Activation and repression of transcription by homoeodomain-containing proteins that bind a common site

Abstract

The product of the fushi tarazu (ftz) gene is shown to be a site-dependent activator of transcription. In vitro-defined binding sites act as ftz-dependent enhancers in cultured cells. Another homoeodomain-containing protein, the engrailed gene product, competes for homoeodomainbinding sites and counteracts ftz activation.GENETIC analysis in Drosophila melanogaster has identified a group of regulatory genes that participate in embryonic pattern formation 1,2 . Many of these genes encode a 60 amino-acid sequence, the homoeodomain, that is highly conserved through evolution [3][4][5] . This domain contains a putative helix-turn-helix motif 6 such as that found in a number of bacterial regulators 7 , and possesses a sequence-specific DNA-binding activity 8,9 . Numerous regulatory interactions occur among homoeodomain-encoding genes during development. For example, a combination of segmentation and homoeotic gene products, many of which contain homoeodomains 1 , specify the developmental fates of cells in striped patterns 10,11 . Furthermore, homoeodomain-containing proteins (homoeodomain proteins) regulate both each other 12 and downstream (`cytodifferentiation' 13 ) genes. Consequently, it has been attractive to think of homoeodomain proteins as direct regulators of transcription. Due to the complexity of cross-regulatory interactions however, mutations altering or eliminating one homoeodomain protein generally change the expression patterns of many other regulators, confounding efforts to distinguish direct from indirect regulation. Additionally, suspected target genes contain large cis-acting control regions that respond, directly or indirectly, to complex combinations of regulators [14][15][16][17] . As a result, identification of target genes directly regulated by homoeodomain proteins has been problematic.Previous studies have documented instances in which homoeodomain proteins control transcription. The enhancer activity of a DNA fragment from the ftz gene, a homoeodomaincontaining member of the pair-rule class of segmentation genes, suggested that the ftz gene product (ftz) activates its own expression in the embryo 18 . In a different approach a Drosophila tissue culture system was used to show that the Ultrabithorax (Ubx) gene product induces expression of its own promoter, and inhibits the Antennapedia (Antp) (P1) promoter (M. Krasnow and D. S. Hogness, personal communication, see ref. 19 for summary). In these cases however, the complexity of the regulatory circuitry and of the responding control regions leaves open the possibility that the homoeodomain proteins produce these transcriptional effects through intermediaries. To circumvent the complexities of natural target genes, we chose to use binding sites identified in vitro to build homoeodomain-responsive promoters. We find that ftz is a potent activator of transcription from these promoters in cultured cells.During sequential subdivision of the Drosophila embryo to produce segmentally repeating patterns, combinations of regulatory gene prod...

A sequence-specific DNA-binding protein that activates fushi tarazu segmentation gene expression.

“…The 413-amino-acid ftz polypeptide contains a homeo domain, suggesting that the protein can bind to DNA (for review, see Gehring and Hiromi 1986;Gehring 1987;Scott et al 1990). Furthermore, the ftz gene is able to activate transcription of a reporter gene when introduced into cultured Drosophila cells (Jaynes and O'Farrell 1988;Han et al 1989;Winslow et al 1989) or in the yeast Saccharornyces cerevisiae (Fitzpatrick and Ingles 1989). The results suggest that the ftz protein functions as a transcription factor in these assays.…”

Modulation of the IgH enhancer's cell type specificity through a genetic switch.

“…In higher eukaryotes, this is best exemplified by the competition among homeo box proteins in Drosophila and by competition among steroid receptor proteins in mammalian cells. In transfection experiments, the Drosophila even-skipped (eve] and engrailed [en] homeo box proteins are potent repressors of the stimulatory activities of both z2 and fushi tarazu homeo box proteins (Jaynes and OTarrell 1988;Han et al 1989). Although all of these proteins are able to bind similar DNA sequences, it is not yet clear whether the repressive activity of eve and en are merely a consequence of passive competition for activator-binding sites or a consequence of more interactive inhibitory mechanisms.…”

Pax2 modulates proliferation during specification of the otic and epibranchial placodes

“…2D, E; n¼4/6). A second approach to reduce activity was the construction of a repressor form of Pax2; fusing the open reading frame of Pax2 to the powerful transcriptional repressor module from the engrailed transcription factor (Pax2-EnR) (Jaynes and O'Farrell, 1988). As Pax2 normally functions as an activator, such a fusion construct acts to completely repress endogenous Pax2 activity (Conlon Fig.…”