Ecdysteroid-regulated gene expression in Drosophila melanogaster

The expression of insecticide resistance-related cytochrome P450 forms is regulated by molting hormone in Drosophila melanogaster

The expression and enzymatic activities of insecticide resistance-related cytochrome P450B are increased by the treatment with 20-hydroxyecdysone (20HE) in D. melanogaster Oregon R flies. We have explored the role of this hormone in the maintenance of P450B basal expression. Arrest of ecdysone synthesis led to a decrease in CYP6A2 mRNA level, as well as in P450B expression and activities. This effect occurred both in insecticide susceptible (ecd1) and resistant (IRED) strains carrying the temperature-sensitive ecd mutation. The role of the 20HE in the regulation of cytochrome P450-mediated insecticide resistance has been proposed.

The role of the BR-C locus on the expression of genes located at the ecdysone-regulated 3C puff of Drosophila melanogaster

During the third larval instar, the steroid moulting hormone ecdysone activates three temporally distinct puff sets on the D. melanogaster salivary gland polytene chromosome: the so-called intermoult, early and late puffs. Hormonal regulation of intermoult puffs is quite complex and, so far, largely not understood. In order to further investigate this aspect, we have analysed the effects of mutations in a key regulator of the ecdysone response at the onset of metamorphosis, the Broad-Complex (BR-C) locus, on the expression of genes mapping at the 3C intermoult puff. On the basis of an accurate examination of 3C intermoult gene activity in single, carefully staged, third instar larvae of wild-type and BR-C mutant strains, we were able to subdivide these genes into two groups. Each group is characterised by a different temporal expression profile, so that at the beginning of the wandering stage the transcription of the first group declines as group II transcription is induced. Interestingly, the BR-C locus appears to play a regulatory role in establishing this transcriptional switch. By using mutants of each of the three lethal complementation groups, we precisely defined the role of BR-C functions in this developmental transition and we show that this locus also plays an essential role in the early pre-metamorphic hormonal response.

Regulatory aspects of esterase 6 activity variation in sibling Drosophila species

Esterase 6 in Drosophila melanogaster, Drosophila simulans and Drosophila mauritiana is produced in several life stages and diverse tissues, but the major pulse of expression is in the sperm ejaculatory duct of adult males. Comparison of EST6 activity levels among several lines of D. melanogaster, D. simulans and D. mauritiana reveals two major quantitative differences among the species. First, newly eclosed females of both D. simulans and D. mauritiana show significantly higher EST6 activity than those of D. melanogaster. Secondly, 5-day-old adult D. simulans have significantly higher activities than D. mauritiana in both sexes and significantly higher activity than D. melanogaster in males. The genetic bases of the differences between D. melanogaster and the other species are investigated through germ line transfer of the D. simulans and D. mauritiana Est-6 genes plus 1.2 kb of 5' and 0.2 kb of their 3' flanking sequences into D. melanogaster. The newly eclosed female activities of the transformants resemble those of the two donor species, suggesting that the interspecific differences in this aspect of expression are due to cis-inherited factors contained within the transferred DNA. In contrast, the 5-day adult activity of the D. simulans transgene resembles the recipient species, D. melanogaster, suggesting that the difference between D. simulans and D. melanogaster in this aspect of expression is due to trans-acting factors. We also find that third instar larval activities of the D. simulans transgene and 5-day male activities of the D. mauritiana transgene are lower than those of either parental species, suggesting that not all the promoter elements relevant to these aspects of expression are included in the transferred DNA.

Elk-1 protein domains required for direct and SRF-assisted DNA-binding

The Ets-related Elk-1 protein can bind to purine-rich DNA target sites in a sequence specific fashion and, in addition, can form a ternary complex with the c-fos serum response element (SRE) and the serum response factor (SRF). We demonstrate that Elk-1 can readily interchange between its different interaction partners. The amino terminal ETS-domain of Elk-1 was shown to be necessary and sufficient for direct DNA-binding activity. For ternary complex formation with the SRE and SRF, both the Elk-1 ETS-domain as well as flanking sequences up to amino acid 169 were required. Removal of sequences between the ETS-domain and amino acids 137-169 did not abolish ternary complex formation. This suggests the Elk-1 region spanning amino acids 137-169 to contain a protein-protein interaction domain. Furthermore, we have shown that a single amino acid exchange introduced into the ETS-domain can drastically alter the direct DNA-binding affinity of Elk-1 without severely affecting SRF-assisted binding to the SRE. Thus, Elk-1 requires different propensities of the ETS-domain to exert its different modes of DNA sequence recognition.

