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| Limits: Publication Date from 1984/01/01 to 1984/12/31 |
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Rearrangements of highly polymorphic regions near telomeres of Saccharomyces cerevisiae.
Horowitz H, Thorburn P, Haber JE.
We have examined the mitotic and meiotic properties of telomeric regions in various laboratory strains of yeast. Using a sequence (Y probe) derived from a cloned yeast telomere (J. Szostak and E. Blackburn, Cell 29:245-255, 1982), we found that various strains of Saccharomyces cerevisiae show extensive polymorphisms of restriction endonuclease fragment length. Some of the variation in the lengths of telomeric fragments appears to be under the control of a small number of genes. When DNA from various strains was digested with endonuclease KpnI, nearly all of the fragments homologous to the Y probe were found to be of different size. The pattern of fragments in different strains was extremely variable, with a greater degree of polymorphism than that observed for fragments containing the mobile TY1 element. Tetrad analysis of haploid meiotic segregants from diploids heterozygous for many different Y-homologous KpnI fragments revealed that most of them exhibited Mendelian (2:0) segregation. However, only a small proportion of these fragments displayed the obligate 2:2 parental segregation expected of simple allelic variants at the same chromosome end. From the segregations of these fragments, we concluded that some yeast telomeres lack a Y-homologous sequence and that the chromosome arms containing a Y-homologous sequence are different among various yeast strains. Regions near yeast telomeres frequently undergo rearrangement. Among eight tetrads from three different diploids, we have found three novel Y-homologous restriction fragments that appear to have arisen during meiosis. In all three cases, the appearance of a new fragment was accompanied by the loss of another band.(ABSTRACT TRUNCATED AT 250 WORDS)
PMID: 6392854 [PubMed - indexed for MEDLINE]
Transfer of yeast telomeres to linear plasmids by recombination.
Dunn B, Szauter P, Pardue ML, Szostak JW.
We have characterized two types of recombination events between linear plasmids and yeast chromosomal telomere-adjacent sequences (Y' elements). In one type of event, a linear plasmid restriction-cut within a Y' element regains the missing Y' DNA, and may also acquire additional Y' elements. This process is similar to the healing of broken chromosomes by recombination. In a second type of event, terminally added C1-3A sequences on the linear plasmid interact with C1-3A sequences located just internal to the chromosomal Y' elements, resulting in the addition of one to four Y' elements to the plasmid. Similar recombination events occurring between different chromosome ends could lead to the dispersal and amplification of telomere-adjacent sequences.
PMID: 6091911 [PubMed - indexed for MEDLINE]
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Unusual DNA sequences associated with the ends of yeast chromosomes.
Walmsley RW, Chan CS, Tye BK, Petes TD.
The genome of the yeast Saccharomyces cerevisiae, like those of other eukaryotes, contains multiple sequences that hybridize with a poly(GT) probe. We have shown previously that some of the sequences that hybridize with the poly(GT) probe are located near the tips of the yeast chromosomes. We report here that many of the remaining poly(GT)-hybridizing sequences are associated with a family of putative replication origins localized near the chromosome ends. These sequences have the general form poly(C1-3A), similar to sequences reported to occur at the tips of chromosomes in the accompanying paper. In addition to poly(C1-3A) tracts, yeast cells contain tracts of alternating C and A bases, similar to those seen in mammalian genomes. These results are used as the basis for a new model of telomere replication.
PMID: 6377091 [PubMed - indexed for MEDLINE]
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DNA sequences of telomeres maintained in yeast.
Shampay J, Szostak JW, Blackburn EH.
Telomeres, the ends of eukaryotic chromosomes, have long been recognized as specialized structures. Their stability compared with broken ends of chromosomes suggested that they have properties which protect them from fusion, degradation or recombination. Furthermore, a linear DNA molecule such as that of a eukaryotic chromosome must have a structure at its ends which allows its complete replication, as no known DNA polymerase can initiate synthesis without a primer. At the ends of the relatively short, multi-copy linear DNA molecules found naturally in the nuclei of several lower eukaryotes, there are simple tandemly repeated sequences with, in the cases analysed, a specific array of single-strand breaks, on both DNA strands, in the distal portion of the block of repeats. In general, however, direct analysis of chromosomal termini presents problems because of their very low abundance in nuclei. To circumvent this problem, we have previously cloned a chromosomal telomere of the yeast Saccharomyces cerevisiae on a linear DNA vector molecule. Here we show that yeast chromosomal telomeres terminate in a DNA sequence consisting of tandem irregular repeats of the general form C1-3A. The same repeat units are added to the ends of Tetrahymena telomeres, in an apparently non-template-directed manner, during their replication on linear plasmids in yeast. Such DNA addition may have a fundamental role in telomere replication.
