A P element containing suppressor of hairy-wing binding regions has novel properties for mutagenesis in Drosophila melanogaster

Genetic characterization of cytological region 77A-D harboring the presenilin gene of Drosophila melanogaster

We performed a systematic lethal mutagenesis of the genomic region uncovered by Df(3L)rdgC-co2 (cytological interval 77A-D) to isolate mutations in the single known Presenilin (Psn) gene of Drosophila melanogaster. Because this segment of chromosome III has not been systematically characterized before, inter se complementation testing of newly recovered mutants was carried out. A total of 79 lethal mutations were isolated, representing at least 17 lethal complementation groups, including one corresponding to the Psn gene. Fine structure mapping of the genomic region surrounding the Psn transcription unit by transgenic rescue experiments allowed us to localize two of the essential loci together with Psn within an approximately 12-kb genomic DNA region. One of these loci, located 3' to Psn, encodes a Drosophila protein related to the yeast 60S ribosomal protein L10 precursor. We also determined which of the newly recovered lethal mutant groups correspond to previously isolated lethal P-element insertions, lethal inversion breakpoints, and lethal polo gene mutants. Point mutations were identified in all five recovered Psn alleles, one of which results in a single amino acid substitution G-E at a conserved residue in the C-terminal cytoplasmic tail of the protein, suggesting an important functional role for this C-terminal domain of Presenilin. In addition, some viable mutations were recovered in the screen, including new alleles of the clipped and inturned loci.

Enhancer blocking by the Drosophila gypsy insulator depends upon insulator anatomy and enhancer strength

Insulators are specialized DNA sequences that prevent enhancer-activated transcription only when interposed between an enhancer and its target promoter. The Drosophila gypsy retrotransposon contains an insulator composed of 12 degenerate binding sites for the Suppressor of Hairy-wing [Su(Hw)] protein that are separated by AT-rich DNA possessing sequence motifs common to matrix/scaffold attachment regions (MARs/SARs). To further understand mechanisms of insulator function, the parameters required for the gypsy insulator to prevent enhancer-activated transcription were examined. Synthetic binding regions were created by reiteration of a single Su(Hw) binding site that lacked the MAR/SAR motifs. These synthetic binding regions reconstituted insulator activity, suggesting that the property of enhancer blocking may be distinct from matrix association. We found that the number and spacing of Su(Hw) binding sites within the gypsy insulator, as well as the strength of the enhancer to be blocked, were important determinants of insulator function. These results provide a link between transcription and insulation, suggesting that these processes may be mechanistically interconnected.

An essential role for the Drosophila Pax2 homolog in the differentiation of adult sensory organs

The adult peripheral nervous system of Drosophila includes a complex array of mechanosensory organs (bristles) that cover much of the body surface of the fly. The four cells (shaft, socket, sheath, and neuron) which compose each of these organs adopt distinct fates as a result of cell-cell signaling via the Notch (N) pathway. However, the specific mechanisms by which these cells execute their conferred fates are not well understood. Here we show that D-Pax2, the Drosophila homolog of the vertebrate Pax2 gene, has an essential role in the differentiation of the shaft cell. In flies bearing strong loss-of-function mutations in the shaven function of D-Pax2, shaft structures specifically fail to develop. Consistent with this, we find that D-Pax2 protein is expressed in all cells of the bristle lineage during the mitotic (cell fate specification) phase of bristle development, but becomes sharply restricted to the shaft and sheath cells in the post-mitotic (differentiative) phase. Two lines of evidence described here indicate that D-Pax2 expression and function is at least in part downstream of cell fate specification mechanisms such as N signaling. First, we find that the lack of late D-Pax2 expression in the socket cell (the sister of the shaft cell) is controlled by N pathway activity; second, we find that loss of D-Pax2 function is epistatic to the socket-to-shaft cell fate transformation caused by reduced N signaling. Finally, we show that misexpression of D-Pax2 is sufficient to induce the production of ectopic shaft structures. From these results, we propose that D-Pax2 is a high-level transcriptional regulator of the shaft cell differentiation program, and acts downstream of the N signaling pathway as a specific link between cell fate determination and cell differentiation in the bristle lineage.

Cohabitation of insulators and silencing elements in yeast subtelomeric regions

In budding yeast, the telomeric DNA is flanked by a combination of two subtelomeric repetitive sequences, the X and Y' elements. We have investigated the influence of these sequences on telomeric silencing. The telomere-proximal portion of either X or Y' dampened silencing when located between the telomere and the reporter gene. These elements were named STARs, for subtelomeric anti-silencing regions. STARs can also counteract silencer-driven repression at the mating-type HML locus. When two STARs bracket a reporter gene, its expression is no longer influenced by surrounding silencing elements, although these are still active on a second reporter gene. In addition, an intervening STAR uncouples the silencing of neighboring genes. STARs thus display the hallmarks of insulators. Protection from silencing is recapitulated by multimerized oligonucleotides representing Tbf1p- and Reb1p-binding sites, as found in STARs. In contrast, sequences located more centromere proximal in X and Y' elements reinforce silencing. They can promote silencing downstream of an insulated expressed domain. Overall, our results suggest that the silencing emanating from telomeres can be propagated in a discontinuous manner via a series of subtelomeric relay elements.

