Xiro-1 controls mesoderm patterning by repressing bmp-4 expression in the Spemann organizer

Abstract

Th e Iroquoi s gene scod eforho- meodomai nprotein stha thav ebee nimplicated intheneura ldevelopmen tof Drosophil a an dver- tebrates .Wesho w her eforthefirs ttim ethat Xiro-1 ,on eofthe Xenopu sIroquoi s genes ,isex- presse dintheSpeman norganize rfro m thestart ofgastrulatio n an d tha titsoverexpressio n in- duce sa secondar y axi saswel lastheectopic expressio n ofsevera lorganize rgenes ,suc h as chordin ,goosecoid ,an d Xlim-1 .Ou rresult salso indicat etha t Xiro- 1 normall yfunction sasa tran- scriptiona lrepresso rinthemesoderm .Overex- pressio nofXiro- 1orachimeri cfor m fuse dtothe represso rdomai nofEngraile dcaus esimila rphe- notype swhil eoverexpressio n offunctiona lde- rivative sof Xiro- 1 fuse dwit htransactivatio ndo- main s (VP1 6 or E1A ) produc e the opposite effects .Finally ,wesho w tha t Xiro- 1 work sasa represso rof bmp- 4 transcriptio nan dtha titsef- fec tonorganize rdevelopmen tisdependen ton BMP- 4activity .Wepropos etha tthepreviously observe ddow nregulatio nof bmp- 4 inthedorsal mesoder m durin ggastrulatio nca nbeexplained by the represso r activit y of Xiro- 1 described here .Thus , Xiro- 1 seem stohav eatleas ttw odif- feren tfunctions :contro lofneura lplat ean dor- ganize rdevelopment ,bot hofwhic hcoul dbeme- diate d by repressio n of bmp- 4 transcription.