Interactions of zeste mutations with loci exhibiting transvection effects in Drosophila melanogaster

Phase separation drives pairing of homologous chromosomes

Pairing of homologous chromosomes is crucial for ensuring accurate segregation of chromosomes during meiosis. Molecular mechanisms of homologous chromosome pairing in meiosis have been extensively studied in the fission yeast Schizosaccharomyces pombe. In this organism, meiosis-specific noncoding RNA transcribed from specific genes accumulates at the respective gene loci, and chromosome-associated RNA-protein complexes mediate meiotic pairing of homologous loci through phase separation. Pairing of homologous chromosomes also occurs in somatic diploid cells in certain situations. For example, somatic pairing of homologous chromosomes occurs during the early embryogenesis in diptera, and relies on the transcription-associated chromatin architecture. Earlier models also suggest that transcription factories along the chromosome mediate pairing of homologous chromosomes in plants. These studies suggest that RNA bodies formed on chromosomes mediate the pairing of homologous chromosomes. This review summarizes lessons from S. pombe to provide general insights into mechanisms of homologous chromosome pairing mediated by phase separation of chromosome-associated RNA-protein complexes.

Polytene Chromosome Structure and Somatic Genome Instability

Polytene chromosomes have for 80 years provided the highest resolution view of interphase genome structure in an animal cell nucleus. These chromosomes represent the normal genomic state of nearly all Drosophila larval and many adult cells, and a better understanding of their striking banded structure has been sought for decades. A more recently appreciated characteristic of Drosophila polytene cells is somatic genome instability caused by unfinished replication (UR). Repair of stalled forks generates enough deletions in polytene salivary gland cells to alter 10%-90% of the DNA strands within more than 100 UR regions comprising 20% of the euchromatic genome. We accurately map UR regions and show that most approximate large polytene bands, indicating that replication forks frequently stall near band boundaries in late S phase. Chromosome conformation capture has recently identified dense topologically associated domains (TADs) in many genomes and most UR bands are similar or slightly smaller than a cognate Drosophila TAD. We argue that bands serve the evolutionarily ancient function of coordinating genome replication with local gene activity. We also discuss the relatively recent evolution of polyteny and somatic instability in Diptera and propose that these processes helped propel the amazing success of two-winged flies in becoming the most ecologically diverse insect group, with 200 times the number of species as mammals.

Polyteny: still a giant player in chromosome research

In this era of high-resolution mapping of chromosome territories, topological interactions, and chromatin states, it is increasingly appreciated that the positioning of chromosomes and their interactions within the nucleus is critical for cellular function. Due to their large size and distinctive structure, polytene chromosomes have contributed a wealth of knowledge regarding chromosome regulation. In this review, we discuss the diversity of polytene chromosomes in nature and in disease, examine the recurring structural features of polytene chromosomes in terms of what they reveal about chromosome biology, and discuss recent advances regarding how polytene chromosomes are assembled and disassembled. After over 130 years of study, these giant chromosomes are still powerful tools to understand chromosome biology.

The Functionality and Evolution of Eukaryotic Transcriptional Enhancers

Enhancers regulate precise spatial and temporal patterns of gene expression in eukaryotes and, moreover, evolutionary changes in these modular cis-regulatory elements may represent the predominant genetic basis for phenotypic evolution. Here, we review approaches to identify and functionally analyze enhancers and their transcription factor binding sites, including assay for transposable-accessible chromatin-sequencing (ATAC-Seq) and clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9, respectively. We also explore enhancer functionality, including how transcription factor binding sites combine to regulate transcription, as well as research on shadow and super enhancers, and how enhancers can act over great distances and even in trans. Finally, we discuss recent theoretical and empirical data on how transcription factor binding sites and enhancers evolve. This includes how the function of enhancers is maintained despite the turnover of transcription factor binding sites as well as reviewing studies where mutations in enhancers have been shown to underlie morphological change.

Structure of Zeste-DNA Complex Reveals a New Modality of DNA Recognition by Homeodomain-Like Proteins

Drosophila Zeste is a DNA binding protein important for chromatin-targeted regulation of gene expression. It is best studied in the context of transvection-a mechanism of interallelic gene regulation involving paired chromosomes-and repression of the expression of white by Zeste mutants. Both of these functions depend on the DNA binding and self-association properties of Zeste, but the underlying structural basis remains unknown. Here we report the crystal structure of the DNA binding domain of Zeste in complex with a 19-bp DNA duplex containing the consensus recognition sequence motif. The structure reveals a helix-turn-helix Myb/homeodomain-like fold with the Zeste-specific insertion sequence forming a short helix and a long loop. Direct base contacts by the major groove binding helix principally account for the sequence-specific recognition, and backbone contacts via the Zeste-specific insertion are mainly responsible for the length requirement and the orientation of DNA. Our structural and biochemical characterizations of the DNA binding property of Zeste uncover an altered DNA binding modality of homeodomain-like proteins, and the structural information should facilitate the unraveling of the intricate mechanism of Zeste in regulation of gene expression.

Transvection is common throughout the Drosophila genome

Higher-order genome organization plays an important role in transcriptional regulation. In Drosophila, somatic pairing of homologous chromosomes can lead to transvection, by which the regulatory region of a gene can influence transcription in trans. We observe transvection between transgenes inserted at commonly used phiC31 integration sites in the Drosophila genome. When two transgenes that carry endogenous regulatory elements driving the expression of either LexA or GAL4 are inserted at the same integration site and paired, the enhancer of one transgene can drive or repress expression of the paired transgene. These transvection effects depend on compatibility between regulatory elements and are often restricted to a subset of cell types within a given expression pattern. We further show that activated UAS transgenes can also drive transcription in trans. We discuss the implication of these findings for (1) understanding the molecular mechanisms that underlie transvection and (2) the design of experiments that utilize site-specific integration.

Nonclassical regulation of transcription: interchromosomal interactions at the malic enzyme locus of Drosophila melanogaster

Regulation of transcription can be a complex process in which many cis- and trans-interactions determine the final pattern of expression. Among these interactions are trans-interactions mediated by the pairing of homologous chromosomes. These trans-effects are wide ranging, affecting gene regulation in many species and creating complex possibilities in gene regulation. Here we describe a novel case of trans-interaction between alleles of the Malic enzyme (Men) locus in Drosophila melanogaster that results in allele-specific, non-additive gene expression. Using both empirical biochemical and predictive bioinformatic approaches, we show that the regulatory elements of one allele are capable of interacting in trans with, and modifying the expression of, the second allele. Furthermore, we show that nonlocal factors--different genetic backgrounds--are capable of significant interactions with individual Men alleles, suggesting that these trans-effects can be modified by both locally and distantly acting elements. In sum, these results emphasize the complexity of gene regulation and the need to understand both small- and large-scale interactions as more complete models of the role of trans-interactions in gene regulation are developed.

