Identification of an upstream regulatory element reveals a novel requirement for Ind activity in maintaining ind expression

A maternally established gradient of nuclear Dorsal protein is the first step in subdivision of the Drosophila neurectoderm into stripes of homeodomain gene expression. Dorsal in combination with the EGF and TGFβ signaling pathways are key regulators of the expression of the genes ventral nervous system defective (vnd), intermediate neuroblasts defective (ind), and muscle segment homeobox (msh) in the developing neurectoderm. These three genes encode homeodomain transcription factors that can repress each other, which ensures adjacent, non-overlapping expression domains. Expression of vnd, ind, and msh is maintained after decline in EGF and TGFβ signaling, but the relevant positive transcriptional regulators have not yet been defined. Here, we show that Ind can bind DNA with the same sequence specificity as its murine ortholog Gsh1. We have identified a novel upstream regulatory element at the ind locus containing predicted Ind binding sites, and we show that Ind activity is both necessary and sufficient for reporter gene expression from this element. We conclude that Ind can act as a transcriptional activator, and that positive autoregulation of Ind is a mechanism for persistent ind expression within the developing embryonic nervous system.