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siRNAsiRNA与miRNA的区别1）来源不同：miRNAsderivefromgenomiclocidistinctfromotherrecognizedgenes,whereassiRNAsoftenderivefrommRNAs,transposons,viruses,orheterochromaticDNA2）结构不同：miRNAsareprocessedfromtranscriptsthatcanformlocalRNAhairpinstructures,whereassiRNAsareprocessedfromlongbimolecularRNAduplexesorextendedhairpins.3）加工不同：AsinglemiRNA:miRNA*duplexisgeneratedfromeachmiRNAhairpinprecursormolecule,whereasamultitudeofsiRNAduplexesaregeneratedfromeachsiRNAprecursormolecule,leadingtomanydifferentsiRNAsaccumulatingfrombothstrandsofthisextendeddsRNA.4）保守性不同：miRNAsequencesarenearlyalwaysconservedinrelatedorganisms,whereasendogenoussiRNAsequencesarerarelyconserved.ThesetypesofdifferencesarethebasisofpracticalguidelinesfordistinguishingandannotatingnewlydiscoveredmiRNAsandendogenoussiRNAs.siRNA的种类1)naturalcis-actingRNA—na-ciRNA;2)naturalcis-antisenseRNA(压力诱导siRNA);3)repeat-associatedshortinterferingRNAs(rasi-RNAs);4)heterochromatinsiRNA—hcsiRNA;5scanRNA(scnRNA).naturalcis-actingRNA源于外源入侵的病毒产生的小分子双链RNA：1）植物外源入侵的病毒产生的小分子双链RNA；2）无脊椎动物外源入的侵病毒产生的小分子双链RNA。植物病毒入侵诱导产生的初生siRNA植物病毒入侵诱导产生的次生siRNAThefigureshowshowprimaryviRNAs(1st)areamplifiedintosecondaryviRNAs(2nd)intheRDR6-dependentpathway.Aberrant(ab)viralmRNAslackingacaporpolyAtail(AAA)canenterRNA-dependentRNApolymerasepathwaysindependentlyof1stviRNAsynthesis.ADCL4-dependentsilencingsignal(arbitrarilydepictedasfree21nucleotideviRNAs)movesthroughtheplasmodesmata(P)toimmunizeneighboringcells.Movementmaybeenhancedthroughfurtherroundsofamplificationinvolvingviraltranscriptsthatenterimmunizedcells.VSRsandpotentialendogenoussilencingsuppressors(red)representgeneticratherthandirectphysicalinteractionswithhostsilencingcomponents.果蝇与线虫病毒入侵诱导产生的siRNA(A)TheDrosophilapathwayisconceptuallysimilartoalinearantiviralsilencingPathwayinplants.(B)AntiviralsilencinginC.eleganshasbeeninferredthroughstudiesofartificialinfectionsystems.ALG,RDE-1,andSAGOarewormAGOsthatrecruitmiRNAs,1stsiRNAs,and2ndsiRNAs,respectively.RRF-1isthoughttoproduce2ndsiRNAsortocopyRNAs(cRNAs)directlyfromRDE3-stabilizedtemplates.SID-1maypossiblytakeupviraldsRNAsintocells.Thesepathwayscanbedisruptedatmultiplepointsbyendogenoussuppressors(red).Cell,130:413-426,2007植物siRNA在细胞间的转移初生25ntsiRNA在细胞间的转移距离较近,仅在数个细胞之间.次生21ntsiRNA可在10-15个细胞之间转移.Naturalcis-antisenseRNADegradationofP5CDHmRNAdecreasesP5CDHactivityandprolinedegradation.Decreasedprolinedegradationcontributestosalt-inducedprolineaccumulationandsalttolerancebut,atthesametime,leadstotheROSAccumulationcausedbybuildupoftheprolinecatabolicintermediateP5C.ThisROSaccumulationispotentiallyharmfulbutisalsoasignaltoinitiatefurtherstressresponses.TheSRO5proteincounteractsP5C-inducedROSaccumulation,thuscompletingaregulatoryloopthatallowsfinetuningofROSaccumulationandstressresponse.植物na-siRNA的加工na-siRNA即naturalcis-actingsiRNA.产生于内源的cis-antisense基因的表达产物.两步生成:24ntna-siRNA→21ntna-siRNA.在拟南芥盐诱导压力环境下产生.作用于自身mRNA.Nat-siRNA的来源Thereare∼2000pairsofnaturalcis-antisensetranscriptsinArabidopsisnat-siRNAscorrespondingtoothercis-antisensetranscriptpairsexistandareDetectedonlyunderinducibleconditions,suggestingthatthistypeofregulationmaynotberestrictedtoasingleexampleofoverlapping.Genomewidepredictionandidentificationofcis-naturalantisensetranscriptsinArabidopsisthaliana.GenomeBiol.6:R30,2006EndogenoussiRNAsderivedfromapairofNaturalcis-antisensetranscriptsregulatesalttoleranceinArabidopsis.Cell123:1279–1291,2005.动物hc-siRNA目前在哺乳动物基因组中尚未发现siRNA.动物siRNA来源于dsRNA,后者在细胞质中经Dicer(RnaseIII)加工成21-22ntsiRNA,随后进一步扩增放大或进入细胞核使染色质异染色质化.Ping-pong’mechanismforrasiRNAbiogenesisAmodelshowingthatthe5′endformationofrepeat-associatedsmallinterferingRNAs(rasiRNAs)associatedwithArgonaute3(AGO3)andAubergine(AUB;orPIWI)aremediatedbyAUB(orPIWI)andAGO3,respectively.Themechanismbywhichthe3′endsofrasiRNAsaregeneratedisstillunknown.Scissorsrepresentendonucleolyticcleavageevents.ThismodelisbasedonthestudiesbyBrenneckeetal(2007)andGunawardaneetal(2007).Rasi-RNA阻止转座子的转座WhilemiRNAactintranslationalrepressionandmRNAcleavageandsiRNAactinmRNAcleavage,rasiRNAacttoregulatechromatinstructureandtranscriptionalsilencing.InDrosophila,mutationsinthePiwiproteinsthatassociatewithrasiRNAleadtosterilityandlossofgermlinecellsinbothmalesandfemales.TransposonrepressionisnotaffectedbythelossofDicerwithinthegermlinecellsrevealingthatthisisthetargetoftherasiRNApathway.SimilartomiRNAandsiRNA,therasiRNAsilencingpathwayisevolutionarilyconservedandhomologydependent.WhentherasiRNApathwayisnotpresent,germlinecellsmayundergoretrotranspositionwhicharesensedasDNAdamageandsignalthecelltoapoptosis.RasiRNAiskeytotheregulatorymechanismofmanyorganismsaspartoftheRNAinterferencepathway.si-RNA与染色质重建EMBOreports(2007)8,723-729Roleofsilencinginheterochromatinformation.TheRNAinterferencemachinery,consistingoftheRNA-inducedinitiationoftranscriptionalgenesilencing(RITS)complex,Dicer-1(DCR1)andtheRNA-directedRNApolymerasecomplex(RDRC),processestranscriptsderivedfromrepeatelementsintosmallinterferingRNAs(siRNAs).ThesesiRNAsmightsilencegeneexpressionpost-transcriptionallyincis.TheClr4com