Chromosomal organization at the level of gene complexes

Metazoan genomes primarily consist of non-coding DNA in comparison to coding regions. Non-coding fraction of the genome contains cis-regulatory elements, which ensure that the genetic code is read properly at the right time and space during development. Regulatory elements and their target genes define functional landscapes within the genome, and some developmentally important genes evolve by keeping the genes involved in specification of common organs/tissues in clusters and are termed gene complex. The clustering of genes involved in a common function may help in robust spatio-temporal gene expression. Gene complexes are often found to be evolutionarily conserved, and the classic example is the hox complex. The evolutionary constraints seen among gene complexes provide an ideal model system to understand cis and trans-regulation of gene function. This review will discuss the various characteristics of gene regulatory modules found within gene complexes and how they can be characterized.

Drosophila SAF-B links the nuclear matrix, chromosomes, and transcriptional activity

Induction of gene expression is correlated with alterations in nuclear organization, including proximity to other active genes, to the nuclear cortex, and to cytologically distinct domains of the nucleus. Chromosomes are tethered to the insoluble nuclear scaffold/matrix through interaction with Scaffold/Matrix Attachment Region (SAR/MAR) binding proteins. Identification and characterization of proteins involved in establishing or maintaining chromosome-scaffold interactions is necessary to understand how the nucleus is organized and how dynamic changes in attachment are correlated with alterations in gene expression. We identified and characterized one such scaffold attachment factor, a Drosophila homolog of mammalian SAF-B. The large nuclei and chromosomes of Drosophila have allowed us to show that SAF-B inhabits distinct subnuclear compartments, forms weblike continua in nuclei of salivary glands, and interacts with discrete chromosomal loci in interphase nuclei. These interactions appear mediated either by DNA-protein interactions, or through RNA-protein interactions that can be altered during changes in gene expression programs. Extraction of soluble nuclear proteins and DNA leaves SAF-B intact, showing that this scaffold/matrix-attachment protein is a durable component of the nuclear matrix. Together, we have shown that SAF-B links the nuclear scaffold, chromosomes, and transcriptional activity.

Effects of chromosomal rearrangements on transvection at the yellow gene of Drosophila melanogaster

Homologous chromosomes are paired in somatic cells of Drosophila melanogaster. This pairing can lead to transvection, which is a process by which the proximity of homologous genes can lead to a change in gene expression. At the yellow gene, transvection is the basis for several examples of intragenic complementation involving the enhancers of one allele acting in trans on the promoter of a paired second allele. Using complementation as our assay, we explored the chromosomal requirements for pairing and transvection at yellow. Following a protocol established by Ed Lewis, we generated and characterized chromosomal rearrangements to define a region in cis to yellow that must remain intact for complementation to occur. Our data indicate that homolog pairing at yellow is efficient, as complementation was disrupted only in the presence of chromosomal rearrangements that break Collapse

Key Words Collapse

MESH Headings

• Animals
• Chromosome Aberrations
• Chromosomes/genetics
• Drosophila Proteins/genetics
• Drosophila melanogaster/genetics
• Female
• Gene Duplication
• Gene Expression Regulation
• Genetic Complementation Test
• Male
• Models, Genetic
• Mutation
• Translocation, Genetic

Collapse

Grants

• R01 GM061936 NIGMS NIH HHS
• R01 GM085169 NIGMS NIH HHS
• GM61936 NIGMS NIH HHS
• GM085169 NIGMS NIH HHS

Collapse

Affiliation(s)

Sharon A Ou Department of Genetics, Harvard Medical School, Boston, MA 02115, USA
Elaine Chang
Szexian Lee
Katherine So
C-ting Wu
James R Morris

Collapse

Genetic analysis of the sexual dimorphism of glass inDrosophila melanogaster

A modifier locus is described that alters the level of phenotypic expression of the third chromosome mutant glass in a sex specific manner. Alternative alleles either confer a sexually dimorphic level of pigment in glass mutants, with the male being greater, or cause similar expression in the two sexes. The alleles are indistinguishable in females but produce the respective phenotypes in males. The gene maps to the tip of theXchromosome at position 0·96 ± 0·11. Cytologically, the locus is present between polytene bands 3A6–8 and 3C2–3 as determined by its inclusion in translocatedXsegments inw+Y,Dp(l;2)w70h31andDp(l;3)w67k27The dimorphic allele is dominant to the nondimorphic condition in males heterozygous for an insertional translocation carrying the dimorphic allele and a normal chromosome carrying the nondimorphic form. The dimorphic allele in two doses in males does not exhibit a dosage effect. The modifier phenotype is unaffected in twoXflies by the presence of the transformer mutation.

Enhancer blocking and transvection at the Drosophila apterous locus

Intra- and interchromosomal interactions have been implicated in a number of genetic phenomena in diverse organisms, suggesting that the higher-order structural organization of chromosomes in the nucleus can have a profound impact on gene regulation. In Drosophila, homologous chromosomes remain paired in somatic tissues, allowing for trans interactions between genes and regulatory elements on the two homologs. One consequence of homolog pairing is the phenomenon of transvection, in which regulatory elements on one homolog can affect the expression of a gene in trans. We report a new instance of transvection at the Drosophila apterous (ap) locus. Two different insertions of boundary elements in the ap regulatory region were identified. The boundaries are inserted between the ap wing enhancer and the ap promoter and have highly penetrant wing defects typical of mutants in ap. When crossed to an ap promoter deletion, both boundary inserts exhibit the interallelic complementation characteristic of transvection. To confirm that transvection occurs at ap, we generated a deletion of the ap wing enhancer by FRT-mediated recombination. When the wing-enhancer deletion is crossed to the ap promoter deletion, strong transvection is observed. Interestingly, the two boundary elements, which are inserted approximately 10 kb apart, fail to block enhancer action when they are present in trans to one another. We demonstrate that this is unlikely to be due to insulator bypass. The transvection effects described here may provide insight into the role that boundary element pairing plays in enhancer blocking both in cis and in trans.

Pairing between gypsy insulators facilitates the enhancer action in trans throughout the Drosophila genome

The Suppressor of the Hairy wing [Su(Hw)] binding region within the gypsy retrotransposon is the best known chromatin insulator in Drosophila melanogaster. According to previous data, two copies of the gypsy insulator inserted between an enhancer and a promoter neutralize each other's actions, which is indicative of an interaction between the protein complexes bound to the insulators. We have investigated the role of pairing between the gypsy insulators located on homologous chromosomes in trans interaction between yellow enhancers and a promoter. It has been shown that trans activation of the yellow promoter strongly depends on the site of the transposon insertion, which is evidence for a role of surrounding chromatin in homologous pairing. The presence of the gypsy insulators in both homologous chromosomes even at a distance of 9 kb downstream from the promoter dramatically improves the trans activation of yellow. Moreover, the gypsy insulators have proved to stabilize trans activation between distantly located enhancers and a promoter. These data suggest that gypsy insulator pairing is involved in communication between loci in the Drosophila genome.

