THE HELIX-LOOP-HELIX EXTRAMACROCHAETAE PROTEIN IS REQUIRED FOR PROPERSPECIFICATION OF MANY CELL-TYPES IN THE DROSOPHILA EMBRYO

The Drosophila Extramacrochaetae protein antagonizes the proneural fun ction of the Achaete and Scute proteins in the generation of the adult fly sensory organs. Extramacrochaetae sequesters these basic-region-h elix-loop-helix transcription factors as heterodimers inefficient for binding to DNA. We show that, during embryonic development, the extram acrochaetae gene is expressed in complex patterns that comprise deriva tives of the three embryonic layers. Expression of extramacrochaetae o ften precedes and accompanies morphogenetic movements. It also occurs at regions of specialized cell-cell contact and/or cell recognition, l ike the epidermal part of the muscle attachment sites and the differen tiating CNS. The insufficiency of extramacrochaetae affects most tissu es where it is expressed. The defects suggest faulty specification of different cell types and result in impairment of processes as diverse as cell proliferation and commitment, cell adhesion and cell recogniti on. If Extramacrochaetae participates in cell specification by dimeriz ing with basic-region-helix-loop-helix proteins, the variety of defect s and tissues affected by the insufficiency of extramacrochaetae sugge sts that helix-loop-helix proteins are involved in many embryonic deve lopmental processes.