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B Byers

Publications and source records attributed to B Byers.

At least 55 records · Page 3Linked to original sources
PubMed1987

Structural comparison of the yeast cell division cycle gene CDC4 and a related pseudogene.

The function of the cell division cycle gene, CDC4, is required in Saccharomyces cerevisiae for progression beyond the G1 phase of the cell cycle. The wild-type gene was isolated from a plasmid library by selection for complementation of a recessive, temperature-sensitive allele. Hybridization of genomic sequences with the cloned gene revealed the presence of a duplicated sequence. Both CDC4 and the duplicated sequence were subjected to DNA sequence analysis. These analyses revealed (1) that CDC4 contains a large open reading frame encoding a protein of 779 amino acids, and (2) that the duplicated sequence bears strong homology with the carboxy-terminal segment of this open reading frame. Presence of a nonsense codon within the duplicated sequence suggested that it does not encode a functional product. Disruption of the duplicated sequence within the yeast genome provided a more critical test for function. The absence of any detectable phenotype for this disruption confirms that the sequence should be considered a pseudogene. The marker inserted to disrupt the sequence also served to map the duplication and to establish that it is not genetically linked to CDC4. The structural features determined suggest evolutionary relationships between these genes as well as between the CDC4 product and other proteins.

Base Sequence↗
PubMed1986

Yeast gene required for spindle pole body duplication: homology of its product with Ca2+-binding proteins.

Saccharomyces cerevisiae strains bearing temperature-sensitive alleles of the cell division cycle gene CDC31 are specifically defective in duplication of the spindle pole body, the microtubule-organizing center of yeast. To define the function encoded by CDC31 more fully, we have isolated genomic clones of the gene by selection for complementation of a temperature-sensitive allele. The locus from which the clone was derived was marked by integration of a nutritional marker and found by meiotic mapping to cosegregate with CDC31. The polypeptide sequence of the open reading frame in the CDC31 gene was determined and compared with the sequences of other known proteins. This revealed significant homology with the calmodulins and other members of the Ca2+-binding protein family. On the basis of comparison with these related proteins, it is evident that the CDC31 gene product has at least two binding sites for Ca2+ and is also homologous with other regions of the calmodulin sequence. We propose that Ca2+ fluxes within the yeast cell play a key role in spindle pole body duplication and consequently in the organization of the microtubule arrays.

Amino Acid Sequence↗
PubMed1985

Perceptual interference and hemispheric specialization.

Two experiments evaluated the effect of stimuli presented at fixation on the recognition of faces or random shapes presented to the left or right visual half-field (VF). Increasing the processing demands of the center stimulus produced a large, linear decrease in recognition from both VFs for both faces and shapes. Recognition of random shapes was decreased more in the right visual field by center digits and in the left VF by center faces and shapes. In addition, interference was found between the VF faces and the center digits to the left of fixation. It was concluded that differences in the processing capacity of the two hemispheres are a function of the verbal-nonverbal nature of the stimuli at a later stage in processing but that the two hemispheres may also differ along other perceptual dimensions at an earlier stage of visual recognition.

Attention↗
PubMed1984

Enhanced gene conversion and postmeiotic segregation in pachytene-arrested Saccharomyces cerevisiae.

Previous study has demonstrated that incubation of yeast cells of strain AP-1 in sporulation medium at 36 degrees permits them to begin meiosis but that they become arrested at pachytene and undergo enhanced intragenic recombination between ade2 heteroalleles. Tetrad analysis was undertaken to characterize the altered program of meiotic recombination more widely. In one set of experiments, pachytene-arrested cells were permitted to resume sporulation upon transfer to the permissive temperature. In the resulting asci, both postmeiotic segregation and gene conversion were increased several-fold at a number of loci relative to unarrested controls, whereas reciprocal recombination increased two- to threefold. Another set of experiments analyzed the genetic consequences of inducing the pachytene-arrested cells to revert directly to mitotic growth without completion of meiosis. The appearance of homozygous sectors from heterozygous markers revealed that these cells had become committed to appreciable recombination but that reciprocal exchange was less frequent than in normal asci. Taken together, the data indicate that pachytene arrest rendered the cells committed to enhanced recombination upon resumption of sporulation but that most of the crossing over did not occur until release from the arrest.--The genetic basis of pachytene arrest by AP-1 was investigated by mating each of its parents with progeny of strain Y55, which is able to sporulate at 36 degrees. Both of these diploids sporulated at 36 degrees, and asci from the one studied further exhibited 2:2 segregation of the sporulation defect, indicating that pachytene arrest is dependent on a recessive, temperature-sensitive allele at a chromosomal locus.

