A position effect variegation model of chromatin compaction

A position effect variegation (PEV) model of chromatin compaction is proposed, based on the concept of a statistical distribution of compaction protein (CP) molecules around compaction initiation centers (CICs). The principles of the model are as follows: (1) CICs are present in both hetero- and euchromatin, and (2) different CP molecules interact not only with DNA but also with one another, forming a multimeric complex. When a certain level of DNA-protein binding is exceeded, heterochromatic domains are formed. The model suggests that continuous and discontinuous chromatin compaction resulting from PEV is due to an irregular CIC distribution along the chromosome.     

Genetic modification of heterochromatic association and nuclear organization in Drosophila

Heterochromatin is the highly compact, usually pericentromeric, region of eukaryotic chromosomes. Unlike the more gene-rich euchromatin, heterochromatin remains condensed during interphase, when it is sequestered to the periphery of the nucleus. Here we show, by using fluorescent in situ hybridization to interphase diploid nuclei of Drosophila, that the insertion of heterochromatin into a euchromatic gene, which results in position-effect variegation (PEV), also causes the aberrant association of the gene and its homologous copy with heterochromatin. In correlation with the gene's mutant variegating phenotype, the cytological association of the heterochromatic region is affected by chromosomal distance from heterochromatin and by genic modifiers of PEV. Proteins that are thought to be involved in the formation of heterochromatin can therefore influence the interphase nuclear position of a chromosomal region. This suggests that heterochromatin and proteins involved in its formation provide a structural framework for the interphase nucleus.     

Treatment of temporomandibular disorders: arthroscopy, orthodontics, kinesitherapy]

In(1LR)pn2a is a pericentric inversion with a euchromatic breakpoint in the 2E polytene region and a heterochromatic breakpoint in the right arm of the X chromosome. It is associated with position-effect variegation (PEV) of the pn, wapl, Pgd and other vital loci of the 2E region, which are relocated near the bulk of the X heterochromatin. Cytological analysis showed that the rearrangement brings the 1A-2E euchromatic segment directly into contact with a major portion of the h34 block, a heterochromatic region that is positively stained by the N-banding technique and contains the AAGAG satellite sequences. Molecular cloning revealed the presence of a new junction between euchromatin and AAGAG satellite sequences and demonstrated that the euchromatic breakpoint of In(1LR)pn2a lies in the vinculin gene. In the X ray-induced secondary rearrangement In(1LR)r30, consisting of a pericentric inversion superimposed on In(1LR)pn2a, the h34 material remains associated with the 2E region but is separated from the rest of the X heterochromatin. In this case, the pn, wapl and Pgd loci no longer variegate, suggesting that the satellite-containing h34 region is not able per se to induce detectable PEV on the adjacent euchromatic genes.     

Segregation distortion in Drosophila melanogaster: genomic organization of Responder sequences

The heterochromatic Responder (Rsp) locus of Drosophila melanogaster is the target of the two distorter loci Sd and E(SD). Rsp is located in a specific heterochromatic region of the second chromosome and is made up of AT-rich satellite sequences whose abundance is related to its sensitivity to the distorter chromosomes. Here we report that a cluster of Rsp sequences is also located in the third chromosome. The third-chromosome cluster has the same flanking sequences as the clone originally used to identify the Rsp elements, and one of the flanking sequences is a rearranged 412 retrotransposon. The presence of a second, unlinked Rsp-sequence cluster makes re-interpretation necessary for some earlier experiments in which segregation of the third chromosome had not been followed and raises interesting possibilities for the origin of the Rsp locus.     

The role of tetrahydrofolate dehydrogenase in the hepatic supply of tetrahydrobiopterin in rats

Potential epidemiological valency (PEV) of the natural foci of tick-borne encephalitis was characterized by the quantity of the infected ticks per unit of the area of the focal territory, and also by the section of the pathway of man's passage along this territory, at which encounter of a single viroforic tick is possible. PEV indices reflected the epidemiological intensity of the natural foci of infection both in the mastered and in the poorly developed areas, and therefore could be used for the epidemiological prognosis on tick-borne encephalitis and in epidemiological search of this infection on the newly-developing territories. The extent of PEV reduction as a result of anti-tick treatment of forests endemic by tick-borne encephalitis which as caricides with a prolonged residual action of the tick-vectors offered a possibility of assessing the epidemiological efficacy of the measures carried out the most objectively.     

