The Drosophila nuclear receptors FTZ-F1α and FTZ-F1β compete as monomers for binding to a site in the fushi tarazu gene

The striped pattern of fushi tarazu (ftz) expression found in the blastoderm of the Drosophila melanogaster embryo is generated largely through complex interactions between multiple transcription factors that bind to the zebra element of the ftz gene. A motif in the zebra element, the FTZ-F1 recognition element (FIRE), has been shown to bind a transcription factor, FTZ-F1α, that is a member of the nuclear receptor family. We recently identified a second, related member of this family, FTZ-F1β, that also binds to this motif. To investigate the possibility that FTZ-F1α and FTZ-F1β coregulate ftz transcription through the FIRE, we have studied the DNA binding properties of FTZ-F1α and FTZ-F1β. We demonstrate that recombinant FTZ-F1α and FTZ-F1β proteins produce similar in vitro DNase I footprint patterns on a 14-nucleotide region of the zebra element and bind to this site with similar affinities and sequence specificities. Using wild-type and N- terminally truncated receptors, we have determined that FTZ-F1α and FTZ- F1β both bind as monomers to the 9-bp FIRE in the zebra element, as well as to an imperfect inverted FIRE repeat present in the Drosophila alcohol dehydrogenase gene. A polyclonal antibody raised against FTZ-F1β identifies a predominant FIRE-binding component in embryonic nuclear extracts. Although FTZ-F1α is also present in these extracts. FTZ-F1α and FTZ-F1β do not appear to form heterodimers with each other. Cotransfection assays in mammalian cell culture indicate that both receptors contribute to the net transcriptional activity of a reporter gene through their direct interaction with the FIRE. These data suggest that FTZ-F1α and FTZ-F1β likely coregulate common target genes by competition for binding to a 9-bp recognition element.