Alleles↗
PubMed1983

Separation of branched from linear DNA by two-dimensional gel electrophoresis.

A general method for separating branched DNA molecules, such as replication forks and recombination intermediates, from linear forms has been developed. Using as a model a stable X-shaped molecule constructed in vitro, it was found that this branched form migrated more slowly during agarose gel electrophoresis than did a linear form of the same mass. Higher agarose concentrations and higher electrophoretic voltages enhanced the extent of retardation. These properties provided the basis for an electrophoretic method of separating branched from linear molecules by variation of agarose concentration and voltage over two dimensions. In the first dimension, concentration and voltage were low; in the second, both parameters were increased, thereby forcing X-shaped molecules to migrate to positions distinct from a diagonal arc of linear molecules. In addition, two-dimensional electrophoresis was capable of separating X-shaped forms of different mass from each other, as well as from linear molecules.

Chemical Phenomena↗
PubMed1982

Reversible pachytene arrest of Saccharomyces cerevisiae at elevated temperature.

The temperature sensitivity of sporulation in a well-characterized yeast strain lacking any known temperature sensitive genes has been investigated. Cytological observations by electron microscopy demonstrate that cells incubated in sporulation medium at a temperature inhibitory to sporulation became arrested in meiotic prophase. The stage of arrest was identified as pachytene by the presence of duplicated (but unseparated) spindle pole bodies and synaptonemal complex. Transfer of the arrested culture to lower temperature permitted resumption of meiosis and sporulation; transfer to vegetative medium resulted in reversion to mitotic division. Genetic analysis of cells that had reverted to mitosis revealed that commitment to intragenic recombination had occurred by the time of arrest. Prolonged incubation at the elevated temperature resulted in the enhancement of intragenic recombination above normal levels, suggesting that some aspect of recombination continued to occur during the pachytene arrest. Evidence is presented that DNA replication, although depressed overall in the arrested cultures, had occurred to completion in many arrested cells.

DNA Replication↗
PubMed1982

Meiotic cytology of Saccharomyces cerevisiae in protoplast lysates.

This report describes cytological features of meiosis in Saccharomyces cerevisiae prepared for electron microscopy by lysis of protoplasts or nuclei on an aqueous surface. Whereas the chromatin of cells lysed before or after meiotic prophase was widely dispersed, pachytene bivalents appeared as discrete, elongate masses of compact chromatin. These bivalents were of nearly uniform thickness; they ranged in length from about 0.6 micrograms to 4.0 micrograms, with a median of 1.6-1.8 micrograms. Enzymatic digestion of chromosomal DNA removed the chromatin to reveal the underlying synaptonemal complex. The lysis of partially purified nuclei was less disruptive and thereby revealed the regular association of the telomeres with fragments of the nuclear envelope. In tetraploid cells, pachytene lysates contained quadrivalents characterized by the close apposition of chromatin masses of similar length. One or more points of intimate association appear to represent sites of exchange between pairing partners. The departure of the diploid cells from pachytene was accompanied by the renewed association of spindle microtubules with the chromosomes shortly before the diplotene chromosomes decondensed. Later, the successive meiotic divisions were identified by the appearance of a single spindle for meiosis I and of two spindles for meiosis II.

Chromatin↗
PubMed1980

Diploid spore formation and other meiotic effects of two cell-division-cycle mutations of Saccharomyces cerevisiae.

The meiotic effects of two cell-division-cycle mutations of Saccharomyces cerevisiae (cdc5 and cdc14) have been examined. These mutations were isolated by L.H. HARTWELL and his colleagues and characterized as defective in mitosis, causing a temperature-sensitive arrest in late nuclear division. When subjected to the restrictive temperature in meiosis, diploid cells homozygous for either of these mutations generally proceeded through premeiotic DNA synthesis and commitment to meiotic levels of recombination, but then arrested at a stage following spindle pole body (SPB) duplication and separation. The two SPBs lacked the interconnection by spindle microtubules typical of the complete meiosis I spindle. Challenge of these homozygotes by a semi-restrictive temperature often caused the production of asci containing two diploid spores. Genetic analysis of the viable pairs of spores revealed that each spore had become homozygous for centromere-linked markers significantly more frequently than for distal markers, indicating that the two spores each contained pairs of sister centromeres that had co-segregated in the reductional division of meiosis I. Ultrastructural analysis of the cdc5 homozygote demonstrated that these cells had completed meiosis I and formed two meiosis II spindles, but that the latter remained unusually short. This resulted in the encapsulation of both poles of each spindle within a single spore wall. These mutations therefore are defective in both meiotic divisions, as well as in the mitotic division described originally.

