ReviewR2R3MYBTranscriptionFactors–FunctionsoutsidetheDNA-BindingDomainPeterS.Millard,1,2BirtheB.Kragelund,3andMeikeBurow1,2,*Severaltranscriptionfactor(TF)families,includingtheMYBfamily,regulateawidearrayofbiologicalprocesses.TFscontainDNA-bindingdomains(DBDs)andregulatoryregions;althoughinformationonproteinstructureisscarceforplantMYBTFs,variousinsilicomethodssuggestthatthenon-MYBregionscontainextensiveintrinsicallydisorderedregions(IDRs).AlthoughIDRsdonotfoldintostableglobularstructures,theycomprisefunctionalregionsincludinginteractionmotifs,andrecentresearchhasshownthatIDRsperformcrucialbiologicalroles.Wemapheredomainorganization,disorderpredictions,andfunctionalregionsacrosstheentireArabidopsisthalianaR2R3MYBTFfamily,andhighlightwhereanincreasedresearchfocuswillbenecessarytoshapeanewunderstandingofstructure–functionrelationshipsinplantTFs.ExpansionandFunctionalDiversificationofthePlantMYBProteinFamilyMYB(myeloblastosis)proteinsaretranscriptionfactors(TFs)thatwerefirstidentifiedinavianmyeloblastosisvirus,buthavesincebeenfoundinalleukaryotes[1].Whereasunikonts(thephy-logeneticgroupthatincludesfungiandanimals)usuallyencodelessthanahandfulofuniqueMYBgenes[2],thefamilyhasgreatlyexpandedintheplantlineage,whereindividualspeciescontainupto~200uniqueMYBgenes[3].MYBTFscontainaMYBDNA-bindingdomain(DBD)thatcomprises1–4imperfectMYBrepeats[2].Eachrepeatcovers50–55residuesthatfoldintothreeα-helices,thesecondandthirdofwhichformahelix–turn–helixstructure[2,4].MYBTFsaresubdividedbasedonthenumberofrepeats,andvertebrateMYBTFshavethreerepeats[5]orfourandahalfrepeats[6].However,thediversityofMYBdomainorganizationisincreasedintheplantlineage,andplantMYBTFscompriseeitherone,two,three,orfourrepeats[2].Inpar-ticular,thenumberofMYBTFscontainingtwoMYBrepeatsintheirDBDsisenormouslyex-pandedintheplantlineage(Figure1A).ThesearegiventhenameR2R3MYBproteinsbecausetheirtworepeatsaremostsimilartothesecondandthirdrepeatsofvertebrateMYBTFs[3,5].ManyplantspeciesencodeN100uniqueR2R3MYBTFs(Fi...
