Modifier mutations of position‐effect variegation (PEV) represent a useful tool for a genetic and molecular dissection of genes connected with chromatin regulation in Drosophila. The Su(var)3‐9 gene belongs to the group of haplo suppressor loci which manifest a triplo enhancer effect. Mutations show a strong suppressor effect even in the presence of PEV enhancer mutations, indicating a central role of this gene in the regulation of PEV. By molecular analysis, Su(var)3‐9 could be correlated with a 2.4 kb transcript which encodes a putative protein of 635 amino acids containing a chromo domain and a region of homology to Enhancer of zeste and trithorax, two antagonistic regulators of the Antennapedia and Bithorax gene complexes, as well as to the human protein ALL‐1/Hrx which is implicated in acute leukemias. This region of homology is found in all four proteins at the C‐terminus. The homology of Su(var)3‐9 to both negative (Polycomb and Enhancer of zeste) and positive (trithorax) regulators of the Antennapedia and Bithorax complexes also suggests similarities in the molecular processes connected with stable transmission of a determined state and the clonal propagation of heterochromatinization.
In insects, peptidergic neurons of the central nervous system regulate the synthesis of the main developmental hormones. Neuropeptides involved in this neuroendocrine cascade have been identified in lepidopterans and dictyopterans. Since these organisms are not suitable for genetic research, we identified peptidergic brain neurons innervating the ring gland in Drosophila melanogaster. In larvae of Drosophila, ecdysteroids and juvenile hormones are produced by the ring gland, which is composed of the prothoracic gland, the corpus allatum, and the corpora cardiaca. Using the GAL4 enhancer trap system, we mapped those neurons of the central nervous system that innervate the ring gland. Eleven groups of neurosecretory neurons and their target tissues were identified. Five neurons of the lateral protocerebrum directly innervate the prothoracic gland or corpus allatum cells of the ring gland and are believed to regulate ecdysteroid and juvenile hormone titers. Axons of the circadian pacemaker neurons project onto dendritic fields of these five neurons. This connection might be the neuronal substrate of the circadian rhythms of molting and metamorphosis in Drosophila. Most of the neurons presented here have not been described before. The enhancer trap lines labeling them will be valuable tools for the analysis of neuronal as well as genetic regulation in insect development.
Pipsqueak (Psq) belongs to a family of proteins defined by a phylogenetically old protein-protein interaction motif. Like the GAGA factor and other members of this family, Psq is an important developmental regulator in Drosophila, having pleiotropic functions during oogenesis, embryonic pattern formation, and adult development. The GAGA factor controls the transcriptional activation of homeotic genes and other genes by binding to control elements containing the GAGAG consensus motif. Binding is associated with formation of an open chromatin structure that makes the control regions accessible to transcriptional activators. We show here that Psq contains a novel DNA-binding domain, which binds, like the GAGA factor zinc finger DNA-binding domain, to target sites containing the GAGAG consensus motif. Binding is suppressed, as in the GAGA factor and other proteins of the family, by the associated protein-protein interaction motif. The DNA-binding domain, which we call the Psq domain, is identical with a previously identified region consisting of four tandem repeats of a conserved 50-amino acid sequence, the Psq motif. The Psq domain seems to be structurally related to known DNA-binding domains, both in its repetitive character and in the putative three-␣-helix structure of the Psq motif, but it lacks the conserved sequence signatures of the classical eukaryotic DNA-binding motifs. Psq may thus represent the prototype of a new family of DNA-binding proteins. Members of the BTB1 /POZ protein family play important roles in development and reproduction of Drosophila melanogaster. These proteins contain a protein-protein interaction motif that was first identified in zinc finger proteins encoded by the Drosophila Broad-Complex and tramtrack genes (1, 2), and later also in the bric à brac gene product (3). The domain, which was thereupon designated as BTB (Broad-Complex, Tramtrack, Bric à brac) domain (3), has since been found in proteins of a variety of species, as diverse as slime molds (4) and humans (for a review, see Ref. 5). Many of these proteins are DNA-binding C 2 H 2 zinc finger proteins, but the presence of the domain in a family of pox virus proteins (6) soon indicated that coupling to a DNA-binding domain is not mandatory. The domain is therefore also referred to as the POZ (pox virus, zinc finger) domain (7). In BTB/POZ proteins that contain a zinc finger DNA-binding motif, DNA binding is strongly inhibited by the BTB/POZ domain. This inhibitory effect on DNA binding is also observed in chimeric proteins in which the BTB/POZ domain is associated with a heterologous DNA-binding domain, for instance a POU domain (7). Inhibition of DNA binding appears to be the result of oligomerization through proteinprotein interactions mediated by the BTB/POZ domain.The tendency of BTB/POZ proteins to oligomerize in solution and their localization in distinct nuclear substructures (7-10) suggests that they might act by modifying chromatin structure (5). In fact, such a mode of action is supported by different lines of eviden...
The steroid hormone 20‐hydroxyecdysone controls both induction and repression of the Drosophila ‘intermolt gene’ Sgs‐4. We show here that the ecdysone receptor binds to two sites, element I and element II, in the regulatory region of Sgs‐4. A functional analysis revealed that element II appears to be of no importance for Sgs‐4 expression, while element I proved to be an ecdysone response element that is necessary, but not sufficient, for induction of Sgs‐4 expression. Our results provide no evidence that repression of Sgs‐4 expression is mediated by one of the two receptor binding sites. In the close vicinity of elements I and II, we detected two binding sites of secretion enhancer binding protein 3 (SEBP 3). Like receptor element I, one of these sites also proved to be necessary, but not sufficient, for expression of Sgs‐4. Therefore, induction of Sgs‐4 requires binding of both ecdysone receptor and SEBP 3 to a complex hormone response unit, which also contains binding sites for a third factor, SEBP 2. The SEBP 2 sites coincide with binding sites of products of the Broad‐Complex locus, which has been implicated recently with transduction of the hormonal signal. Thus, the available data suggest that induction of Sgs‐4, and possibly other ‘intermolt genes’, is a combination of a primary and a secondary response to the hormone.
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