PMID: 6330571 [PubMed - indexed for MEDLINE]
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Elaboration of telomeres in yeast: recognition and modification of termini from Oxytricha macronuclear DNA.
Pluta AF, Dani GM, Spear BB, Zakian VA.
The termini of macronuclear DNA molecules from the protozoan Oxytricha fallax share a common sequence and structure, both of which differ markedly from those deduced for yeast telomeres. Despite these differences, terminal restriction fragments from O. fallax macronuclear DNA can support telomere formation in yeasts. Two linear plasmids (LYX-1 and LYX-2) constructed by ligating BamHI-digested total Oxytricha macronuclear DNA to a yeast vector were analyzed. One end of LYX-1 and both ends of LYX-2 are derived from the Oxytricha DNA that encodes rRNA (rDNA) whereas the other end of LYX-1 is from an Oxytricha fragment other than rDNA. After propagation in yeast, both ends of LYX-1 and LYX-2 retain the C4A4 repeat characteristic of the O. fallax terminal sequence. In addition, both ends of both plasmids acquire 300-1000 base pairs of DNA containing the sequence (C-A)n, a sequence found near the termini of yeast chromosomes. Thus, at least two different Oxytricha termini display distinctive properties in yeast cells in that linear plasmids containing them are not degraded nor are they integrated into chromosomal DNA. These Oxytricha termini may act directly as telomeres in yeast; alternatively, the Oxytricha DNA may serve as a signal that results in the elaboration of a yeast telomere on the ciliate DNA.
PMID: 6324194 [PubMed - indexed for MEDLINE]
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The mechanism of meiotic homologue pairing.
Maguire MP.
Homologous chromosome pairing involves the moving together of matching chromosomes or chromosome segments across substantial distances within a nucleus. Although the time in the life cycle of initial association of homologues varies among organisms, it may well be that similar underlying mechanisms for its occurrence prevail throughout sexually reproducing eukaryotes. The means by which pairing its accomplished is in no case understood. In the apparent absence of a long range specific force of attraction, simple partial models have been proposed which relay for the most part upon interactions of chromosome ends (telomeres) with specialized portions of the nuclear envelope. While such interactions, as well as the persistence of chromosome orientation established by mitotic anaphase poleward movement of centromere regions, may provide in many cases for closer than random positioning of some parts of homologues, the distances remaining to be traversed are still long range in physical-chemical terms. Also, the specific pairing observed in some kinds of rearranged segments is not facilitated by such circumstances, even if synapsis is initiated at available homologous telomere pairs and proceeds to completion by a "zip-up" process. A unified, more complex model is considered which is designed to accommodate the various relevant findings. It invokes the interaction of intranuclear structures with intercalary and/or terminal chromosomal pairing sites, e.g. filamentous structures which specifically bind to these, and a contractile system involving proteins such as actin and myosin to draw homologues together.
PMID: 6423910 [PubMed - indexed for MEDLINE]
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DNA rearrangements of the variable surface antigen genes of the trypanosomes.
Murphy WJ, Brentano ST, Rice-Ficht AC, Dorfman DM, Donelson JE.
The trypanosome genome contains several hundred (and perhaps several thousand) genes for the trypanosome variable surface glycoproteins (VSGs). In an individual trypanosome only one of these genes is expressed at a given instant; the others are transcriptionally silent. This differential gene expression is responsible for the sequential antigenic variation displayed by trypanosomes. It is mediated by two types of genomic rearrangements of these VSG genes. The best understood rearrangement type is the formation of a transcriptionally-active expression-linked extra copy (ELC) of a transcriptionally-silent basic copy (BC) gene. This duplication and translocation event places the ELC near a chromosomal end (a telomere) where it is apparently located downstream from a strong promotor. Some VSG genes are not expressed via this ELC mechanism. These genes, which seem to already be near telomeres, are activated by a different non-duplication associated ( NDA ) type of mechanism. We have used recombinant DNA techniques to clone and determine the sequences of genes expressed by both the ELC and NDA mechanisms. Comparison of these sequences reveals that sequences flanking the VSG coding regions are similar. This indicates that there is a sequence correlation between the two mechanisms of expression. We have also shown that when bloodstream trypanosomes expressing a specific VSG via the ELC mechanism are established in culture, the resultant procyclic trypanosomes rapidly stop synthesizing the VSG mRNA (and the VSG) but retain the ELC of the VSG gene. This demonstrates that transcription of an ELC can cease without the loss of that ELC and may indicate the presence of other factors regulating VSG gene transcription.
