POSITIONING ADJACENT PAIR-RULE STRIPES IN THE POSTERIOR DROSOPHILA EMBRYO

We present a genetic and molecular analysis of two hairy (h) pair-rule stripes in order to determine how gradients of gap proteins position adjacent stripes of gene expression in the posterior of Drosophila emb ryos. We have delimited regulatory sequences critical for the expressi on of h stripes 5 and 6 to 302 bp and 526 bp fragments, respectively, and assayed the expression of stripe-specific reporter constructs in s everal gap mutant backgrounds. We demonstrate that posterior stripe bo undaries are established by gap protein repressors unique to each stri pe: h stripe 5 is repressed by the giant (gt) protein on its posterior border and h stripe 6 is repressed by the hunchback (hb) protein on i ts posterior border. Interestingly, Kruppel (Kr) limits the anterior e xpression limits of both stripes and is the only gap gene to do so, in dicating that stripes 5 and 6 may be coordinately positioned by the Kr repressor. In contrast to these very similar cases of spatial repress ion, stripes 5 and 6 appear to be activated by different mechanisms. S tripe 6 is critically dependent upon knirps (kni) for activation, whil e stripe 5 likely requires a combination of activating proteins (gap a nd non-gap). To begin a mechanistic understanding of stripe formation, we locate binding sites for the Kr protein in both stripe enhancers. The stripe 6 enhancer contains higher affinity Kr-binding sites than t he stripe 5 enhancer, which may allow for the two stripes to be repres sed at different Kr protein concentration thresholds. We also demonstr ate that the kni activator binds to the stripe 6 enhancer and present evidence for a competitive mechanism of Kr repression of stripe 6.