Molecular characterization of a novel amplicon at 1q21-q22 frequently observed in human sarcomas

Suppressors of position-effect variegation (Su(var)s) in Drosophila melanogaster are usually studied in the presence of chromosomal rearrangements, which exhibit variegated expression of euchromatic genes moved near to, or heterochromatic genes moved away from, centromeric heterochromatin. However, the effects of Su(var) mutations on heterochromatic gene expression in the absence of a variegating re-arrangement have not yet been defined. Here we present a number of results which suggest that Su(var) gene products can interact to affect the expression of the light gene in its normal heterochromatic location. We initially observed that eye pigment was reduced in several Su(var) double mutants; the phenotype resembled that of light mutations and was more severe when only one copy of the light gene was present. This reduced pigmentation could be alleviated by a duplication for the light gene or by a reduction in the amount of cellular heterochromatin. In addition, the viability of most Su(var) double mutant combinations tested was greatly reduced in a genetic background of reduced light gene dosage, when extra heterochromatin is present. We conclude that Su(var) gene products can affect expression of the heterochromatic light gene in the absence of any chromosomal rearrangements. However, it is noteworthy that mutations in any single Su(var) gene have little effect on light expression; we observe instead that different pairings of Su(var) mutations are required to show an effect on light expression. Interestingly, we have obtained evidence that at least two of the second chromosome Su(var) mutations are gain-of-function lesions, which also suggests that there may be different modes of interaction among these genes. It may therefore be possible to use this more sensitive assay of Su(var) effects on heterochromatic genes to infer functional relationships among the products of the 50 or more known Su(var) loci.     

The pyroelectric vidicon camera as a medical thermograph

The operating principles of a pyroelectric vidicon (PEV) tube as an infra red (IR) image detector tube are explained. Characteristic requirements such as pedestal operation and sensitivity to time varying scenes are outlined. Of the possible read-out modes, the synchronous chopper mode is considered. The PEV camera properties are discussed in terms of measurement errors. The largest such as pedestal variations, noise and thermal blurring can be reduced to an acceptable level by image processing. A future PEV camera system with specifications comparable to, or even better than, those of existing commerical medical IR scanners seems to be possible.     

In vivo analysis of scaffold-associated regions in Drosophila: a synthetic high-affinity SAR binding protein suppresses position effect variegation

Scaffold-associated regions (SARs) were studied in Drosophila melanogaster by expressing a synthetic, high-affinity SAR-binding protein called MATH (multi-AT-hook), which consists of reiterated AT-hook peptide motifs; each motif is known to recognize a wide variety of short AT-rich sequences. MATH proteins were expressed specifically in the larval eye imaginal discs by means of the tetracycline-regulated transactivation system and tested for their effect on position effect variegation (PEV). MATH20, a highly potent SAR ligand consisting of 20 AT-hooks, was found to suppress whitemottled 4 variegation. This suppression required MATH20 expression at an early larval developmental stage. Our data suggest an involvement of the high AT-rich SARs in higher order chromatin structure and gene expression.     

Aggregation of heterochromatin regions of chromosomes in Drosophila melanogaster neuroblasts

Interaction of heterochromatic regions was studied during interphase and mitosis. The interphasic heterochromatin unites, producing 1-8 H-chromocentres. A lack of synapsis between heterochromatic regions in prophase is shown to be a result of hypotonic treatment. It is mentioned that adhesion of sister chromatids was used for heterochromatin localization. The results are discussed in connection with the supposed interaction between heterochromatin and membrane.     

