COMPLEX REGULATORY REGION MEDIATING TAILLESS EXPRESSION IN EARLY EMBRYONIC PATTERNING AND BRAIN-DEVELOPMENT

tailless encodes a transcription factor expressed in multiple domains in the developing embryo, Early and transient expression at the poster ior pole is required to establish a domain from which the eighth abdom inal segment, telson and posterior gut arise, Just a few nuclear cycle s later, a brain-specific domain is initiated at the anterior; express ion in this domain is maintained with complex modulations throughout e mbryogenesis. Expression of tailless in this domain is required to est ablish the most anterior region of the brain, To understand the functi on and regulation of these different domains of expression, we provide a detailed description of tailless expression in brain neuroblasts an d show that this expression is not detectably regulated by the head ga p genes buttonhead or orthodenticle, by the proneural gene lethal of s cute or by tailless itself, We show that approximately 6 kb of sequenc ed upstream regulatory DNA can drive lacZ expression in a pattern that mimics the full tailless embryonic expression pattern. Within this se quence we identify multiple modules responsible for different aspects of the tailless pattern, In addition to identifying additional torso r esponse elements that mediate early blastoderm polar expression, we sh ow that the complex brain expression pattern is driven by a combinatio n of modules; thus expression at a low level throughout the brain and at a high level in the dorsal medial portion of the brain and in the o ptic lobe, as well as neuroblast-specific repression are mediated by d ifferent DNA regions.