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Biomedical subjects

J R Simon

Publications and source records attributed to J R Simon.

At least 55 records · Page 3Linked to original sources
PubMed1990

Transformation and recombination in rad mutants of Saccharomyces cerevisiae.

Disruption/deletion mutations in genes of the RAD52 epistasis group of Saccharomyces cerevisiae were examined for their effects on recombination between single- and double-stranded circular DNA substrates and chromosomal genes in a transformation assay. In rad50 mutants there was a small reduction in recombination with single-stranded DNA at the leu2-3, 112 allele; in addition there was an almost complete elimination of recombination at trp1-1 for both single- and double-stranded DNA. Reintroduction of a wild-type RAD50 gene on a replicating plasmid carrying CEN4 restored recombinational competence at trp1-1, indicating that rad50 is defective in gene replacement of this allele. In rad52 mutants a reduction of 30%-50% in recombination involving either single- or double-stranded circular DNA was observed in each experiment when compared to the wild type. This reduction of recombination in rad52 mutants was similar for recombination at the ura3-52 mutant locus where only integration events have been observed, and at the trp1-1 mutant locus, where recombination occurs predominantly by gene replacement. Neither the rad54 nor the rad57 mutations had a significant effect on recombination with single- or double-stranded DNA substrates.

Genes, Fungal↗
PubMed1990

Effect of conflicting cues on information processing: the 'Stroop effect' vs. the 'Simon effect'.

This study examined the relationship between two sources of interference in human information processing: the Stroop effect and the Simon effect. Forty subjects pressed a left- or right-hand key in response to a Stroop color word located on the left or right side of a screen. For one group, ink color was the relevant cue and, for another group, word meaning was the relevant cue. Independent variables were: congruence, i.e., agreement or lack thereof between the ink color and meaning of the Stroop word; spatial correspondence, i.e., agreement or lack thereof between the location of the Stroop word and the location of the key used to make the response; and stimulus duration, i.e., 400 or 100 ms. Each of these variables had a significant effect on RT, and there were no significant interactions. According to Sternberg's additive-factor logic, these findings suggest that the Stroop effect (congruence) and the Simon effect (spatial correspondence) involve separate stages of processing. If one assumes that manipulation of stimulus duration effects the encoding stage, then results also suggest that neither the Stroop effect nor the Simon effect involves the stimulus encoding stage.

Attention↗
PubMed1990

The structure of microtubule ends during the elongation and shortening phases of dynamic instability examined by negative-stain electron microscopy.

Microtubules (MTs) are dynamic polymers that can exist in phases of elongation and rapid-shortening at steady-state. These phases have been observed in vitro and in living cells, and this property of MTs has been termed 'dynamic instability'. The purpose of this study was to use negative-stain electron microscopy (EM) to test if there are structural differences between the ends of MTs in the elongation and shortening phases, which could provide insight into the mechanisms of dynamic instability. MTs in the elongation phase were obtained by seeding either highly purified porcine brain tubulin (PC-tubulin) or tubulin containing microtubule-associated proteins (MTP), from isolated Tetrahymena axonemes. The results are that, in addition to intact cylindrical MTs, a significant fraction of the tubulin polymer in the elongation phase occurred as sheets of parallel protofilaments, as found in previous investigations with self-assembled MTs. Therefore, sheet formation is an intrinsic property of MT assembly that does not depend on the tubulin purity or the method of nucleation. Also, since sheets lack helical symmetry, at least a fraction of tubulin polymers seeded from axonemes did not assemble by helical addition of tubulin dimers to the ends, an assumption often made in mathematical models of dynamic instability. Sheets and intact MTs that were seeded from isolated axonemes, emanated both from the intact MT wall of the axoneme A-subfiber and from the incomplete wall of the B-subfiber. Therefore, axoneme seeds do not provide a homogeneous nucleation site for tubulin growth, or produce a homogeneous population of tubulin polymers under our conditions. Previous evidence has indicated that MT disassembly can occur by a segmental release of tubulin oligomers from the ends and at sites along the length of MTs. However, these studies were performed with MTP, and disassembly was induced by cold depolymerization. We examined MT shortening under conditions that closely represent shortening via dynamic instability, namely isothermal dilution at 37 degrees C of self-assembled MTs. This was compared with the morphology of cold-disassembled MTs. The cold-depolymerization of MTs composed of MTP showed rings and protofilament curls as previously observed using similar methods. Surprisingly, cold-depolymerization of MTs assembled from PC-tubulin induced not only shortening, but also the opening of a large fraction of MTs into sheets, suggesting that the MT lattice contains a cold-labile seam. Under conditions that mimic stochastic shortening, MTs were intact, closed cylinders with ends that were approximately blunt. Therefore, rapid shortening occurs at the ends of the MT, without a long-range disruption of the MT wall. In conclusion, MTs in the elongation phase can have highly irregular ends and need not elongate by a helical assembly process. Conversely, MTs in the shortening phase can have relatively blunt, even ends and can depolymerize in a relatively uniform fashion.

