Dorsal-Ventral Patterning and Gene Regulation in the Early Embryo of Drosophila melanogaster

In order for an embryo to develop and form properly, the anterior-posterior and dorsal-ventral axes must be specified. This is accomplished by controlled regulation of gene expression that allows for the activation and repression of tissue specific genes. Patterning of the Drosophila dorsal-ventral axis is an excellent model for understanding how axis specification is controlled. The dorsal-ventral axis is patterned by a nuclear gradient of Dorsal that is highest in ventral regions of the embryo. Dorsal activates genes in a concentration dependent manner to establish early patterning of the embryo. The patterns are refined by interactions between Dorsal and other activators as well as repressors in both dorsal and ventral regions of the embryo. Until recently there was only evidence for repressors acting in ventral regions of the embryo but our studies and other recent studies have provided evidence to suggest that several repressors act in dorsal regions of the embryo to refine Dorsal target genes. Here we show that an element, the A-box, previously identified in the cis-regulatory module (CRM) of the gene intermediate neuroblast defective (ind), is necessary and sufficient to mediate dorsal repression and is also involved in activation of ind. We conducted an affinity chromatography assay and identified factors that bound the A-box element. One of the factors that bound to this element, Grh, activates ind. We also identified several chromatin-remodeling factors that may function to silence ind in dorsal regions of the embryo. Our results also indicate that a second tier of repression that is independent of the A-box element, mediates repression of ind via Dpp-signaling. We extended our studies to the CRMs of ventral neuroblast defective (vnd) and short gastrulation (sog). Using a chimeric CRM repression assay, we found that strong and weak dorsal repression are also mediated by the vnd and sog CRMs, respectively. This suggests that limiting amounts of Dorsal are not sufficient to establish the dorsal borders of dorsal-ventral patterning genes as was previously believed, and rather, repressors are used to establish these borders.