Transvection Effect Research Articles

Impact of interactions between Su(Hw)-dependent insulators on the transvection effect in <I>Drosophila melanogaster</i>

Transvection is a phenomenon of interallelic communication in which enhancers can activate a specific promoter located on a homologous chromosome. Insulators play a significant role in ensuring functional interactions between enhancers and promoters. In the presented work, we created the model where two or three copies of the insulator are located next to enhancers and promoters localized on homologous chromosomes. Using the Su(Hw) insulator as model, we showed that the functional interaction between a pair of insulators promotes enhancer-promoter trans interactions. The interaction between the three insulators, on the contrary, can lead to the formation of chromatin loops that sterically hinder the full enhancer-promoter interaction. The results of the work suggest the participation of insulators in the regulation of homologous chromosome pairing and in communication between distant genomic loci.

Impact of Interactions between Su(Hw)-Dependent Insulators on the Transvection Effect in Drosophila melanogaster.

Transvection is a phenomenon of interallelic communication in which enhancers can activate a specific promoter located on a homologous chromosome. Insulators play a significant role in ensuring functional interactions between enhancers and promoters. In the presented work, we created a model where two or three copies of the insulator are located next to enhancers and promoters localized on homologous chromosomes. Using the Su(Hw) insulator as a model, we showed that the functional interaction between a pair of insulators promotes enhancer-promoter trans-interactions. The interaction between the three insulators, on the contrary, can lead to the formation of chromatin loops that sterically hinder the full enhancer-promoter interaction. The results of the work suggest the participation of insulators in the regulation of homologous chromosome pairing and in communication between distant genomic loci.

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.

13 Transvection in Drosophila

Pairing-dependent interallelic complementation was first described for the Ultrabithorax gene of the bithorax-complex in Drosophila by Lewis and cited as an example of a new phenomenon that Lewis called the "trans-vection effect." Several different kinds of pairing-dependent gene expression have been observed in Drosophila, and it is now clear that a variety of different molecular mechanisms probably underlie the changes in gene expression that are observed after disrupting chromosome pairing. Transvection in the bithorax-complex appears to result from the ability of cis-regulatory elements to regulate transcription of the promoter on the homologous chromosome. The same phenomenon appears to be responsible for pairing-dependent interallelic complementation at numerous other genes in Drosophila. Some transvection effects are dependent on the presence of wild-type or specific mutant forms of the protein encoded by the zeste trans-regulatory gene, but other transvection effects are zeste-independent. The ease with which chromosome aberrations can disrupt transvection also varies widely among different genes.

Intranuclear ion fountains as regulators of genome work: fountain hypothesis of dominance and some epigenetic effects

It is hypothesized that gene function is modulated through the action of the ion channels of the internal nuclear membrane, and that this underlies the phenomenon of dominance and some epigenetic effects. The topographic specificity essential to gene regulation by injecting portions of ions--ion fountains--is ensured by special fountain RNAs (fRNAs), which operate as double-stranded molecules, and fions, the fRNA-binding sites on the DNA. Every specific fion.fRNA complex then binds with the protein of an ion channel in the inner nuclear membrane, whereby this channel briefly opens to dispense, e.g., Ca2+, Zn2+, or K+ from the perinuclear cistern into the nucleus, as defined by the specificity of fRNA, fion, and the channel chosen by them. Fions may be situated both in introns of genes and at their flanks, even quite far from the target gene. Allelic fions, that is, fions located in homologous sites of homologous chromosomes but differing in the capacity of binding different fRNAs, will unequally influence the ionic surroundings of their structural genes. Ion channels can provide the dominant allele with an ionic atmosphere dissimilar from that of the recessive allele. Distinctions in the nature, number, and location of fions may be the main reason why the dominant and recessive alleles of a structural gene differ in activity even though their other properties are identical. Unequal changes occurring in the vicinity of the alleles may involve chromatin configuration, transcriptional activity of the gene, mRNA processing and lifespan. Isolation of structural genes in a chromosome by long intergene spacers, and large distances between chromosomes in the nucleus prevent undesirable interference of ion fountains. Fions may be key components of many enhancers and silencers. The performance of fions may be affected by pairing of homologous nucleotide sequences of chromosomes, which generates a number of epigenetic effects, such as transvection and gene position effect. Excessive approach of a structural gene to inadequate ion fountains may cause unscheduled chromatin compaction and gene suppression.