Transvection at the vestigial locus of Drosophila melanogaster

Transvection is a phenomenon wherein gene expression is effected by the interaction of alleles in trans and often results in partial complementation between mutant alleles. Transvection is dependent upon somatic pairing between homologous chromosome regions and is a form of interallelic complementation that does not occur at the polypeptide level. In this study we demonstrated that transvection could occur at the vestigial (vg) locus by revealing that partial complementation between two vg mutant alleles could be disrupted by changing the genomic location of the alleles through chromosome rearrangement. If chromosome rearrangements affect transvection by disrupting somatic pairing, then combining chromosome rearrangements that restore somatic pairing should restore transvection. We were able to restore partial complementation in numerous rearrangement trans-heterozygotes, thus providing substantial evidence that the observed complementation at vg results from a transvection effect. Cytological analyses revealed this transvection effect to have a large proximal critical region, a feature common to other transvection effects. In the Drosophila interphase nucleus, paired chromosome arms are separated into distinct, nonoverlapping domains. We propose that if the relative position of each arm in the nucleus is determined by the centromere as a relic of chromosome positions after the last mitotic division, then a locus will be displaced to a different territory of the interphase nucleus relative to its nonrearranged homolog by any rearrangement that links that locus to a different centromere. This physical displacement in the nucleus hinders transvection by disrupting the somatic pairing of homologous chromosomes and gives rise to proximal critical regions.

Transvection effects in Drosophila

An unusual feature of the Diptera is that homologous chromosomes are intimately synapsed in somatic cells. At a number of loci in Drosophila, this pairing can significantly influence gene expression. Such influences were first detected within the bithorax complex (BX-C) by E.B. Lewis, who coined the term transvection to describe them. Most cases of transvection involve the action of enhancers in trans. At several loci deletion of the promoter greatly increases this action in trans, suggesting that enhancers are normally tethered in cis by the promoter region. Transvection can also occur by the action of silencers in trans or by the spreading of position effect variegation from rearrangements having heterochromatic breakpoints to paired unrearranged chromosomes. Although not demonstrated, other cases of transvection may involve the production of joint RNAs by trans-splicing. Several cases of transvection require Zeste, a DNA-binding protein that is thought to facilitate homolog interactions by self-aggregation. Genes showing transvection can differ greatly in their response to pairing disruption. In several cases, transvection appears to require intimate synapsis of homologs. However, in at least one case (transvection of the iab-5,6,7 region of the BX-C), transvection is independent of synapsis within and surrounding the interacting gene. The latter example suggests that transvection could well occur in organisms that lack somatic pairing. In support of this, transvection-like phenomena have been described in a number of different organisms, including plants, fungi, and mammals.

Transvection and silencing of the Scr homeotic gene of Drosophila melanogaster

The Sex combs reduced (Scr) gene specifies the identities of the labial and first thoracic segments in Drosophila melanogaster. In imaginal cells, some Scr mutations allow cis-regulatory elements on one chromosome to stimulate expression of the promoter on the homolog, a phenomenon that was named transvection by Ed Lewis in 1954. Transvection at the Scr gene is blocked by rearrangements that disrupt pairing, but is zeste independent. Silencing of the Scr gene in the second and third thoracic segments, which requires the Polycomb group proteins, is disrupted by most chromosomal aberrations within the Scr gene. Some chromosomal aberrations completely derepress Scr even in the presence of normal levels of all Polycomb group proteins. On the basis of the pattern of chromosomal aberrations that disrupt Scr gene silencing, we propose a model in which two cis-regulatory elements interact to stabilize silencing of any promoter or cis-regulatory element physically between them. This model also explains the anomalous behavior of the Scx allele of the flanking homeotic gene, Antennapedia. This allele, which is associated with an insertion near the Antennapedia P1 promoter, inactivates the Antennapedia P1 and P2 promoters in cis and derepresses the Scr promoters both in cis and on the homologous chromosome.

Terminal retrotransposons activate a subtelomeric white transgene at the 2L telomere in Drosophila

Genetically marked P elements inserted into the subtelomeric satellites of Drosophila show repression and variegation of the reporter gene. One such white+ reporter, inserted between the subtelomeric satellite and the terminal HeT-A array in the left arm of chromosome 2 (2L), is sensitive to its context; changes in the structure of the telomere region can be identified by changes in eye color. Addition of HeT-A or TART elements to the 2L terminus increases w+ expression, and loss of sequence from the end decreases expression. This indicates that the telomeric retrotransposons in Drosophila have an activating influence on the repressed subterminal reporter gene. Changes in eye color due to altered expression of the transgene also allow the detection of interactions between homologous telomeres. The 2L arms that terminate in long HeT-A/TART arrays showed increased expression of the subterminal w+ transgene when the terminal repeats on the homologue are absent or markedly shorter. We propose that the chromatin structure of the terminal HeT-A/TART array and the activity of a putative promoter/enhancer element on HeT-A are affected by telomeric interactions. Such trans-activation may reflect control over HeT-A transcription and, thus, transposition activity.

Xenopus Enhancer of Zeste (XEZ); an anteriorly restricted polycomb gene with a role in neural patterning

We have identified the Xenopus homologue of Drosophila Enhancer of Zeste using a differential display strategy designed to identify genes involved in early anterior neural differentiation. XEZ codes for a protein of 748 amino acids that is very highly conserved in evolution and is 96% identical to both human and mouse EZ(H)2. In common with most other Xenopus Pc-G genes and unlike mammalian Pc-G genes, XEZ is anteriorly restricted. Zygotic expression of XEZ commences during gastrulation, much earlier than other anteriorly localized Pc-G genes; expression is restricted to the anterior neural plate and is confined later to the forebrain, eyes and branchial arches. XEZ is induced in animal caps overexpressing noggin; up-regulation of XEZ therefore represents a response to inhibition of BMP signalling in ectodermal cells. We show that the midbrain/hindbrain junction marker En-2,and hindbrain marker Krox-20, are target genes of XEZ and that XEZ functions to repress these anteroposterior marker genes. Conversely, XEZ does not repress the forebrain marker Otx-2. XEZ overexpression results in a greatly thickened floor of the forebrain. These results implicate an important role for XEZ in the patterning of the nervous system.

