A dual role for the protein kinase shaggy in the repression of achaete-scute.

achaete and scute are expressed in a spatially restricted pattern and provide neural potential to cells. The domains of expression depend partly on extramacrochaetae whose product is itself spatially restricted and acts as a negative post-translational regulator of achaete and scute. The protein kinase shaggy also represses achaete and scute at many sites but may act via intermediate transcription factors. However shaggy and extramacrochaetae act synergistically and molecular studies suggest that they may be part of the same pathway. shaggy is functionally homologous to the mammalian glycogen synthase kinase-3 and analogy with the known physiology of this enzyme, suggests that this function of shaggy may result from the "constitutive" activity. At the site where a single neural precursor will develop, achaete and scute are initially expressed in a group of equivalent cells. The genes Notch and Delta are part of a lateral signal required to single out one precursor cell and to silence achaete and scute expression in the other cells. shaggy is required downstream of Notch for transduction of the inhibitory signal. This second role of shaggy may be due to modulation of enzymatic activity during signalling.