generally not positioned appropriately to cleave mRNAs that are targeted by micro rnas once bound to targets MI risks appear to recruit additional factors including in particular a protein that contains multiple glycine tryptophan repeats which is known as TRNC six in humans this protein is thought to be involved in repressing translation and in destabilizing the mRNA through mechanisms that remain poorly understood what is clear is that the repression mechanism does not depend on the sisal activity of the Argonaut protein in contrast to micro RNAs si RNAs are not generally encoded in the genome as specific genes instead si RNAs are derived from double-stranded RNA that comes from several different sources endogenous duplex RNA can arise from the normal transcription of genomic loci that have extensive hairpin structures or from the annealing of sense and antisense RNAs that have both been transcribed from a given locus exogenous sources of double-stranded RNA include viral RNAs and duplex structures that have been synthetically introduced into cells by scientists for experimental purposes irrespective of their origins duplex RNAs become substrates for an RNA processing reaction akin to the one described for the micro RNAs but without the first nuclear DRA show dependent cleavage only dicing by the Dicer enzyme is needed the cytoplasmic enzymes sequentially cleaved the long duplex structures approximately every 20 to 25 base pairs the cutting frame is set by interactions between the double-stranded RNA and the paths region of the Dicer protein the products of the Dicer reaction are short duplex RNAs similar to the mere mere star duplexes but fully base paired along the length once the SI r SI r star duplex has been generated it also is loaded into the appropriate Argonaut protein of the si R star passenger strand is selectively removed for these fully complementary si R si R star duplexes the endogenous endonuclease activity of the Pirie domain of the Argonaut protein catalyzes cleavage of the passenger strand the cleared strands are then easily unwound and released this mechanism makes use of the RNase H fold of the PV domain and acidic residues that are poised in the active site for such catalysis the resulting single stranded si risk complexes can now scan of their fully complementary target RNAs in contrast the situation with the mi risk complexes fully complementary binding to the target RNAs leads the three prime end of the si RNA being displaced from the pals domain of the Argonaut as extended u plex nucleic acid forms this results in conformational changes that activate the cleavage activity of the RNAs HPV domain the cleavage reaction is referred to as slicing products are the slicing reaction are thought to immediately become targets for the exosome and other standard RNA decay pathways in the cell

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Keywords: yt:quality=high, video, oxford university, Oxford university press, education, publishing, scholarship, oxford, oup, oup academic, Oxford academic, Molecular Biology (Field Of Study), regulatory RNAs, small RNAs, siRNA, miRNA, craig

Id: 5YsTW5i0Xro

Length: 6min 50sec (410 seconds)

Published: Tue Aug 12 2014