Effect of ecd1 mutation on the expression of genes mapped at the Drosophila melanogaster 3C11-12 intermoult puff

The Drosophila melanogaster ecd1 mutation causes a severe temperature-sensitive deficiency in the titre of the steroid hormone ecdysone. This mutation was used to investigate the role of ecdysone in both the transcription of the genes mapped at the 3C11-12 intermoult puff region and the puff formation. Thoroughly synchronized ecd1 larvae were shifted to the non-permissive temperature at various times of the development; after 24 or 48 h, the levels of the transcripts derived from Sgs-4, Pig-1 and ng-1, the three genes located at the 3C11-12 polytene bands, were determined. The results showed that the levels of the transcripts encoded by Pig-1 and ng-1 are unaffected by the drop in the ecdysone titre occurring in non-permissive conditions whereas the amount of Sgs-4 mRNA is greatly reduced. These data clearly indicate that transcription of the three genes mapped within the puff region is affected differently by the hormone. Furthermore, ecd1 larvae cultured at the non-permissive temperature show a prominent puff at the 3C11-12 polytene bands, indicating that ecdysone is not essential for puff induction and that puff size is not simply correlated with high-level Sgs-4 transcription.

Genes for Drosophila small heat shock proteins are regulated differently by ecdysterone

Genes for small heat shock proteins (hsp27 to hsp22) are activated in late third-instar larvae of Drosophila melanogaster in the absence of heat stress. This regulation has been simulated in cultured Drosophila cells in which the genes are activated by the addition of ecdysterone. Sequence elements (HERE) involved in ecdysterone regulation of the hsp27 and hsp23 genes have been defined by transfection studies and have recently been identified as binding sites for ecdysterone receptor. We report here that the hsp27 and hsp23 genes are regulated differently by ecdysterone. The hsp27 gene is activated rapidly by ecdysterone, even in the absence of protein synthesis. In contrast, high-level expression of the hsp23 gene begins only after a lag of about 6 h, is dependent on the continuous presence of ecdysterone, and is sensitive to low concentrations of protein synthesis inhibitors. Transfection experiments with reporter constructs show that this difference in regulation is at the transcriptional level. Synthetic hsp27 or hsp23 HERE sequences confer hsp27- or hsp23-type ecdysterone regulation on a basal promoter. These findings indicate that the hsp27 gene is a primary, and the hsp23 gene is mainly a secondary, hormone-responsive gene. Ecdysterone receptor is implied to play a role in the regulation of both genes.

Genes for Drosophila small heat shock proteins are regulated differently by ecdysterone

Genes for small heat shock proteins (hsp27 to hsp22) are activated in late third-instar larvae of Drosophila melanogaster in the absence of heat stress. This regulation has been simulated in cultured Drosophila cells in which the genes are activated by the addition of ecdysterone. Sequence elements (HERE) involved in ecdysterone regulation of the hsp27 and hsp23 genes have been defined by transfection studies and have recently been identified as binding sites for ecdysterone receptor. We report here that the hsp27 and hsp23 genes are regulated differently by ecdysterone. The hsp27 gene is activated rapidly by ecdysterone, even in the absence of protein synthesis. In contrast, high-level expression of the hsp23 gene begins only after a lag of about 6 h, is dependent on the continuous presence of ecdysterone, and is sensitive to low concentrations of protein synthesis inhibitors. Transfection experiments with reporter constructs show that this difference in regulation is at the transcriptional level. Synthetic hsp27 or hsp23 HERE sequences confer hsp27- or hsp23-type ecdysterone regulation on a basal promoter. These findings indicate that the hsp27 gene is a primary, and the hsp23 gene is mainly a secondary, hormone-responsive gene. Ecdysterone receptor is implied to play a role in the regulation of both genes.

