Regulation and function of Scr, exd, and hth in the Drosophila salivary gland

Salivary gland formation in the Drosophila embryo is dependent on the homeo tic gene Sex combs reduced (Scr). When Scr function is missing, salivary gl ands do not form, and when SCR is expressed everywhere in the embryo, saliv ary glands form in new places. Scr is normally expressed in all the cells t hat form the salivary gland. However, as the salivary gland invaginates, Sc r mRNA and protein disappear. Homeotic genes, such as Scr, specify tissue i dentity by regulating the expression of downstream target genes. For many h omeotic proteins, target gene specificity is achieved by cooperatively bind ing DNA with cofactors, Therefore, it is likely that SCR also requires a co factor(s) to specifically bind to DNA and regulate salivary gland target ge ne expression. Here, we show that two homeodomain-containing proteins encod ed by the extradenticle (exd) and homothorax (hth) genes are also required for salivary gland formation. exd and hth function at two levels: (1) exd a nd hth are required to maintain the expression of Scr in the salivary gland primordia prior to invagination and (2) exd and hth are required in parall el with Scr to regulate the expression of downstream salivary gland genes. We also show that Scr regulates the nuclear localization of EXD in the sali vary gland primordia through repression of homothorax (hth) expression, lin king the regulation of Scr activity to the disappearance of Scr expression in invaginating salivary glands. (C) 2000 Academic Press.