Identical transacting factor requirement for knirps and knirps-related Gene expression in the anterior but not in the posterior region of the Drosophila embryo.

The Drosophila genes knirps (kni) and knirps-related (knrl) are located within the 77E1,2 region on the left arm of the third chromosome. They encode nuclear hormone-like transcription factors containing almost identical Cys2/Cys2 DNA-binding zinc finger motifs which bind to the same target sequence. kni is a member of the gap class of segmentation genes, and its activity is required for the normal establishment of the abdomen. The function of knrl is still unknown; however, a possible gap gene function in the abdominal region of the embryo can be excluded. Both genes are initially expressed in three identical regions of the blastoderm embryo: in an anterior cap domain, in an anterior stripe and in a posterior broad band linked to the kni gap gene function. The transacting factor requirement for the expression of kni and knrl is identical for the two anterior domains but different, although similar, for the posterior domain of expression in the blastoderm. Both the anteroposterior morphogen bicoid and the dorsoventral morphogen dorsal are necessary but not sufficient for the activation of the two genes in the anterior cap domain, suggesting they act together to bring about its normal spatial limits.