A Quantitative Measure of the Mitotic Pairing of Alleles in Drosophila melanogaster and the Influence of Structural Heterozygosity

In Drosophila there exist several examples of gene expression that can be modified by an interaction between alleles; this effect is known as transvection. The inference that alleles interact comes from the observations that homologous chromosomes pair in mitotically dividing cells, and that chromosome rearrangements can alter the phenotype produced by a pair of alleles. It is thought that heterozygous rearrangements impede the ability of alleles to pair and interact. However, because the existing data are inconsistent, this issue is not fully settled. By measuring the frequency of site-specific recombination between homologous chromosomes, we show that structural heterozygosity inhibits the pairing of alleles that lie distal to a rearrangement breakpoint. We suggest that some of the apparent conflicts may owe to variations in cell-cycle lengths in the tissues where the relevant allelic interactions occur. Cells with a longer cell cycle have more time to establish the normal pairing relationships that have been disturbed by rearrangements. In support, we show that Minute mutations, which slow the rate of cell division, partially restore a transvection effect that is disrupted by inversion heterozygosity.

Regulation of polyhomeotic transcription may involve local changes in chromatin activity in Drosophila

The polyhomeotic (ph) gene of Drosophila is a member of the Polycomb group of genes and encodes a chromatin protein required for negative regulation of homeotic genes and other loci, in particular the ph locus itself. We have studied the genetic control of ph transcription during development. Early ph expression is under the control of bicoid and engrailed as activators and of oskar as an inhibitor. The negative autoregulation of ph starts at the blastoderm stage and is partly mediated by a transvection effect. As the number of functional copies of ph increases in the same genome, a concomitant reduction of the transcription of each copy is observed. This regulation is ensured, likely at the chromatin level, positively by the trithorax group and negatively by the Polycomb group gene products like a homeotic gene, but it occurs in the same cells. We propose that an equilibrium between these two states of chromatin activity ensures an accurate level of ph transcription.

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.

Different effects of 93D on 87C heat shock puff activity in Drosophila melanogaster and D. simulans

It is known that treatment of the 93D heat shock locus in Drosophila melanogaster with two inducers, eg. benzamide (BM) or colchicine plus heat shock, causes the 93D puff in polytene nuclei to regress and, at the same time, the 87A and 87C heat shock puffs to be expressed unequally. In view of the close phylogenetic relationship and similar banding pattern of polytene chromosomes of the sibling species D. melanogaster and D. simulans, we examined the expression of the 93D puff and its effects on the transcriptional activity of the 87A and 87C heat shock puffs in salivary glands of D. simulans and D. melanogaster x D. simulans hybrid larvae by autoradiography. As in D. melanogaster, the 93D puff was selectively induced by BM or colchicine and regressed when heat shock treatment was given in addition in D. simulans and in the interspecific hybrid. However, unlike in D. melanogaster, the relative activity of the 87A and 87C heat shock puffs on D. simulans chromosomes remained equal in glands exposed to heat shock in combination with BM or colchicine. In the nuclei of interspecific hybrids, the 87A and 87C loci on the two homologues responded in the same way as those of the respective parents. There was no evidence of a transvection effect. It is known that the 87C locus of D. simulans differs from that of D. melanogaster in not carrying the heat-inducible αβ sequences and therefore, it is proposed that the 93D effect on 87C puff activity in D. melanogaster is mediated via the αβ sequences. However, the role of possible structural differences in the 93D locus of the two species cannot be ruled out.

