SETTING LIMITS ON HOMEOTIC GENE-FUNCTION - RESTRAINT OF SEX-COMBS-REDUCED ACTIVITY BY TEASHIRT AND OTHER HOMEOTIC GENES

Each of the homeotic genes of the HOM or HOX complexes is expressed in a limited domain along the anterior-posterior axis. Each homeotic pro tein directs the formation of characteristic structures, such as wings or ribs. In flies, when a heat shock-inducible homeotic gene is used to produce a homeotic protein in all cells of the embryo, only some ce lls respond by altering their fates. We have identified genes that lim it where the homeotic gene Sex combs reduced (Scr) can affect cell fat es in the Drosophila embryo. In the abdominal cuticle Scr is prevented from inducing prothoracic structures by the three bithorax complex (B X-C) homeotic genes. However, two of the BX-C homeotic genes, Ultrabit horax (Ubx) and abdominal-A (abd-A), have no effect on the ability of Scr to direct the formation of salivary glands. Instead, salivary glan d induction by Scr is limited in the trunk by the homeotic gene teashi rt (tsh) and in the last abdominal segment by the third BX-C gene, Abd ominal-B (AbdB). Therefore, spatial restrictions on homeotic gene acti vity differ between tissues and result both from the regulation of hom eotic gene transcription and from restraints on where homeotic protein s can function.