Cell Cycle↗
PubMed1979

Blindness secondary to steroid injections into the nasal turbinates.

Two cases of inadvertent intra-arterial injection into the anterior turbinates for treatment of allergic rhinitis resulted in retrograde flow of methylprednisolone acetate in the ophthalmic artery. Both patients demonstrated embolization of the retinal circulation, and one of the choroidal circulation as well. Visual outcome was favorable, although one patient showed a permanent paracentral scotoma. Fundus photographs clearly showed intra-arterial obstruction of arterioles that cleared within several days. Precautions can be taken for the prevention of this complication.

Adult↗
PubMed1979

Occurrence of crossed strand-exchange forms in yeast DNA during meiosis.

The crossed strand-exchange form (Holliday structure, half chiasma) has been predicted as an intermediate in the genetic recombination of eukaryotes. We report here the detection of this form in the yeast plasmid, 2-micron DNA, isolated during meiosis. Physical mapping has previously suggested that two forms of 2-micron DNA arise because of recombination between inverted repeat regions. After appropriate digestion with restriction endonuclease, a crossed strand-exchange form intermediate in this recombination would yield an X-shaped form resistant to loss by branch migration because of nonhomology in sequences flanking the region of homology. We first generated this X-shaped form artificially by reannealing melted restriction fragments of 2-micron DNA. This enabled us to develop a procedure for the physical separation of the X-shaped form by agarose gel electrophoresis. We then used this electrophoretic procedure to isolate a naturally occurring form of identical structure from the 2-micron DNA of meiotic cells. Electron microscopy demonstrated that the exchange junction had the expected configuration of strands and indicated that the junction occurred within the region of homology.

Crossing Over, Genetic↗
PubMed1978

Meiotic effects of DNA-defective cell division cycle mutations of Saccharomyces cerevisiae.

The meiotic effects of several cell division cycle (cdc) mutations of Saccharomyces cerevisiae have been investigated by electron microscopy and by genetic and biochemical methods. Diploid strains homozygous for cdc mutations known to confer defects on vegetative DNA synthesis were subjected to restrictive conditions during meiosis. Electron microscopy revealed that all four mutants were conditionally arrested in meiosis after duplication of the spindle pole bodies but before spindle formation for the first meiotic division. None of these mutants became committed to a recombination or contained synaptonemal complex at the meiotic arrest.--The mutants differed in their ability to undergo premeiotic DNA synthesis under restrictive conditions. Both cdc8 and cdc21, which are defective in the propagation of vegetative DNA synthesis, also failed to undergo premeiotic DNA synthesis. The arrest of these mutants at the stage before meiosis I spindle formation could be attributed to the failure of DNA synthesis because inhibition of synthesis by hydroxyurea also caused arrest at this stage.--Premeiotic DNA synthesis occurred before the arrest of cdc7, which is defective in the initiation of vegetative DNA synthesis, and of cdc2, which synthesizes vegetative DNA but does so defectively. The meiotic arrest of cdc7 homozygotes was partially reversible. Even if further semiconservative DNA replication was inhibited by the addition of hydroxyurea, released cells rapidly underwent commitment to recombination and formation of synaptonemal complexes. The cdc7 homozygote is therefore reversibly arrested in meiosis after DNA replication, whereas vegetative cultures have previously been shown to be defective only in the initation of DNA synthesis.

Cell Cycle↗
PubMed1978

Stable denaturation of chromosomal DNA from Saccharomyces cerevisiae during meiosis.

Partial denaturation of Saccharomyces cerevisiae chromosomal DNA was found to occur spontaneously during meiosis. Short regions of strand separation (300 base pairs long) were seen in DNA molecules prepared for electron microscopy by the aqueous spreading technique. These regions were clustered along the DNA. The time course of their appearance indicated that the denatured regions were present during the periods of premeiotic DNA replication and recombination. A similar pattern of denaturation was also detected in the DNA from vegetatively grown cells of a conditional cdc8 mutant, which is defective in DNA replication.

Chromosomes↗
PubMed1978

The role of spindle pole bodies and modified microtubule ends in the initiation of microtubule assembly in Saccharomyces cerevisiae.