Local transposition of a hobo element within the decapentaplegic locus of Drosophila

We have efficiently mobilized a phenotypically silent hobo transgene inserted within the cis-regulatory heldout region of the decapentaplegic (dpp) locus in Drosophila melanogaster. The goal of our experiment was to identify germline transmission of a local transposition event within the dpp locus that meets two specific criteria. First, excision of the hobo construct does not generate an adult mutant phenotype, suggesting minimal alteration to the original site of insertion. Second, we required a new insertion of the hobo transgene into the Haploinsufficient region of the locus approximately 25 kb away. Genetic and molecular criteria are used to evaluate candidate germlines. In a pilot study, this local transposition event occurred independently in two individuals. Both of the transposition events appear to be new insertions into the dpp transcription unit. One insertion is between the two protein-coding exons, and the other is in the 3'-untranslated region of exon three. Strains carrying these insertions are valuable new reagents for the analysis of dpp function and molecular evolution. These results further support the use of the hobo system as an important tool in Drosophila genetics.

Homologous association of the Bithorax-Complex during embryogenesis: consequences for transvection in Drosophila melanogaster

Transvection is the phenomenon by which the expression of a gene can be controlled by its homologous counterpart in trans, presumably due to pairing of alleles in diploid interphase cells. Transvection or trans-sensing phenomena have been reported for several loci in Drosophila, the most thoroughly studied of which is the Bithorax-Complex (BX-C). It is not known how early trans-sensing occurs nor the extent or duration of the underlying physical interactions. We have investigated the physical proximity of homologous genes of the BX-C during Drosophila melanogaster embryogenesis by applying fluorescent in situ hybridization techniques together with high-resolution confocal light microscopy and digital image processing. The association of homologous alleles of the BX-C starts in nuclear division cycle 13, reaches a plateau of 70% in postgastrulating embryos, and is not perturbed by the transcriptional state of the genes throughout embryogenesis. Pairing frequencies never reach 100%, indicating that the homologous associations are in equilibrium with a dissociated state. We determined the effects of translocations and a zeste protein null mutation, both of which strongly diminish transvection phenotypes, on the extent of diploid homologue pairing. Although translocating one allele of the BX-C from the right arm of chromosome 3 to the left arm of chromosome 3 or to the X chromosome abolished trans-regulation of the Ultrabithorax gene, pairing of homologous alleles surprisingly was reduced only to 20–30%. A zeste protein null mutation neither delayed the onset of pairing nor led to unpairing of the homologous alleles. These data are discussed in the light of different models for trans-regulation. We examined the onset of pairing of the chromosome 4 as well as of loci near the centromere of chromosome 3 and near the telomere of 3R in order to test models for the mechanism of homologue pairing.

A proline-rich region in the Zeste protein essential for transvection and white repression by Zeste

The DNA-binding protein encoded by the zeste gene of Drosophila activates transcription and mediates interchromosomal interactions such as transvection. The mutant protein encoded by the zeste1 (z1) allele retains the ability to support transvection, but represses white. Similar to transvection, repression requires Zeste-Zeste protein interactions and a second copy of white, either on the homologous chromosome or adjacent on the same chromosome. We characterized two pseudorevertants of z1 (z1-35 and z1-42) and another zeste mutation (z78c) that represses white. The z1 lesion alters a lysine residue located between the N-terminal DNA-binding domain and the C-terminal hydrophobic repeats involved in Zeste self-interactions. The z78c mutation alters a histidine near the site of the z1 lesion. Both z1 pseudorevertants retain the z1 lesion and alter different prolines in a proline-rich region located between the z1 lesion and the self-interaction domain. The pseudorevertants retain the ability to self-interact, but fail to repress white or support transvection at Ultrabithorax. To account for these observations and evidence indicating that Zeste affects gene expression through Polycomb group (Pc-G) protein complexes that epigenetically maintain chromatin states, we suggest that the regions affected by the z1, z78c, and pseudorevertant lesions mediate interactions between Zeste and the maintenance complexes.

A Polycomb and GAGA dependent silencer adjoins the Fab-7 boundary in the Drosophila bithorax complex

The homeotic genes of the Drosophila bithorax complex are controlled by a large cis-regulatory region that ensures their segmentally restricted pattern of expression. A deletion that removes the Frontabdominal-7 cis-regulatory region (Fab-7') dominantly transforms parasegment 11 into parasegment 12. Previous studies suggested that removal of a domain boundary element on the proximal side of Fab-7' is responsible for this gain-of-function phenotype. In this article we demonstrate that the Fab-7' deletion also removes a silencer element, the iab-7 PRE, which maps to a different DNA segment and plays a different role in regulating parasegment-specific expression patterns of the Abd-B gene. The iab-7 PRE mediates pairing-sensitive silencing of mini-white, and can maintain the segmentally restricted expression pattern of a BXD, Ubx/lacZ reporter transgene. Both silencing activities depend upon Polycomb Group proteins. Pairing-sensitive silencing is relieved by removing the transvection protein Zeste, but is enhanced in a novel pairing-independent manner by the zeste' allele. The iab-7 PRE silencer is contained within a 0.8-kb fragment that spans a nuclease hypersensitive site, and silencing appears to depend on the chromatin remodeling protein, the GAGA factor.

Topological constraints on transvection between white genes within the transposing element TE35B in Drosophila melanogaster

The transposable element TE35B carries two copies of the white (w) gene at 35B1.2 on the second chromosome. These w genes are suppressed in zeste-1 (z1) mutant background in a synapsis-dependent manner. Single-copy derivatives of the original TE35B stock give red eyes when heterozygous, but zeste eyes when homozygous. TE35B derivatives carrying single, double or triple copies of w were crossed to generate flies carrying from two to five ectopic w genes. Within this range, z1-mediated suppression is insensitive to copynumber and does not distinguish between w genes that are in cis or in trans. Suppression does not require the juxtaposition of even numbers of w genes, but is extremely sensitive to chromosomal topology. When arranged in a tight cluster, in triple-copy TE derivatives, w genes are nonsuppressible. Breakpoints falling within TE35B and separating two functional w genes act as partial suppressors of z1. Similarly, breakpoints immediately proximal or distal to both w genes give partial suppression. This transvection-dependent downregulation of w genes may result from mis-activation of the X-chromosome dosage compensation mechanism.