Synapsis-dependent allelic complementation at the decapentaplegic gene complex in Drosophila melanogaster.

Allelic complementation at the decapentaplegic gene complex (dpp: 2-4-0, cytogenetic location: polytene chromosome bands 22F1-3) of Drosophila melanogaster frequently occurs between site mutations. Two specific instances of allelic complementation are shown to be dependent upon normal somatic chromosome synapsis of homologous dpp genes. Numerous strains have been identified that bear lesions that disrupt allelic complementation when heterozygous with structurally normal chromosomes; each of these 57 strains contains a gross chromosomal rearrangement with a break on chromosome 2. The properties of the rearrangements carried by 50 of these strains are consonant with the idea that their effects are due to a disruption of somatic chromosome synapsis in the dpp region of chromosome arm 2L. In double heterozygotes of simple two-break rearrangements, allelic complementation is restored (presumably through the restoration of structural homozygosity). The types of rearrangements that disrupt complementation have properties very similar to those of rearrangements that disrupt the transvection effect at bithorax [Lewis, E. B. (1954) Am. Nat. 88, 225-239]. The existence of synapsis-dependent allelic complementation is a demonstration of the physiological importance of nuclear organization in gene expression.

Genetic analysis of the larval secretion gene Sgs-4 and its regulatory chromosome sites in Drosophila melanogaster.

Larval salivary gland secretion from seven wild-type stocks of Drosophila melanogaster was electrophoretically analyzed. Considerable variability occurs in the X-chromosomally coded secretion protein 4, both qualitatively, as expressed by differences in electrophoretic mobilities, and quantitatively as seen by its relative amount in the secretion. Drosophila stocks with "normal" amounts of protein 4 show approximately 80-90% dosage compensation in the males, whereas in two stocks with lower amounts of protein 4 there is no indication of dosage compensation. Genetic analysis showed that the properties of secretion protein 4 and the level of expression of the Sgs-4 gene are controlled by the X-chromosome. Recombination experiments indicate that the stock-specific characteristics of protein 4 are properties of the structural gene Sgs-4 itself or of a chromosome region immediately adjacent to Sgs-4. One recombinant (R+79), manifesting an intermediate level of dosage compensation, indicates that a chromosome segment closely distal to Sg-4 is responsible for the regulation of the gene and for dosage compensation in particular. Accordingly, Sgs-4 must be transcribed from distal to proximal. Its position on the genetic map is 3.6. Two stocks, Hikone-R and Kochi-R, which were originally described as 0-mutants produce very low amounts of a specific secretion protein, 4 h, as revealed by a transvection effect and also by fluorography of overloaded gels.

The interaction of two complex loci, zeste and bithorax in Drosophila melanogaster.

It has been found that certain alleles of the zeste locus (z(a) 1-1.0) have no phenotype of their own, but interact with certain alleles at the bithorax locus (bx 3-58.8). This interaction takes the form of an enhancement of the homeotic bx phenotype to a more extreme form-i.e., the metathorax is transformed into mesothorax in varying degrees depending on the bx allele used. This enhancement is somewhat reminiscent of the transvection effect described by Lewis (1954). The characterization of the interaction thus far has shown that the enhancement only effects bx alleles which arise spontaneously, whereas the origin of the z(a) allele is unimportant. The gene claret nondisjunctional was used for the production of gynandromorphs which showed that the enhancing ability of z(a), like the eye pigment change caused by z, is autonomous. The enhancement of one specific allele (bx(34e)), which is temperature-sensitive, has allowed a delineation of the temperature-sensitive period of the bithorax locus to a period extending from the middle of the second larval instar to the middle of the third larval instar. These results, as well as those of other enhancer and suppressor systems in Drosophila, have revealed the possibility of the involvement of heterocyclic compounds in the control of cell determination and fate in Drosophila melanogaster.