Characterization of a specific ecdysteroid receptor-DNA complex reveals common properties for invertebrate and vertebrate hormone-receptor/DNA interactions

A gel-filtration assay has been developed with which the specific interaction between Drosophila melanogaster ecdysteroid receptor and the 20-hydroxy-ecdysone responsive element of the hsp27 gene promoter region was characterized in terms of complex formation, saturation of DNA binding and the apparent molecular mass of the complex. The hsp27 DNA-binding sequence for ecdysteroid receptor in vitro was delimited by footprinting and mutational analysis. The combined results show that ecdysteroid receptor binds as a dimer to an imperfect palindromic sequence (GGTTCAATGCACT) closely resembling the structures of the different vertebrate steroid-hormone-responsive elements reported so far.

A unique zinc finger protein is associated preferentially with active ecdysone-responsive loci in Drosophila

Using an immunochemical approach, we have identified a unique antigen, PEP (protein on ecdysone puffs), which is associated in third-instar larvae with the active ecdysone-regulated loci on polytene chromosomes; PEP is not associated with most intermolt puffs and is found on some, but not all, heat shock-induced puffs. The distribution pattern changes with changing puffing patterns in the developmental program. We have screened an expression library and recovered a cDNA clone encoding PEP. PEP possesses multiple potential nucleic acid- and protein- binding regions: a glycine- and asparagine-rich amino terminus, four zinc finger motifs, two very acidic segments, two short basic stretches, and an alanine- and proline-rich carboxyl terminus. The Pep gene maps by in situ hybridization to the cytological locus 74F, adjacent to the early ecdysone-responsive region; however, the gene is not regulated by ecdysone at the level of transcription. The pattern of Pep expression through development suggests that maternal Pep gene transcripts are supplied to the embryo, and that the abundance of Pep gene transcripts decreases to a lower, fairly constant level thereafter. This unusual protein may play a role in the process of gene activation, or possibly in RNA processing, for a defined set of developmentally regulated loci.

Identification of ecdysone response elements by analysis of the Drosophila Eip28/29 gene

We have identified ecdysone-response elements (EcREs) by studying regulation of the steroid-responsive Drosophila Eip28/29 gene. First, functional assays of deletion mutants identified large sequence regions required for the response; then a blotting method using the specifically labeled steroid receptor as probe identified receptor-binding regions. Three short receptor-binding regions near Eip28/29 have been identified: Prox and Dist [521 and 2295 nucleotides, respectively, downstream of the poly(A) site] are probably required for the Eip28/29 response in cell lines; Upstream (-440) is unnecessary for that response. We have also demonstrated that an EcRE-containing region from hsp27 contains a receptor-binding site. Each of these four receptor-binding regions functions as an EcRE when placed upstream of an ecdysone nonresponsive promoter and each contains an imperfect palindrome, suggesting the consensus 5'-RG(GT)TCANTGA(CA)CY-3'. Furthermore, a synthetic 15-bp fragment containing an imperfect palindrome similar to the consensus is a fully functional EcRE. The presence of any of the EcREs leads, in the absence of hormone, to depressed gene expression. When hormone is added, it relieves this repression and causes additional activation. The similarity of the EcRE sequence to response elements for estrogen, thyroid hormone, and retinoic acid receptors suggests that the steroid receptors and their signal transduction mechanisms have been strongly and broadly conserved.

Molecular characterization of the Drosophila melanogaster urate oxidase gene, an ecdysone-repressible gene expressed only in the malpighian tubules

The urate oxidase (UO) gene of Drosophila melanogaster is expressed during the third-instar larval and adult stages, exclusively within a subset of cells of the Malpighian tubules. The UO gene contains a 69-base-pair intron and encodes mature mRNAs of 1,224, 1,227, and 1,244 nucleotides, depending on the site of 3' endonucleolytic cleavage prior to polyadenylation. A direct repeat, 5'-AAGTGAGAGTGAT-3', is the proposed cis-regulatory element involved in 20-hydroxyecdysone repression of the UO gene. The deduced amino acid sequences of UO of D. melanogaster, rat, mouse, and pig and uricase II of soybean show 32 to 38% identity, with 22% of amino acid residues identical in all species. With use of P-element-mediated germ line transformation, 826 base pairs 5' and approximately 1,200 base pairs 3' of the D. melanogaster UO transcribed region contain all of the cis elements allowing for appropriate temporal regulation and Malpighian tubule-specific expression of the UO gene.

