THE ACHAETE-SCUTE COMPLEX AS AN INTEGRATING DEVICE

A classical model to study pattern formation is provided by the epider mal sensory organs (bristles and other sensilla) that cover the body o f Drosophila. Many of these sensory organs (SOs) arise in very constan t positions. How are these positions specified? To a large extent, the y are defined by the highly resolved sites of expression of the proneu ral genes of the achaete-scute complex (AS-C). These genes, which conf er to cells the capacity to become SO precursors, attain their resolve d patterns of expression by means of many position-specific enhancers located within the non-transcribed AS-C DNA. Each enhancer drives expr ession at one or very few sites. Evidence is growing that the enhancer s interact with combinations of activators and repressors (prepattern) distributed in partially overlapping domains which are larger than th e AS-C expressing sites. ASC transcription is activated only at sites with appropriate combinations of factors. Thus, the AS-C integrates th e positional information embodied in the relatively broad distribution s of prepattern factors and creates a sharper and topographically more precise pattern.