CIS-REGULATION OF ACHAETE AND SCUTE - SHARED ENHANCER-LIKE ELEMENTS DRIVE THEIR COEXPRESSION IN PRONEURAL CLUSTERS OF THE IMAGINAL DISCS

The pattern of bristles and other sensory organs on the adult cuticle of Drosophila is prefigured in the imaginal discs by the pattern of ex pression of the proneural achaete (ac) and scute (sc) genes, two membe rs of the ac-sc complex (AS-C). These genes are simultaneously express ed by groups of cells (the proneural clusters) located at constant pos itions in discs. Their products (transcription factors of the basic-he lix-loop-helix family) allow cells to become sensory organ mother cell s (SMCs), a fate normally realized by only one or a few cells per clus ter. Here we show that the highly complex pattern of proneural cluster s is constructed piecemeal, by the action on ac and sc of site-specifi c, enhancer-like elements distributed along most of the AS-C (similar to 90 kb). Fragments of AS-C DNA containing these enhancers drive repo rter lacZ genes in only one or a few proneural clusters. This expressi on is independent of the ac and sc endogenous genes, indicating that t he enhancers respond to local combinations of factors (prepattern). We show further that the cross-activation between ac and sc, discovered by means of transgenes containing either ac or sc promoter fragments l inked to lacZ and thought to explain the almost identical patterns of ac and sc expression, does not occur detectably between the endogenous ac and sc genes in most proneural clusters. Our data indicate that co expression is accomplished by activation of both ac and sc by the same set of position-specific enhancers.