The spindle poles of the budding yeast, Saccharomyces cerevisiae, have been removed from mitotic and meiotic cells by osmotic lysis of spheroplasts. The spindle pole bodies (SPBs)--diskoidal structures also termed 'spindle plaques'--have been analysed for their ability to potentiate the polymerization of microtubules in vitro. Free SPBs were completely deprived of any detectable native microtubules by incubation in the absence of added tubulin and were then challenged with chick neurotubulin, which had been rendered partially defective in self-initiation of repolymerization. Electron microscopy revealed that these SPBs served as foci for the initiation of microtubule polymerization in vitro. Because the attached microtubules elongated linearly with time but did not increase in numbers after the first stage of the reaction, it is apparent that there are a limited number of sites for initiation. The initiating potential of the SPBs was found to be inhibited by enzymic hydrolysis of protein but not of DNA. The microtubule end proximal to the site of initiation on the SPB is distinguished by a 'closed' appearance because of a terminal component which is continuous with the microtubule wall, whereas the distal end has the 'open' appearance characteristic of freely repolymerized neurotubules. SPBs which were partially purified on sucrose gradients retained their ability to initiate the assembly of microtubules with the same structural differentiation of their ends. The occurrence of closed proximal ends on native yeast microtubules suggests that closed ends may play a role in the initiation of microtubule polymerization in vivo, as well as in vitro.

Cell Differentiation↗
PubMed1977

Pattern dystrophy of the pigment epithelium.

Three generations of a family showed alterations in the retinal pigment epithelium along with mild visual disturbance. Younger members had granular pigment dispersed beneath the fovea while older members showed a reticular pigment pattern. The electro-oculogram was subnormal in most affected individuals, but visual fields, color vision, dark adaptation, and the electroretinogram were normal. The pigmentary changes in these patients are similar to those seen in other pigment epithelial dystrophies such as reticular dystrophy and butterfly-shaped dystrophy. Until these disorders can be separated pathophysiologically, we suggest the definition of a more inclusive clinical term, "pattern dystrophies of the pigment epithelium.

Adolescent↗
PubMed1975

The isolation and characterization of nuclear ghosts from cultured HeLa cells.

Macromolecular complexes, which appear as ghosts when viewed by phase contrast microscopy, have been isolated from the nuclei of HeLa cells grown in culture. The preparation of these ghosts involves a detergent wash which removes the unit membranes of the nuclear envelop structure but leaves intact both the nuclear pores and the dense structure conferring nuclear margins (possibly the dense lamella). Detergent-washed nuclei are subsequently treated with 0.5 M MgCl2 and fractionated on continuous sucrose gradients containing 0.5 M MgCl2. The ghosts are recovered as a sharp band at an apparent sucrose density of 47-52% and consist of 72% protein, 10% phospholipid, 14% DNA, And 4% RNA. The release of the majority of intranuclear components is indicated by the large loss of nuclear DNA (95%), RNA (71%), and protein (87%) contrasted to the small loss of phospholipid (27%) druing the conversion of detergent washed nuclei to isolated ghosts. Sodium dodecyl sulfate-polyacrylamide gel patterns of the ghost proteins consist of two major bands with approximate molecular weights of 20,000 and 35,000. The isolation of ghosts with a similar density and protein composition from nondetergent-washed nuclei indicates that the ghost is not an artifact induced by the detergent treatment. The absence of cytoplasmic contamination in the preparations of detergent washed nuclei and nuclear ghosts was demonstrated by chemical, enzymatic, and electron microscope studies. We suggest that the isolated ghosts represent a structural macromolecular complex which underlies and is probably attached to the inner nuclear membrane of intact nuclei. The possible additional presence of intranuclear network proteins has not been excluded.

Amino Acids↗
PubMed1975

Electron microscopic observations on the meiotic karyotype of diploid and tetraploid Saccharomyces cerevisiae.

Certain strains of Saccharomyces cerevisiae contain visible segments of synaptonemal complex which are apparent components of bivalents in pachytene of meiotic prophase. The synaptonemal complex has the typical width in the frontal plane but is unusually thin in the sagittal plane, thus accounting for its poor visibility. Amorphous densities situated adjacent to the central element occur at intervals suggesting their coincidence with sites of crossing over. Reconstruction of the synaptonemal complex from serial sections has permitted karyotypic analysis. The number of segments of synaptonemal complex and the distribution of their legths is consistent with the genetic map. Two, possibly three, segments enter the nucleolus as if bearing sequences encoding ribosomal RNA. Reconstruction of tetraploid nuclei reveals an approximate doubling of the diploid chromosome number and confirms the pattern of nucleolar entry. Quadrivalent pairing is evident between the pairs of synaptonemal complex segments in the tetraploid nuclei.

Cell Nucleus↗