Segmental determination in Drosophila conferred by hunchback (hb), a repressor of the homeotic gene Ultrabithorax (Ubx).

Abstract

The activity of homeotic genes in Drosophila cells determines segment-specific morphogenesis. Here, we provide evidence that the product of hunchback (hb), a segmentation gene, acts as a direct repressor or "silencer" od the homeotic gene Ultrabithorax (Ubx) Segmental determination is conferred by the activity of homeotic genes such as Ultrabithorax (Ubx), whose product is necessary (3, 4) and to some extent sufficient (5,6) to control the morphogenesis of segment-specific features. The Ubx gene is activated in a midembryonic region of the blastoderm embryo (7), and its continuous activity in the descendants of these embryonic cells is needed until differentiation starts (8).Boundaries of early Ubx expression depend on the segmentation gene hunchback (hb) (9-11), whose product carries anteroposterior positional information by virtue of its restricted distribution in the early embryo: hb protein (HB) is found at high levels anteriorly as well as posteriorly in the blastoderm embryo (12), in embryonic regions that are nearcomplementary to the early Ubx expression domain (7). HB binding sites were found within Ubx control elements, and these apparently mediate repression of the Ubx gene at the blastoderm stage (13,14). HB fades to undetectable levels soon after this stage (12), and hb function becomes dispensable from there onwards (10,15,16). Maintenance of Ubx expression boundaries throughout later stages depends on the function of a group ofgenes (4, 17, 18) of which Polycomb (Pc) is the best known member.Previously, we found that Ubx expression boundaries at advanced embryonic stages are mediated by repressor control elements located in remote regions of the Ubx gene (19). These elements act at long distance to suppress outside the Ubx expression domain the activity of an embryonic Ubx enhancer element (termed BXD) which, in the absence of the repressor elements, confers a Ubx-like expression pattern in virtually every parasegnent (ps) from head to tail. One of the repressor elements contains HB binding sites (14). Here, we provide evidence that HB acts directly to repress BXDmediated activity. As this repression occurs at long distance, we shall refer to it as "silencing" (20). Moreover, we show that the silencing activity from HB binding sites is evident at late embryonic stages during which HB is no longer detectable or required. We discuss a mechanism by which HB may effect Ubx repression and thus confer segmental determination in a "hit-and-run" fashion. MATERIALS AND METHODSPlasmids. The basic BXD construct (based on a Carnegie 20 transformation vector) containing the minimal BXD control fragment linked to the proximal Ubx promoter and a f3-galactosidase (a-gal) gene has been described (19). Unique Not I and Kpn I sites were engineered between the BXD fragment and the proximal Ubx promoter so that fragments and oligonucleotides could be inserted and tested for their silencing capacity. Oligonucleotides (hb-mes, hb-ep, and hb-mut, a mutant derivative of hb-mes) were designed to contain all consec...

Helios, a T cell-restricted Ikaros family member that quantitatively associates with Ikaros at centromeric heterochromatin

“…Such a function would be similar to that proposed for the Drosophila Hunchback protein, which contains zinc finger domains that are highly related to those in Ikaros and Helios. Hunchback acts as a simple activator during embryogenesis and also has been proposed to establish silencing complexes in Drosophila by recruiting Polycomb-group proteins (Zhang and Bienz 1992;Poux et al 1996).…”

Delimiting the conserved features ofhunchbackfunction for the trunk organization of insects

“…The anterior domain of Dm'hb limits the anterior borders of the Dm'Ubx and Dm'Antp expression domains (Irish et al, 1989;Lehmann and Nüsslein-Volhard, 1987;Qian et al, 1991;White and Lehmann, 1986;Zhang and Bienz, 1992). However, the effects caused by this ectopic expression of Hox genes in Dm'hb mutants are often concealed by the segmentation defects, as the Hox genes are ectopically expressed in the segments that are deleted in the larvae (Lehmann and Nüsslein-Volhard, 1987).…”

General transcriptional silencing by a Polycomb response element inDrosophila

“…Importantly, none of the three enhancers used in this study is active in the early embryo. Moreover, these enhancers probably do not contain binding sites for specific transcriptional repressors, such as the gap repressors, which are required for establishment of PcG silencing at some PREs in the early embryo (Zhang and Bienz, 1992). We therefore imagine that, in our constructs, PcG silencing complexes assemble by default on the 1.6 kb Ubx PRE in the early embryo and that PcG silencing is thus firmly established by the stage when the imaginal discs enhancers would become active.…”