Animals↗
PubMed1989

A rapid and efficient procedure for transformation of intact Saccharomyces cerevisiae by electroporation.

A rapid and efficient procedure is described for transforming Saccharomyces cerevisiae using electroporation to render intact cells permeable to DNA. The technique uses relatively low voltages and is particularly sensitive to low concentrations of plasmid DNA. At the highest voltage used (400 volts), the frequency of transformation increased with the amount of plasmid DNA between 25 ng and 100 ng. At higher concentrations of DNA (1-1.5 micrograms) electroporation yielded one-third to one-half the number of transformants obtained with a standard lithium acetate pretreatment. Because this method requires neither pretreatment of cells nor addition of polyethylene glycol (PEG), it has several advantages over currently used transformation procedures.

Electric Stimulation↗
PubMed1989

Activity-wheel stress and serotonergic hypersensitivity in rats.

Adult male Wistar rats were subjected to activity wheel stress: unlimited access to an activity wheel for up to twelve days and food for 30 to 60 min each day. Each treated rat was paired with a control, the latter being housed in home cages and given sufficient food to maintain a weight similar to the stressed partner. All rats were previously trained on a variable interval schedule for milk reinforcement. When the activity of the stressed rat increased rapidly then decreased suddenly, the pair was decapitated for biochemical analysis. Levels of the serotonin metabolite, 5-hydroxyindoleacetic acid, decreased by 50%, and the Bmax for ketanserin binding increased by 19% in frontal cortical homogenates from the stressed rats when compared to controls. These data support the concept that stress increases the sensitivity of central serotonin receptors.

Animals↗
PubMed1989

Restoration of high affinity choline uptake in the hippocampal formation following septal cell suspension transplants in rats with fimbria-fornix lesions.

High affinity choline uptake (HACU) was investigated in the hippocampal formation following fetal septal cell suspension transplants into rats with fimbria-fornix lesions. Nine-14 weeks after transplantation, HACU was markedly decreased in hippocampi from animals with fimbria-fornix lesions; this decrease was ameliorated by fetal septal cells transplanted into the host hippocampus. HACU related to septal transplantation was activated in vitro by K+, and in vivo by the administration of scopolamine and picrotoxin. These findings suggest that fetal septal cell transplantation can restore HACU in the host hippocampus following fimbria-fornix lesions, and that HACU related to the graft has pharmacological properties similar to those of the normal adult HACU system. The activation of HACU by picrotoxin, a gamma-aminobutyric acid (GABA) antagonist, suggests that transplanted cholinergic neurons receive either direct or indirect functional input from GABAergic afferents from the transplant and/or host hippocampus. Lesions of the fimbria-fornix also resulted in an increased binding to muscarinic receptors in the dorsal hippocampus. This increase in binding was not significantly ameliorated by intrahippocampal grafts of cholinergic neurons.