PMID: 6737319 [PubMed - indexed for MEDLINE]
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Healing of broken linear dicentric chromosomes in yeast.
Haber JE, Thorburn PC.
In yeast, meiotic recombination between a linear chromosome III and a haploid-viable circular chromosome will yield a dicentric, tandemly duplicated chromosome. Spores containing apparently intact dicentric chromosomes were recovered from tetrads with three viable spores. The spore containing the dicentric inherited URA3 (part of the recombinant DNA used to join regions near the ends of the chromosome into a circle) as well as HML, HMR and MAL2 (located near the two ends of a linear but deleted from the circle). The Ura+ Mal+ colonies were highly variegated, giving rise to as many as seven distinctly different stable ("healed") derivatives, some of which were Ura+ Mal+, others Ura+ Mal- and others Ura- Mal+. The colonies were also sectored for five markers (HIS4, LEU2, CRY1, MAT and THR4) initially heterozygous in the tandemly duplicated dicentric chromosome.--Southern blot and genetic analyses have demonstrated that these stable derivatives arose from mitotic breakage have demonstrated that these stable derivatives arose from mitotic breakage of the dicentric chromosome, followed by one of several different healing events. The majority of the stable derivatives contained circular or linear chromosomes apparently resulting from homologous recombination between a broken chromosome end and a homologous region on the other end of the original dicentric duplicated chromosome. A smaller proportion of events resulted in apparently uniquely healed linear chromosomes in which the broken chromosome acquired a new telomere. In two instances we recovered chromosome III partially duplicated with a novel right end. We have also found one derivative that had also experienced rearrangement of repeated DNA sequences found adjacent to yeast telomeres.
PMID: 6365688 [PubMed - indexed for MEDLINE]
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Cloning of the vaccinia virus telomere in a yeast plasmid vector.
DeLange AM, Futcher B, Morgan R, McFadden G.
The vaccinia virus DNA telomere, which contains a covalently closed hairpin structure, has been cloned in a yeast plasmid vector. Restriction mapping indicates that the cloned vaccinia telomere is maintained in yeast not in its native hairpin configuration but as an inverted repeat structure, within a circular plasmid, with the sequences of the viral hairpin now at the axis of symmetry of an imperfect palindrome. As such, the cloned telomere resembles the telomeric replicative intermediate observed during vaccinia virus DNA replication. Small deletions and duplications in the viral inverted repeats of different clones suggest a model in which the observed circular plasmids were generated in yeast by the replication of hybrid linear DNA molecules consisting of the linearized yeast vector flanked by two hairpin-containing vaccinia termini.
PMID: 6325299 [PubMed - indexed for MEDLINE]
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Ultrastructure of a temperature-induced Balbiani ring in Chironomus thummi.
Santa-Cruz MC, Morcillo G, Diez JL.
A Balbiani ring-like structure (T-BR III) is induced at the right telomere on chromosome III by a 35 degrees C heat-shock. The location of T-BR III was identified in 3 micron-semithin sections that were afterwards resectioned to obtain ultrathin sections. These were stained either by uranyl acetate-lead citrate or PTA. The puff appeared composed of different structures: small compact chromatin bodies, loose chromatin with an apparently fibrillar organization, and granules. The granules, 200-250 A in diameter, appeared either in linear arrays or in a clustered form. The three components described above were interspersed within the T-BR without a compartmentalized organization. EDTA preferential ribonucleoprotein staining technique evidenced an EDTA-positive material within the T-BR that corresponded to 200-250 A granules as well as apparently fibrillar structures. However, EDTA did not completely stain some clustered granules. Neither free nor clustered granules were found in T-BRs formed in the presence of actinomycin D. The significance of the different T-BR structures in relation to the transcriptional activity of the puff is discussed.
PMID: 6241831 [PubMed - indexed for MEDLINE]
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