Transcriptional regulation of a pair-rule stripe in Drosophila.

The periodic, seven-stripe pattern of the primary pair-rule gene even-skipped (eve) is initiated by crude, overlapping gradients of maternal and gap gene proteins in the early Drosophila embryo. Previous genetic studies suggest that one of the stripes, stripe 2, is initiated by the maternal morphogen bicoid (bcd) and the gap protein hunchback (hb), while the borders of the stripe are formed by selective repression, involving the gap protein giant (gt) in anterior regions and the Krüppel (Kr) protein in posterior regions. Here, we present several lines of evidence that are consistent with this model for stripe 2 expression, including in vitro DNA-binding experiments and transient cotransfection assays in cultured cells. These experiments suggest that repression involves a competition or short-range quenching mechanism, whereby the binding of gt and Kr interferes with the binding or activity of bcd and hb activators at overlapping or neighboring sites within the eve stripe 2 promoter element. Such short-range repression could reflect a general property of promoters composed of multiple, but autonomous regulatory elements.