CHARACTERIZATION OF THE CIS-REGULATORY REGION OF THE DROSOPHILA HOMEOTIC GENE SEX COMBS REDUCED

The Drosophila homeotic gene Sex combs reduced (Scr) controls the segm ental identity of the labial and prothoracic segments in the embryo an d adult. It encodes a sequence-specific transcription factor that cont rols, in concert with other gene products, differentiative pathways of tissues in which Scr is expressed. During embryogenesis, Scr accumula tion is observed in a discrete spatiotemporal pattern that includes th e labial and prothoracic ectoderm, the subesophageal ganglion of the v entral nerve cord and the visceral mesoderm of the anterior and poster ior midgut. Previous analyses have demonstrated that breakpoint mutati ons located in a 75-kb interval, including the Scr transcription unit and 50 kb of upstream DNA, cause Scr misexpression during development, presumably because these mutations remove Scr cis-regulatory sequence s from the proximity of the Scr promoter. To gain a better understandi ng of the regulatory interactions necessary for the control of Scr tra nscription during embryogenesis, we have begun a molecular analysis of the Scr regulatory interval. DNA fragments from this 75-kb region wer e subcloned into P-element vectors containing either an Scr-lacZ or hs p70-lacZ fusion gene, and patterns of reporter gene expression were as sayed in transgenic embryos. Several fragments appear to contain Scr r egulatory sequences, as they direct reporter gene expression in patter ns similar to those normally observed for Scr, whereas other DNA fragm ents direct Scr reporter gene expression in developmentally interestin g but non-Scr-like patterns during embryogenesis. Scr expression in so me tissues appears to be controlled by multiple regulatory elements th at are separated, in some cases, by more than 20 kb of intervening DNA . Interestingly, regulatory sequences that direct reporter gene expres sion in an Scr-like pattern in the anterior and posterior midgut are i mbedded in the regulatory region of the segmentation gene fushi tarazu (ftz), which is normally located between 10 and 20 kb 5' of the Scr t ranscription start site. This analysis provides an entry point for the study of how Scr transcription is regulated at the molecular level.