Identifying Targets of FTZ and FTZ-F1

Homeobox (Hox) proteins regulate cell fate and segmental identity throughout the animal kingdom. Yet, given the homeodomain's promiscuous DNA binding activity in vitro, it is unclear how they select downstream target genes. The product of fushi tarzu (ftz) is a Hox protein responsible for the development of alternate parasegments within the Drosophila embryo. It has been shown that FTZ interacts with a cofactor, the orphan nuclear receptor FTZ-F1, to cooperatively bind DNA and synergistically regulate transcription (1, 2). ftz-f1 is maternally deposited and ubiquitously expressed, while ftz is zygotically expressed in seven stripes at the blastoderm stage. The phenotypes of ftz and ftz-f1 null mutants are identical, demonstrating an in vivo requirement for their functional interaction in promoting the development of alternate parasegments. The two best characterized FTZ/FTZ-F1targets are engrailed (en) and ftz itself. FTZ-F1 interacts with FTZ to positively autoregulate ftz gene expression; similarly, FTZ and FTZ-F1 coordinately regulate alternate stripes of en (3). In order to extend this list, the genome of Drosophila melanogaster was screened for FTZ and FTZ-F1 binding sites with known cooperative spacing and orientations. Genes having clusters of five or more FTZ/FTZ-F1 sites within 20 kb of their transcription start sites were selected for further analysis. Two candidate target genes, Dsulf1 and apt, were identified with this approach (4). We are currently extending this search for realizator genes using Affymetrix microarrays to identify genes that are differentially expressed in wild-type embryos and ftz-f1 mutants through successive stages of cellularization and gastrulation.