Reduction of transcription by homologue asynapsis in Drosophila imaginal discs

The interactions between enhancers and promotor elements that control gene expression are generally considered to act in cis only, but genetic studies suggest that they can also function in trans between non-contiguous DNA molecules. Termed transvection, such trans interactions have been proposed to be responsible for several examples of intragenic complementation in Drosophila. Transvection is thought to depend on the physical proximity of sister chromosomes, because it is inhibited when chromosome rearrangements reduce the pairing of homologues. This led to the suggestion that transvection occurs when enhancer elements on one chromosome regulate expression on the other, with the pairing dependence resulting from a need for proximity between the two copies of the gene. Here we have analysed the levels of transcription from both alleles of the Drosophila Ultrabithorax (Ubx) gene, and report that the predictions of this simple model are not supported. Our findings indicate a more complex level of trans regulation that may have implications for the aetiology of genetic disorders that are influenced by chromosome rearrangements.

Interallelic complementation at the suppressor of forked locus of Drosophila reveals complementation between suppressor of forked proteins mutated in different regions

The Su(f) protein of Drosophila melanogaster shares extensive homologies with proteins from yeast (RNA14) and man (77 kD subunit of cleavage stimulation factor) that are required for 3' end processing of mRNA. These homologies suggest that su(f) is involved in mRNA 3' end formation and that some aspects of this process are conserved throughout eukaryotes. We have investigated the genetic and molecular complexity of the su(f) locus. The su(f) gene is transcribed to produce three RNAs and could encode two proteins. Using constructs that contain different parts of the locus, we show that only the larger predicted gene product of 84 kD is required for the wild-type function of su(f). Some lethal alleles of su(f) complement to produce viable combinations. The structures of complementing and noncomplementing su(f) alleles indicate that 84-kD Su(f) proteins mutated in different domains can act in combination for partial su(f) function. Our results suggest protein-protein interaction between or within wild-type Su(f) molecules.

Mutations of zeste that mediate transvection are recessive enhancers of position-effect variegation in Drosophila melanogaster

Evidence is presented demonstrating that mutations of zeste, particularly the null state, are strong recessive enhancers of position-effect variegation (PEV) for the white, roughest and Notch loci. The zeste locus encodes a DNA-binding protein that acts as a transcription factor and mediates transvection phenomena at several loci. Its involvement with these seemingly diverse phenomena suggests that the normal zeste product functions in the decondensation of chromatin. A model is presented proposing that zeste is important for opening and stabilizing domains of chromatin, a step in gene determination and the establishment of cell memory. It postulates that chromatin domains that have been structurally modified by chromosomal rearrangement or by insertion of transposable elements are particularly sensitive to the absence or modification of the zeste protein. Such a view unifies the role of zeste in transcription, transvection and PEV.

A genetic analysis of the Suppressor 2 of zeste complex of Drosophila melanogaster

The zeste1 (z1) mutation of Drosophila melanogaster produces a mutant yellow eye color instead of the wild-type red. Genetic and molecular data suggest that z1 achieves this change by altering expression of the wild-type white gene in a manner that exhibits transvection effects. There exist suppressor and enhancer mutations that modify the z1 eye color, and this paper summarizes our studies of those belonging to the Suppressor 2 of zeste complex [Su(z)2-C]. The Su(z)2-C consists of at least three subregions called Psc (Posterior sex combs), Su(z)2 and Su(z)2D (Distal). The products of these subregions are proposed to act at the level of chromatin. Complementation analyses predict that the products are functionally similar and interacting. The alleles of Psc define two overlapping phenotypic classes, the hopeful and hapless. The distinctions between these two classes and the intragenic complementation seen among some of the Psc alleles are consistent with a multidomain structure for the product of Psc. Psc is a member of the homeotic Polycomb group of genes. A general discussion of the Polycomb and trithorax group of genes, position-effect variegation, transvection, chromosome pairing and chromatin structure is presented.

Regulatory autonomy and molecular characterization of the Drosophila out at first gene

Our previous work has shown that the expression of the Drosophila decapentaplegic (dpp) gene in imaginal disks is controlled by a 30 kb array of enhancers located 3' of the dpp coding region. Here, we describe the cloning and characterization of out at first (oaf), a gene located near this enhancer region. Transcription of oaf results in three classes of alternatively polyadenylated RNAs whose expression is developmentally regulated. All oaf transcripts contain two adjacent open reading frames separated by a single UGA stop codon. Suppression of the UGA codon during translation, as seen previously in Drosophila, could lead to the production of different proteins from the same RNA. During oogenesis, oaf RNA is expressed in nurse cells of all ages and maternally contributed to the egg. During embryonic development, zygotic transcription of the gene occurs in small clusters of cells in most or all segments at the time of germband extension and subsequently in a segmentally repeated pattern in the developing central nervous system. The gene is also expressed in the embryonic, larval and adult gonads of both sexes. We also characterize an enhancer trap line with its transposon inserted within the oaf gene and use it to generate six recessive oaf mutations. All six cause death near the beginning of the first larval instar, with two characterized lines showing nervous system defects. Last, we discuss our data in light of the observation that the enhancers controlling dpp expression in the imaginal disks have no effect on the relatively nearby oaf gene.

Cis and trans interactions between the iab regulatory regions and abdominal-A and abdominal-B in Drosophila melanogaster

The infra-abdominal (iab) elements in the bithorax complex of Drosophila melanogaster regulate the transcription of the homeotic genes abdominal-A (abd-A) and Abdominal-B (Abd-B) in cis. Here we describe two unusual aspects of regulation by the iab elements, revealed by an analysis of an unexpected complementation between mutations in the Abd-B transcription unit and these regulatory regions. First, we find that iab-6 and iab-7 can regulate Abd-B in trans. This iab trans regulation is insensitive to chromosomal rearrangements that disrupt transvection effects at the nearby Ubx locus. In addition, we show that a transposed Abd-B transcription unit and promoter on the Y chromosome can be activated by iab elements located on the third chromosome. These results suggest that the iab regions can regulate their target promoter located at a distant site in the genome in a manner that is much less dependent on homologue pairing than other transvection effects. The iab regulatory regions may have a very strong affinity for the target promoter, allowing them to interact with each other despite the inhibitory effects of chromosomal rearrangements. Second, by generating abd-A mutations on rearrangement chromosomes that break in the iab-7 region, we show that these breaks induce the iab elements to switch their target promoter from Abd-B to abd-A. These two unusual aspects of iab regulation are related by the iab-7 breakpoint chromosomes that prevent iab elements from acting on Abd-B and allow them to act on abd-A. We propose that the iab-7 breaks prevent both iab trans regulation and target specificity by disrupting a mechanism that targets the iab regions to the Abd-B promoter.