Orientation of interphase chromosomes as detected by Giemsa C-bands

The orientation of Giemsa C-bands has been studied in mitotic and interphase cells of Allium cepa. A sativum and of Aloe vera. The C-bands in these three species are located at the telomeres, secondary constriction region of the nucleolar chromosomes and the centromeric regions, respectively. Observations in A. cepa and Aloe indicate clearly that the interphase chromosomes are non-random in their orientation and possibly maintain their telophase configuration through the attachment of telomeres and perhaps of kinetochores with the nuclear membrane. Electron micrographs of onion cells also reveal that certain heterochromatic segments are associated with the nuclear membrane.--The nucleolar interstitial C-bands in A. sativum remain free in the nucleoplasm and may come close to each other due to heterochromatic attraction. Such a heterochromatic attraction is also evident between telomeric regions and between centromeres. However, a two by two attachment could not be noticed. A diagrammatic representation of the orientation of interphase chromosomes has been presented.     

Reactivation of silenced, virally transduced genes by inhibitors of histone deacetylase

Retroviral and adeno-associated viral sequences can dramatically silence transgene expression in mice. We now report that this repression also occurs in stably infected HeLa cells when the cells are grown without selection. Expression of a transduced lacZ gene (rAAV/CMVlacZ) is silenced in greater than 90% of cells after 60 days in culture. Surprisingly, high-level expression can be reactivated by treating the cells with sodium butyrate or trichostatin A but not with 5-azacytidine. When cell clones with integrated copies of rAAV/CMVlacZ were isolated, lacZ expression was silenced in 80% of the clones; however, lacZ expression was reactivated in all of the silenced clones by treatment with butyrate or trichostatin A. The two drugs also reactivated a silenced globin gene construct (rAAV/HS2alphabetaAS3) in stably infected K562 cells. Trichostatin A is a specific inhibitor of histone deacetylase; therefore, we propose that hyperacetylation of histones after drug treatment changes the structure of chromatin on integrated viral sequences and relieves repression of transduced genes. The reactivation of silenced, transduced genes has implications for gene therapy. Efficient viral gene transfer followed by drug treatment to relieve suppression may provide a powerful combination for treatment of various genetic and infectious diseases.     

The effect of nitrate and nitrite on the respiration and cytochrome system of Rhizobium lupini]

Chromosome replication has been analysed in four species of Chilocorus. In C. ORBUS Csy., C. tricyclus Smith, and C. hexacyclus Smith, centric regions of all chromosomes are last to replicate, preceded in order by heterochromatic arms and euchromatic arms. In C. stigma Say, very late replication of centric regions can be detected only in otherwise wholly euchromatic chromosomes (= monophasics); in chromosomes with one arm heterochromatic ( = disphasics), these arms are last to replicate. Based on pachytene bivalent morphology and chromosome banding patterns, and supported by autoradiographic data, models are presented for the general organisation of Chilocorus chromosomes. All chromosomes in the first three species are subdivided into euchromatic arm, centric heterochromatin, and either a second euchromatic are (monophasics) or a heterochromatic arm (diphasics). Chilocorus stigma diphasics apparently lack distinct centric organisation, and are therefore divided into euchromatic and heterochromatic arms only.     

How Y chromosomes become genetically inert

We have investigated the mechanistic aspects of inactivation of the major larval cuticle protein genes (Lcp1-4) in Drosophila miranda during Y chromosome evolution. The Lcp genes are located on the X2 and neo-Y chromosomes in D. miranda but are autosomally inherited in all other Drosophila species investigated so far. In the neo-Y chromosome all four Lcp loci are embedded within a dense cluster of transposable elements. The X2 Lcp1-4 loci are expressed, while the Y chromosomal Lcp3 locus shows only reduced activity and the Lcp1, Lcp2, and Lcp4 are completely inactive. Our results suggest that Lcp1 and Lcp3 loci on the degenerating Y chromosome of D. miranda are silenced by neighboring transposable elements. These observations support our assumption that the first step in Y chromosome degeneration is the successive silencing of Y chromosomal loci caused by trapping and accumulation of transposons.