Molecular characterization of the Drosophila melanogaster urate oxidase gene, an ecdysone-repressible gene expressed only in the malpighian tubules

The urate oxidase (UO) gene of Drosophila melanogaster is expressed during the third-instar larval and adult stages, exclusively within a subset of cells of the Malpighian tubules. The UO gene contains a 69-base-pair intron and encodes mature mRNAs of 1,224, 1,227, and 1,244 nucleotides, depending on the site of 3' endonucleolytic cleavage prior to polyadenylation. A direct repeat, 5'-AAGTGAGAGTGAT-3', is the proposed cis-regulatory element involved in 20-hydroxyecdysone repression of the UO gene. The deduced amino acid sequences of UO of D. melanogaster, rat, mouse, and pig and uricase II of soybean show 32 to 38% identity, with 22% of amino acid residues identical in all species. With use of P-element-mediated germ line transformation, 826 base pairs 5' and approximately 1,200 base pairs 3' of the D. melanogaster UO transcribed region contain all of the cis elements allowing for appropriate temporal regulation and Malpighian tubule-specific expression of the UO gene.

Purified glucocorticoid receptors bind selectively in vitro to a cloned DNA fragment that mediates a delayed secondary response to glucocorticoids in vivo

We have identified and characterized a 206-base-pair region downstream from rat alpha 2u-globulin promoter that specifically mediates a delayed secondary response to glucocorticoids. Unlike positive primary glucocorticoid response elements (GREs), this regulatory element, termed delayed sGRE, dictates an inductive process preceded by a time lag of several hours and blocked by the protein synthesis inhibitor cycloheximide. Reminiscent of GREs and negative GREs (nGREs), a delayed sGRE confers hormonal regulation upon a linked heterologous promoter from a downstream position with respect to transcription start site and, remarkably, also interacts selectively with purified glucocorticoid receptor. These results imply that receptor binding to a delayed sGRE in vivo may mediate certain secondary responses to glucocorticoid hormones.

A member of the steroid hormone receptor gene family is expressed in the 20-OH-ecdysone inducible puff 75B in Drosophila melanogaster

Drosophila melanogaster DNA has been cloned which encompasses a major part of the 20-OH-ecdysone inducible puff 75B. One 20-OH-ecdysone responsive transcription unit was detected which is expressed into two transcripts which accumulate upon the incubation of salivary glands of 3rd instar larvae with 20-OH-ecdysone. This accumulation is correlated with the 20-OH-ecdysone induced activity of puff 75B. 75B cDNA analysis indicates that the activity of puff 75B leads to the synthesis of a protein which belongs to the steroid and thyroid hormone receptor superfamily. The highest similarity of the derived 75B protein sequence was found to the DNA and ligand binding domains of human retinoic acid receptor. A study of the tissue distribution in larvae revealed that 75B mRNA is present in most, if not all 20-OH-ecdysone target tissues. It is proposed that 75B protein is a DNA-binding protein playing a key role in mediating the regulation of the larval molt by 20-OH-ecdysone.

Characterization of a putative transcription factor gene expressed in the 20-OH-ecdysone inducible puff 74EF in Drosophila melanogaster

Drosophila melanogaster DNA has been cloned which encompasses the major part of the 20-OH-ecdysone inducible puff 74EF. One 20-OH-ecdysone responsive transcription unit was detected which gives rise to two alternative transcripts. The expression of one transcript in salivary glands of 3rd instar larvae is correlated with the 20-OH-ecdysone induced activity of puff 74EF. Corresponding cDNA analysis indicates that the two transcripts are translated into two different proteins which have alternative amino terminal ends. The carboxy terminal domain of the 74E proteins is similar to the carboxy terminal sequences of ets-oncoproteins suggesting that the 74E proteins represent alternative transcription factors. It is proposed that the activity of the 20-OH-ecdysone inducible puff 74EF leads to a switch in the synthesis of a transcription factor.