Transvection in the iab-5,6,7 region of the bithorax complex of Drosophila: homology independent interactions in trans

The Abdominal-B (Abd-B) gene of the bithorax complex (BX-C) of Drosophila controls the identities of the fifth through seventh abdominal segments and segments in the genitalia (more precisely, parasegments 10-14). Here we focus on iab-5, iab-6 and iab-7, regulatory regions of Abd-B that control expression in the fifth, sixth and seventh abdominal segments (parasegments 10-12). By analysis of partial BX-C deficiencies, we show that these regions are able to promote fifth and sixth abdominal segment identities in the absence of an Abd-B gene in cis. We establish that this ability does not result from cis-regulation of the adjacent abd-A or Ubx genes of the BX-C but rather occurs because the iab-5,6,7 region is able to interact with Abd-B in trans. We demonstrate that this interaction is proximity dependent and is, therefore, a case of what E. B. Lewis has called transvection. Interactions of this type are presumably facilitated by the synapsis of homologues that occurs in somatic cells of Dipterans. Although transvection has been detected in a number of Drosophila genes, transvection of the iab-5,6,7 region is exceptional in two ways. First, interaction in trans with Abd-B does not require that homologues share homologous sequences within, or for some distance to either side of, the BX-C. This is the first case of transvection shown to be independent of local synapsis. A second unusual feature of iab-5,6,7 transvection is that it is remarkably difficult to disrupt by heterozygosity for chromosome rearrangements. The lack of requirement for local synapsis and the tenacity of trans-interaction argue that the iab-5,6,7 region can locate and interact with Abd-B over considerable distance. This is consistent with the normal role of iab-5,6,7, which must act over some 20-60 kb to influence its regulatory target in cis at the Abd-B promoter. Evidence is presented that trans-action of iab-5,6,7 requires, and may be mediated by, the region between distal iab-7 and Abd-B. Also, we show that iab-5,6,7 transvection is independent of the allelic state of zeste, a gene that influences several other cases of transvection. The long-range nature of interactions in trans between iab-5,6,7 and Abd-B suggests that similar interactions could operate effectively in organisms lacking extensive somatic pairing. Transvection may, therefore, be of more general significance than previously suspected.

Transvection at the eyes absent gene of Drosophila

The Drosophila eyes absent (eya) gene is required for survival and differentiation of eye progenitor cells. Loss of gene function in the eye results in reduction or absence of the adult compound eye. Certain combinations of eya alleles undergo partial complementation, with dramatic restoration of eye size. This interaction is sensitive to the relative positions of the two alleles in the genome; rearrangements predicted to disrupt pairing of chromosomal homologs in the eya region disrupt complementation. Ten X-ray-induced rearrangements that suppress the interaction obey the same general rules as those that disrupt transvection at the bithorax complex and the decapentaplegic gene. Moreover, like transvection in those cases, the interaction at eya depends on the presence of normal zeste function. The discovery of transvection at eya suggests that transvection interactions of this type may be more prevalent than generally thought.

Identification of mutations in three genes that interact with zeste in the control of white gene expression in Drosophila melanogaster

Three previously described genes, enhancer of yellow, 1, 2 and 3, are shown to cooperate with the zeste gene in the control of white gene expression. The mutations e(y)1u1, e(y)3u1, and to a lesser extent e(y)2u1, enhance the effect of the zeste null allele zv77h. Different combinations of e(y)1u1, e(y)2u1 and e(y)3u1 mutations with several other z alleles also enhance the white mutant phenotype, but only to levels characteristic of white alleles containing a deletion of the upstream eye enhancer. Loss of zeste protein binding sites from the white locus does not eliminate the effect of e(y)1u1 and e(y)3u1 mutations, suggesting that the products of these genes interact with some other nucleotide sequences. Combinations of either e(y)1u1 or e(y)2u1 mutations with e(y)3u1 are lethal. The products of these three genes may represent, together with zeste, a group of proteins involved in the organization of long-distance interactions between DNA sequences.

The determined state of white expression in the Drosophila eye is modified by zeste1 in the wzm family of mutants

Analysis of the whitezeste mottled (wzm) mutant family suggests that the zeste gene product functions in establishing and stabilizing a transcriptionally active chromatin domain for white locus expression. The z1 mutation reduces expression of paired or proximate copies of white, while single or unpaired copies maintain wild-type levels of expression. The wzm mutation, caused by the insertion of the retrotransposon BEL into the 5' intron of white, alters the zeste-white interaction to produce a mottled eye phenotype in hemizygous z1 wzm males. We have determined the molecular structure of four wzm derivatives. wzl results from the insertion of an additional transposable element into the 5' regulatory region of white. wzvl is a deletion of sequences upstream of the white locus. Two others, whalo and wcres, result from the transposition of wzm plus the entire verticals-roughest region into heterochromatin near the tip of chromosome 3L. They variegate for roughest but not for white; rather, the z1 effect on wzm now causes white expression to become non-autonomous and non-clonal. The analysis of these five mutations shows that the neomorphic zeste1 product, in combination with structural changes imposed by transposons and intercalary heterochromatin, modifies the determination and stability of white expression. We propose that the normal zeste product functions as part of a complex that stimulates transcription by changing chromatin conformation to establish and maintain transcriptionally active domains. The unpairing of homologs is proposed to be one of the initial results of conformational change, providing an explanation for the role of zeste in transvection.

polyhomeotic regulatory sequences induce developmental regulator-dependent variegation and targeted P-element insertions in Drosophila

Variegation of the miniwhite gene is observed in a euchromatic context in transformant lines that contain a P transposon including regulatory sequences of the polyhomeotic (ph) gene upstream of the resident miniwhite gene (P[ph]). This variegated phenotype is not affected by most of the genetic modifiers of heterochromatic position-effect variegation (PEV) nor by removal of the Y chromosome. Interestingly, it is sensitive to ph and Polycomb (Pc) mutations, which are known to affect homeotic gene regulation. Regulatory DNA of ph can also mediate transvection of the miniwhite gene. This transvection is abolished in a ph but not in a zeste mutant background. In addition, P[ph] inserts preferentially in sites corresponding to PH/PC protein-binding sites as defined at the polytene chromosome level. These insertions induce an unusually high proportion of mutations in genes affecting homeotic gene regulation. In particular, one insertion is located within the tramtrack locus, which is thought to regulate fushi tarazu, an Ultrabithorax activator. We suggest that a multimeric complex containing PH and PC proteins, at a minimum, causes a local and clonally inherited heterochromatinization, which maintains the repressed state of transcription of the homeotic genes.

