Seeonlineversionforlegendandreferences.326Cell143,October15,2010©2010ElsevierInc.DOI10.1016/j.cell.2010.09.050SnapShot:NetworkMotifsOrenShovalandUriAlonDepartmentofMolecularCellBiology,WeizmannInstituteofScience,Rehovot76100,Israel326.e1Cell143,October15,2010©2010ElsevierInc.DOI10.1016/j.cell.2010.09.050SnapShot:NetworkMotifsOrenShovalandUriAlonDepartmentofMolecularCellBiology,WeizmannInstituteofScience,Rehovot76100,IsraelTranscriptionregulationandsignalingnetworksarecomposedofrecurringpatternscallednetworkmotifs.Networkmotifsaremuchmoreprevalentinbiologicalnetworksthanwouldbeexpectedbycomparisontorandomnetworksandcomprisealmosttheentirenetworkstructure.Thesamesmallsetofnetworkmotifshasbeenfoundindiverseorgan-ismsrangingfrombacteriatoplantstohumans.Experimentsshowthateachnetworkmotifcancarryoutspecificdynamicfunctionsinthecomputationdonebythecell.Herewereviewthemainclassesofnetworkmotifsandtheirbiologicalfunctions.AutoregulationNegativeautoregulation(NAR)occurswhenatranscriptionfactorrepressesthetranscriptionofitsowngene.(Weusetranscriptiontohelpmakeexamplesconcrete;allcircuitsdescribedherecouldoperatealsoinotherregulatorymodes,e.g.,aproteininhibitingitsownactivitybyautophosphorylation.)ThisoccursinabouthalfoftherepressorsinE.coliandcanspeeduptheresponsetimeofgenecircuitsandreducecell–cellvariationinproteinlevelsthatareduetofluctuationsinproductionrate.Positiveautoregulationoccurswhenatranscriptionfactorenhancesitsownrateofproduction.Responsetimesareslowedandvariationisusuallyenhanced.Thismotif,givensufficientcooperativity,canleadtobimodal(all-or-none)distributions,wheretheconcentrationofXislowinsomecellsbuthighinothers.CascadesCascadesofgeneexpressioncreatesequentialactivationofgenes.Thedownstreamgeneisactivatedwhenitsregulatorreachestherelevantthreshold.Usingnegativeregula-tion,thegenescanbesequentiallyactivatedandrepressed.Positive-FeedbackLoopsDevelopmentaltranscriptionnetworksoftenusepositive-feedbackloopsthataremadeoftwotranscriptionfactorsthatregulateea...
