HOW DOES THE FUSHI-TARAZU GENE ACTIVATE ENGRAILED IN THE DROSOPHILA EMBRYO

In the even-numbered parasegments of the Drosophila embryo, expression of the fushi tarazu (ftz) gene is necessary for transcription of engr ailed (en). Yet those cells expressing ftz(+) in a stripe; only the an teriormost come to express en. One explanation is that the level of ft z(+) might be graded across the stripe and in order to express en, ii would be sufficient for cells to exceed a threshold concentration of F it protein. We use photographs and microspectrophotometry io measure d ifferences in Fit antigen concentration; we do not find a gradient wit hin the Fit stripe. Rather, the stripe appears io contain cells with s imilar amounts of antigen plus a few weakly staining cells that are us ually at the posterior edge. Further, varying the amount of Ftz protei n has no effect on en expression. Finally, embryos lacking the even-sk ipped gene have normal levels of Ftz but do not express en. Our observ ations appear to rule out the threshold hypothesis. (C) 1998 Wiley-Lis s, Inc.