Related chromosome binding sites for zeste, suppressors of zeste and Polycomb group proteins in Drosophila and their dependence on Enhancer of zeste function

Polycomb group genes are necessary for maintaining homeotic genes repressed in appropriate parts of the body plan. Some of these genes, e.g. Psc, Su(z)2 and E(z), are also modifiers of the zeste-white interaction. The products of Psc and Su(z)2 were immunohistochemically detected at 80-90 sites on polytene chromosomes. The chromosomal binding sites of these two proteins were compared with those of zeste protein and two other Polycomb group proteins, Polycomb and polyhomeotic. The five proteins co-localize at a large number of sites, suggesting that they frequently act together on target genes. In larvae carrying a temperature sensitive mutation in another Polycomb group gene, E(z), the Su(z)2 and Psc products become dissociated from chromatin at non-permissive temperatures from most but not all sites, while the binding of the zeste protein is unaffected. The polytene chromosomes in these mutant larvae acquire a decondensed appearance, frequently losing characteristic constrictions. These results suggest that the binding of at least some Polycomb group proteins requires interactions with other members of the group and, although zeste can bind independently, its repressive effect on white involves the presence of at least some of the Polycomb group proteins.

Molecular analysis of the zeste-white interaction reveals a promoter-proximal element essential for distant enhancer-promoter communication

We have analyzed the eye and testis enhancers located 1 kb upstream of the transcription start site of the white gene. Both enhancers confer the corresponding tissue-specific expression on a heterologous promoter as well as on the white promoter. The eye determinant consists of multiple elements, each able to stimulate eye-specific expression. It also contains five binding sites for the zeste protein while the immediately adjacent testis element contains none. Site-directed mutation of these zeste binding sites abolishes the zeste-white interaction but does not significantly affect the eye enhancer activity, indicating that they are not important for the eye enhancer activity per se. Other zeste binding sites just upstream of the promoter are not necessary for the zeste-white interaction. We conclude that the overlap of the eye enhancer with the zeste binding sites is responsible for the zeste-white interaction and explains why this interaction affects eye but not testis expression. Sequence deletion or substitution experiments suggested that the white promoter is internal to the transcription start site; the zeste protein is not required for distant enhancer action but a 95-bp promoter-proximal sequence is essential for distant enhancer-promoter interaction. This element may serve as an anchor to stabilize formation of a loop that brings the enhancer to the vicinity of the promoter.

An unusual genomic position effect on Drosophila white gene expression: pairing dependence, interactions with zeste, and molecular analysis of revertants

The white gene in the AR4-24 P[white,rosy] insertion on chromosome 2 has a novel expression pattern, in which it is repressed in the dorsal half of the eye. X-ray mutagenesis led to the isolation of six revertants mapping to chromosome 2, which are wild type in a zeste+ background, and three extreme derivatives, in which white gene expression is repressed in ventral regions of the eye as well. By Southern blot analyses the breakpoints of five of the revertants and one of the extreme derivatives were mapped in the flanking DNA bordering each side of the AR4-24 insertion. The revertants show some dorsal repression of white in the presence of z1, and by this criterion each is only a partial revertant. The extreme derivatives act not only in cis, but also in trans to repress expression of AR4-24 and its various derivatives. We provide evidence that these trans effects are proximity-dependent effects, possibly mediated by pairing of gene copies, as they do not extend to copies of the white gene located elsewhere in the genome. We show that one extreme derivative, E1, is a small deletion spanning the insertion site at the 5' end of the white gene, and propose that the distance between a negative regulatory element in the 5' flanking DNA and the white promoter influences the degree of the repression.

The homeotic gene Sex combs reduced of Drosophila melanogaster is differentially regulated in the embryonic and imaginal stages of development

The Sex combs reduced (Scr) locus is unique among the genes contained within the Antennapedia complex (ANT-C) of Drosophila melanogaster in that it directs functions that are required for both cephalic and thoracic development in the embryo and the adult. Antibodies raised against protein encoded by Scr were used to follow the distribution of this gene product in embryos and imaginal discs of third instar larvae. Analysis of Scr protein accumulation in embryos hemizygous for breakpoint lesions mapping throughout the locus has allowed us to determine that sequences required for establishment of the Scr embryonic pattern are contained within a region of DNA that overlaps with the identified upstream regulatory region of the segmentation gene fushi tarazu (ftz). Gain-of-function mutations in Scr result in the presence of ectopic sex comb teeth on the first tarsal segment of mesothoracic and metathoracic legs of adult males. Heterozygous combinations of gain-of-function alleles with a wild-type Scr gene exhibit no evidence of ectopic protein localization in the second and third thoracic segments of embryos. However, mesothoracic and metathoracic leg imaginal discs can be shown to accumulate ectopically expressed Scr protein, implying a differential regulation of the Scr gene during these two periods of development. Additionally, we have found that the spatial pattern of Scr gene expression in imaginal tissues involved in the development of the adult thorax is governed in part by synapsis of homologous chromosomes in this region of the ANT-C. However, those imaginal discs that arise anteriorly to the prothorax do not appear to be sensitive to this form of gene regulation. Finally, we have demonstrated that the extent of Scr expression is influenced by mutations at the Polycomb (Pc) locus but not by mutant alleles of the zeste (z) gene. Taken together, our data suggests that Scr gene expression is differentially regulated both temporally and spatially in a manner that is sensitive to the structure of the locus.

A fragment of engrailed regulatory DNA can mediate transvection of the white gene in Drosophila

We have found a fragment of engrailed regulatory DNA that has an unusual effect on expression of a linked marker gene, white, in the P element transposon CaSpeR. Normally, flies homozygous for a given CaSpeR insertion have darker eyes than heterozygotes. However, when a particular engrailed DNA fragment is included in that transposon, homozygotes often have lighter eyes than heterozygotes. Thus, engrailed DNA appears to cause white expression to be repressed in homozygotes. The suppression of white is dependent on the proximity of the two transposons in the genome-either in cis (i.e., on the same chromosome) or in trans (i.e., on homologous chromosomes). Thus, the engrailed fragment is mediating a phenomenon similar to that mediated by the zeste gene at the white locus. However, the interactions we observe do not require, nor are influenced by, mutations of zeste. We suggest that the engrailed DNA contains one or more binding sites for a protein that facilitates interactions between transposons. The normal function of these sites may be to mediate interactions between distant cis-regulatory regions of engrailed, a large locus that extends over 70 kilobases.

Transvection and long-distance gene regulation

Numerous genes contain regulatory elements located many tens of kilobases away from the promoter they control. Specific mechanisms must be required to ensure that such distant elements can find and interact with their proper targets but not with extraneous genes. This review explores the connections between transvection phenomena, the activation of domains of homeotic gene expression, position effect variegation and silencers. These various examples of long-distance effects suggest that, in all cases, related forms of chromatin packaging may be involved.

