Expression, function, and regulation of the hairy segmentation protein in the Drosophila embryo.

The Drosophila pair-rule segmentation gene, hairy, is required for the proper development of alternate embryonic segment primordia and the normal spatial expression of another pair-rule gene, fushi tarazu (ftz). We demonstrate that the product of the hairy gene is localized to the nuclei of cells in eight distinct regions of the early embryo. The major stripes of protein accumulation comprise the posterior half and the adjacent anterior portion of alternate segment primordia; the hairy pattern overlaps transiently with ftz expression in a narrow band of cells. Unlike other pair-rule products that have been studied, hairy is not expressed in the embryonic nervous system. The spatial distribution of the nuclear hairy protein and the pattern of ftz expression in embryos lacking hairy function suggests that hairy plays a direct role in repressing ftz gene expression. The pattern of hairy expression in gap gene mutants that affect ftz expression indicates that the gap genes largely act indirectly upon ftz through their effects on hairy.