THE DROSOPHILA NUCLEAR RECEPTORS FTZ-F1-ALPHA AND FTZ-F1-BETA COMPETEAS MONOMERS FOR BINDING TO A SITE IN THE FUSHI-TARAZU GENE

The striped pattern of fushi tarazu (ftz) expression found in the blas toderm of the Drosophila melanogaster embryo is generated largely thro ugh complex interactions between multiple transcription factors that b ind to the zebra element of the ftz gene. A motif in the zebra element , the FTZ-F1 recognition element (F1RE), has been shown to bind a tran scription factor, FTZ-F1 alpha, that is a member of the nuclear recept or family. We recently identified a second, related member of this fam ily, FTZ-F1 beta, that also binds to this motif. To investigate the po ssibility that FTZ-F1 alpha and FTZ-F1 beta coregulate ftz transcripti on through the F1RE, we have studied the DNA binding properties of FTZ -F1 alpha and FTZ-F1 beta. We demonstrate that recombinant FTZ-F1 alph a and FTZ-F1 beta proteins produce similar in vitro DNase I footprint patterns on a 14-nucleotide region of the zebra element and bind to th is site with similar affinities and sequence specificities. Using wild type and N-terminally truncated receptors, we have determined that FT Z-F1 alpha and FTZ-F1 beta both bind as monomers to the 9-bp F1RE in t he zebra element, as well as to an imperfect inverted F1RE repeat pres ent in the Drosophila alcohol dehydrogenase gene. A polyclonal antibod y raised against FTZ-F1 beta identifies a predominant F1RE-binding com ponent in embryonic nuclear extracts. Although FTZ-F1 alpha is also pr esent in these extracts, FTZ-F1 alpha and FTZ-F1 beta do not appear to form heterodimers with each other. Cotransfection assays in mammalian cell culture indicate that both receptors contribute to the net trans criptional activity of a reporter gene through their direct interactio n with the F1RE. These data suggest that FTZ-F1 alpha and FTZ-F1 beta likely coregulate common target genes by competition for binding to a 9-bp recognition element.