Genetic analysis of the enhancer of zeste locus and its role in gene regulation in Drosophila melanogaster

The Enhancer of zeste [E(z)] locus of Drosophila melanogaster is implicated in multiple examples of gene regulation during development. First identified as dominant gain-of-function modifiers of the zeste1-white (z-w) interaction, mutant E(z) alleles also produce homeotic transformations. Reduction of E(z)+ activity leads to both suppression of the z-w interaction and ectopic expression of segment identity genes of the Antennapedia and bithorax gene complexes. This latter effect defines E(z) as a member of the Polycomb-group of genes. Analysis of E(z)S2, a temperature-sensitive E(z) allele, reveals that both maternally and zygotically produced E(z)+ activity is required to correctly regulate the segment identity genes during embryonic and imaginal development. As has been shown for other Polycomb-group genes, E(z)+ is required not to initiate the pattern of these genes, but rather to maintain their repressed state. We propose that the E(z) loss-of-function eye color and homeotic phenotypes may both be due to gene derepression, and that the E(z)+ product may be a general repressing factor required for both examples of negative gene regulation.

A novel transvection phenomenon affecting the white gene of Drosophila melanogaster

The zeste mutation of Drosophila melanogaster suppresses the expression of white genes in the eye. This suppression is normally dependent on there being two copies of w+ located close to each other in the genome--they may either be in cis (as in a tandem duplication of w+) or in trans, i.e. on homologous chromosomes. Duplicated w+ genes carried by a giant transposing element, TE146(Z), are suppressed by z whether they are in direct (tandem) or inverted order. The tandem form of the TE is very sensitive to a rearrangement on the homologous chromosome--many rearrangements with breakpoints "opposite" the TE's insertion site prevent the interaction between the white genes on a z background. These aberrations act as dominant suppressors of zeste that are specific to the tandemly duplicated form of TE146(Z). The inverted form of the TE146(Z) presumably pairs as a hairpin loop; this is more stable than the tandem form by the criterion that its zeste phenotype is unaffected by any of the aberrations. This effect of rearrangements has been used as the basis for a screen, gamma-ray induced aberrations with at least one breakpoint opposite the TE site were recovered by their suppression of the zeste phenotype.

Transvection in the Ultrabithorax domain of the bithorax complex of Drosophila melanogaster

The phenotypes of several heterozygous combinations of mutations which map within the Ultrabithorax gene of Drosophila melanogaster are modulated by the extent of somatic homologous chromosome pairing, an effect known as transvection. One can discriminate between otherwise phenotypically similar mutations via their transvection behavior. This suggested the existence of previously undetected intragenic functional units. A collection of mutations has been classified into "transvection groups" (in analogy to complementation groups) on the basis of transvection tests with bithorax34e, postbithorax2, and Contrabithorax1 Ultrabithorax1. The conditions necessary for each transvection effect were determined from these transvection groups. The bithorax34e mutation only transvects with Ultrabithorax mutations with a contiguous Ultrabithorax transcriptional unit. In contrast, postbithorax2 transvection requires the distal part of the bithoraxoid region. As expected, Ultrabithorax mutations do not transvect with Contrabithorax1 Ultrabithorax1. However, it appears that this cross activation is not mediated solely through one of the known regulatory regions as mutations in these regions do not consistently block the response.

Cloning and characterization of the segment polarity gene cubitus interruptus Dominant of Drosophila

The segment polarity mutation, cubitus interruptus Dominant (ciD), of Drosophila melanogaster causes defects in the posterior half of every embryonic segment. We cloned sequences from the ciD region on the proximal fourth chromosome by "tagging" the gene with the transposable element P. Genetic and molecular evidence indicates that the P-element insertions, which all occurred within the same restriction fragment, are in 5'-regulatory regions of the ciD gene within 3 kb of the first exon of its transcript. The putative ciD transcript was identified on the basis of its absence in homozygous ciD embryos. Its spatial pattern of expression during development is unusual in that, unlike most other segmentation genes, it exhibits uniform expression throughout cellular blastoderm and gastrulation and does not resolve into a periodic pattern until the end of the fast phase of germ-band elongation when it is present in 15 broad segmentally repeating stripes along the anterior-posterior axis of the embryo. Registration of the ciD stripes of expression relative to the stripes of other segment polarity genes shows that ciD is expressed in the anterior three-quarters of every segment. This registration does not correlate with the pattern defects observed in ciD mutants. Sequence analysis indicates that the protein encoded by the ciD transcript contains a domain of five tandem amino acid repeats that have sequence similarity to the zinc-finger repeats of the Xenopus transcription factor TFIIIA and that share the highest degree of identity with the human zinc-finger protein GLI, which has been found to be amplified in several human glioblastomas.

Mutations in polycombeotic, a Drosophila polycomb-group gene, cause a wide range of maternal and zygotic phenotypes

The polycomb-group genes, a set of genes characterized by mutations that cause similar phenotypes and dosage-dependent interactions, are required for the normal expression of segment-specific homeotic loci. Here we report that polycombeotic (formerly 1(3)1902), originally identified by a lethal mutation that causes a small-disc phenotype, is also a member of this group of essential genes. Adults homozygous for temperature-sensitive pco alleles that were exposed to the restrictive temperature during larval life display the second and third leg to first leg transformation characteristic of polycomb-group mutants. Adult females homozygous for temperature-sensitive alleles exposed to the restrictive temperature during oogenesis produce embryos that show anterior segments with structures normally unique to the eighth abdominal segment, another transformation characteristic of polycomb-group mutants. Mutations in the polycombeotic gene also cause defects not reported for mutations in other polycomb-group genes. Females homozygous for the most extreme temperature-sensitive allele are sterile, and larvae homozygous for null alleles have small imaginal discs and reduced frequencies of mitotic figures in the brain. Dominant mutations originally identified as enhancers or suppressors of zeste are gain-of-function alleles of polycombeotic. The type and variety of defects displayed by different mutations in this gene indicate that the product might be involved in chromosome structure and/or function.

Aristapedioid: a gain of function, homeotic mutation in Drosophila melanogaster

The isolation of gain of function mutations has allowed the identification of a number of genes which are important in the normal development of the organism. We report here the isolation and characterization of Aristapedioid, a gain of function mutation which causes a partial transformation of arista towards tarsus and the loss or decrease in size of the dorso-central and scutellar bristles. Aristapedioid is the result of a P element mediated inversion which juxtaposes unrelated DNA adjacent to Suppressor 2 of zeste, causing a gain of function mutation in that gene.

Homeosis and the interaction of zeste and white in Drosophila

Transvection effects in Drosophila melanogaster suggest a form of gene modulation that is responsive to the proximity of homologous genes. These effects have been well characterized at bithorax and decapentaplegic, and in the interaction between the zeste and white genes. The mechanistic basis for transvection is not known. As part of a genetic analysis of transvection, a study is being made of a class of mutations defined as modifiers of the eye color resulting from the interaction of zeste and white. This report details the observations that several of these mutations also have homeotic effects.