TAFII40 protein is encoded by the e(y)1 gene: biological consequences of mutations

The enhancer of yellow 1 gene, e(y)1, of Drosophila melanogaster has been cloned and demonstrated to encode the TAFII40 protein. The e(y)1 gene is expressed in females much more strongly than in males due to the accumulation of e(y)1 mRNA in the ovaries. Two different e(y)1 mutations have been obtained. The e(y)1(ul) mutation, induced by the insertion of Stalker into the coding region, leads to the replacement of 25 carboxy-terminal amino acids by 17 amino acids encoded by the Stalker sequences and to a decrease of the e(y)1 transcription level. The latter is the main cause of dramatic underdevelopment of the ovaries and sterility of females bearing the e(y)1 mutation. This follows from the restoration of female fertility upon transformation of e(y)1(u1) flies with a construction synthesizing the mutant protein. The e(y)1(P1) mutation induced by P element insertion into the transcribed nontranslated region of the gene has almost no influence on the phenotype of flies. However, in combination with the phP1 mutation, which leads to a strong P element-mediated suppression of e(y)1 transcription, this mutation is lethal. Genetic studies of the e(y)1(u1) mutation revealed a sensitivity of the yellow and white expression to the TAFII40/e(y)1 level. The su(Hw)-binding region, Drosophila insulator, stabilizes the expression of the white gene and makes it independent of the e(y)1(u1) mutation.

Stem cell gene therapy for the beta-chain hemoglobinopathies. Problems and progress

Virus vectors hold great promise for the stem cell gene therapy of beta-chain hemoglobinopathies. However, conventional vectors suffer from low gene transfer rates, low expression levels, and inconsistent or short-lived expression in vivo. In this review we summarize the current status of vector systems for the transduction of hematopoietic stem cells, including the development of novel vector systems and methods for selection of transduced stem cells in vivo. We also summarize efforts to achieve therapeutic expression levels of transferred globin genes with retrovirus vectors, including the manipulation of transcription cassettes, the use of globin gene enhancers, and advances in the use of chromatin insulators for improving the frequency of gene expression following hematopoietic stem cell transduction.

Y-Linked male sterile mutations induced by P element in Drosophila melanogaster

The Y chromosome in Drosophila melanogaster is composed of highly repetitive sequences and is essential only in the male germ line. We employed P-element insertional mutagenesis to induce male sterile mutations in the Y chromosome. By using a combination of two modifiers of position effect variegation, adding an extra Y chromosome and increasing temperature, we isolated 61 P(ry+) elements in the Y chromosome. Six of these Y-linked insertions (approximately 10%) induced male sterile mutations that are mapped to two genes on the long and one on the short arms of the Y chromosome. These mutations are revertible to the wild type in a cell-autonomous and germ-line-dependent manner, consistent with previously defined Y-linked gene functions. Phenotypes associated with these P-induced mutations are similar to those resulting from deletions of the Y chromosome regions corresponding to the male fertility genes. Three alleles of the kl-3 gene on the Y long arm result in loss of the axonemal outer dynein arms in the spermatid tail, while three ks-2 alleles on the Y short arm induce defects at early postmeiotic stages. The recovery of the ms(Y) mutations induced by single P-element insertions will facilitate our effort to understand the structural and functional properties of the Y chromosome.

Genetic lesions in Drosophila behavioural mutants

Over the last 30 years, several hundred behavioural mutants have been isolated in Drosophila. Only a fraction of these are well characterized genetically, behaviourally, and structurally. From six areas of behaviour a set of 24 well-studied mutants was chosen, in which the behavioural defect is probably caused by a central dysfunction and not by an impairment of sensory input or motor output. In all cases, the affected genes can be mutated to more than just a behavioural phenotype. Most genes in the sample are essential. Thus, phenotypic specificity is caused by the specificity of the mutation and not by the gene being a 'behavioural gene'. This study investigates how partial functional inactivation in these loci is brought about genetically. In particular, an attempt is made to discern whether behavioural mutations affect part of a protein's functional repertoire, a subset of protein isoforms, or the spatio-temporal expression of a gene. Not unexpectedly, in view of the predominant use of ethyl methanesulfonate (EMS) as mutagen, the majority of sampled mutations fall into the first two categories. The potentially richest source of genetic versatility, the spatio-temporal modulation of promoter activity by enhancers and silencers, has thus been insufficiently exploited for obtaining behavioural mutants. Various mutagens are reviewed as to their suitability in inducing selective regulatory mutations.

