Pair-rule gene runt restricts orthodenticle expression to the presumptive head of the Drosophila embryo.

Ectopic expression of the pair-rule gene runt in the anterior end of the Drosophila embryo antagonizes transcriptional activation of the head gap gene orthodenticle (otd) by the anterior morphogen bicoid. Here we investigate the relevance of runt's activity as a repressor of otd in normal Drosophila embryogenesis otd expression is activated in the posterior region of embryos that are mutant for runt. This posterior expression domain of otd depends on the activity of the orphan nuclear receptor protein Tailless. Repression of otd by runt does not require the conserved VVVRPY motif that mediates interaction between Runt and the co-repressor protein Groucho. The observed functional interactions between runt and tailless on otd expression may indicate there are other contexts where members of these two families of transcriptional regulators interact to regulate gene expression during development.