Polycomb group repression is blocked by the Drosophila suppressor of Hairy-wing [su(Hw)] insulator

The suppressor of Hairy-wing [SU(HW)] binding region disrupts communication between a large number of enhancers and promoters and protects transgenes from chromosomal position effects. These properties classify the SU(HW) binding region as an insulator. While enhancers are blocked in a general manner, protection from repressors appears to be more variable. In these studies, we address whether repression resulting from the Polycomb group genes can be blocked by the SU(HW) binding region. The effects of this binding region on repression established by an Ultrabithorax Polycomb group Response Element were examined. A transposon carrying two reporter genes, the yellow and white genes, was used so that repression and insulation could be assayed simultaneously. We demonstrate that the SU(HW) binding region is effective at preventing Polycomb group repression. These studies suggest that one role of the su(Hw) protein may be to restrict the range of action of repressors, such as the Polycomb group proteins, throughout the euchromatic regions of the genome.

High genetic instability of heterochromatin after transposition of the LINE-like I factor in Drosophila melanogaster

In the present work, we have asked whether a group of 13 essential genes mapping to the heterochromatin of Drosophila melanogaster chromosome 2 are mutable following transposition of the I factor during I-R hybrid dysgenesis. We found that the frequency of lethal events mapping to chromosome 2 heterochromatin is surprisingly high, despite the low density of genetic functions identified in this region compared with euchromatin. Cytogenetic and molecular analyses indicated that the recovered mutations correspond either to insertions or to rearrangements. Moreover, chromosomes bearing specific heterochromatic lethal mutations were generated by recombination in the heterochromatin. Together, these data indicate that I factors transpose with high frequency into pericentric regions of chromosome 2 and may play a role in the evolution of constitutive heterochromatin.

Gypsy retrotransposon as a tool for the in vivo analysis of the regulatory region of the optomotor-blind gene in Drosophila

We report here a method for the in vivo dissection of the regulatory region of a gene in the Drosophila genome. Our system includes (i) the reporter genes lacZ and white to detect transcriptional enhancer and silencer activities in a target gene, (ii) an efficient way to induce integration of gypsy elements in the genome, and (iii) unidirectional blocking of regulatory activities by the gypsy element, which is dependent on the su(Hw) protein. The optomotor-blind (omb) gene was analyzed. In the omb(P1) line, a P[lacW] construct is inserted about 1.4 kb upstream of the omb transcription start site. The lacZ reporter gene within P[lacW] exhibits the same expression pattern as omb. The white reporter gene is expressed in a "bipolar" pattern. We induced high frequency gypsy mobilization in omb(P1) and identified two lines (D11 and D13-1) with altered eye pigmentation pattern, which is dependent on su(Hw) activity. A gypsy element was found inserted in the first intron of omb in D13-1 and in P[lacW] in D11. These results indicate that it is the blocking of regulatory activities by gypsy that caused the changes in the white reporter gene expression. The effect of these gypsy insertions on the expression patterns allowed us to predict several aspects of the organization of the regulatory elements in the omb locus.

The role of insulator elements in defining domains of gene expression

Insulators are naturally occurring DNA sequences that protect transgenes from genomic position effects, thereby establishing independent functional domains within the chromosome. Recent studies have focused on the identification of the cis and trans requirements for insulator activity. These experiments demonstrate that insulators contain multiple components that cooperate to confer their unique properties. Additionally, they suggest that the mechanism of insulation is related to that of enhancer function. Two models of insulator can be considered: a domain boundary and a transcriptional decoy model.

Trans-suppression of terminal deficiency-associated position effect variegation in a Drosophila minichromosome

Position effect variegation (PEV) is the clonal inactivation of euchromatic or heterochromatic genes that are abnormally positioned within a chromosome. PEV can be influenced by modifiers in trans, including single gene mutations and the total amount of heterochromatin present in the genome. Terminal deletions of a Drosophila minichromosome (Dp1187) dramatically increase PEV of a yellow+ body-color gene located in cis, even when the terminal break is > 100 kb distal to the yellow gene. Here we demonstrate that terminal deficiency-associated PEV can be suppressed by the presence of a second minichromosome, a novel phenomenon termed "trans-suppression." The chromosomal elements responsible for trans-suppression were investigated using a series of minichromosomes with molecularly characterized deletions and inversions. The data suggest that trans-suppression does not involve communication between transcriptional regulatory elements on the homologues, a type of transvection known to act at the yellow locus. Furthermore, trans-suppression is not accomplished by titration through the addition of extra centric heterochromatin, a general mechanism for PEV suppression. We demonstrate that trans-suppression is disrupted by significant changes in the structure of the suppressing minichromosome, including deletions of the yellow region and centric heterochromatin, and large inversions of the centric heterochromatin. We conclude that chromosome pairing plays an important role in trans-suppression and discuss the possibility that terminal deficiency-associated PEV and trans-suppression reflect changes in nuclear positioning of the chromosomes and the gene, and/or the activity and distribution of telomere-binding proteins.

Target site selection in transposition

Transposable elements are discrete mobile DNA segments that can insert into non-homologous target sites. Diverse patterns of target site selectivity are observed: Some elements display considerable target site selectivity and others display little obvious selectivity, although none appears to be truly "random." A variety of mechanisms for target site selection are used: Some elements use direct interactions between the recombinase and target DNA whereas other elements depend upon interactions with accessory proteins that communicate both with the target DNA and the recombinase. The study of target site selectivity is useful in probing recombination mechanisms, in studying genome structure and function, and also in providing tools for genome manipulation.