Animals↗
PubMed1989

The effect of prediction accuracy on choice reaction time.

In this study, we examined the effect of prediction accuracy on reaction time (RT). Subjects performed on three blocks of choice RT trials, all of which involved the mapping of four stimuli (red, green, 1, or 0) onto two response keys. The subjects were told that the four stimuli were equally probable and that their task was to respond to each stimulus onset by pressing the correct key. In one block (stimulus-prediction), the subjects predicted, prior to each trial, the precise stimulus that would appear. In a second block (category-prediction), the subjects predicted the category of the stimulus (i.e., color or digit) that would appear. In a third block (no-prediction), the subjects simply responded to each stimulus without making a prior prediction. In the stimulus-prediction block, RT was faster for correct predictions than for incorrect predictions. In addition, RT was faster on trials in which an incorrect prediction involved the correct category than on trials in which it involved the incorrect category: that is, a "half-wrong" prediction was better than an "all-wrong" prediction. In the category-prediction block, RT was faster when the stimulus category was predicted correctly than when it was not. There was little evidence of a response-facilitation contribution to the correct-prediction effect. These results permit inferences concerning the encoding and organization of information in memory.

Adult↗
PubMed1988

The molecular mobility of alpha-actinin and actin in a reconstituted model of gelation.

Dictyostelium discoideum alpha-actinin (D.d. alpha-actinin) is a calcium and pH-regulated actin-binding protein that can cross-link F-actin into a gel at a submicromolar free calcium concentration and a pH less than 7 [Fechheimer, et al., 1982]. We examined mixtures of actin and D.d. alpha-actinin at four pH and calcium concentrations that exhibited various degrees of gelation or solation. The macroscopic viscosities of these mixtures were measured by falling ball viscometry (FBV) and compared to the translational diffusion coefficients measured by gaussian spot and periodic-pattern fluorescence photobleaching recovery (FPR) of both the actin filaments and D.d. alpha-actinin. A homogeneous, macroscopic gel was not composed of a static actin network. Instead, the filament diffusion coefficient decreased to approximately 65% of the control value. If the D.d. alpha-actinin concentration was increased, the solution became inhomogeneous, consisting of domains of higher actin concentration. These domains were often composed of a static actin network. The mobility of D.d. alpha-actinin consisted of a major fraction that freely diffused and a minor fraction that appeared immobile under the conditions employed. This suggested that D.d. alpha-actinin binding to the actin filaments was static over the time course of measurement (approximately 5 sec). Under solation conditions, there was no apparent interaction of actin with D.d. alpha-actinin. These results demonstrate that 1) actin filaments need not be cross-linked into an immobile, static array in order to have macroscopic properties of a gel; 2) interpretation of the rheological properties of actin:alpha-actinin gels are complicated by spatial heterogeneity of the filament concentration and mobility; and 3) a fraction of D.d. alpha-actinin binds statically to actin in undisturbed gels. The implications of these results are discussed in relation to cytoplasmic structure and contractility.

Actinin↗
PubMed1988

Induction of homologous recombination in Saccharomyces cerevisiae.

We have investigated the effects of UV irradiation of Saccharomyces cerevisiae in order to distinguish whether UV-induced recombination results from the induction of enzymes required for homologous recombination, or the production of substrate sites for recombination containing regions of DNA damage. We utilized split-dose experiments to investigate the induction of proteins required for survival, gene conversion, and mutation in a diploid strain of S. cerevisiae. We demonstrate that inducing doses of UV irradiation followed by a 6 h period of incubation render the cells resistant to challenge doses of UV irradiation. The effects of inducing and challenge doses of UV irradiation upon interchromosomal gene conversion and mutation are strictly additive. Using the yeast URA3 gene cloned in non-replicating single- and double-stranded plasmid vectors that integrate into chromosomal genes upon transformation, we show that UV irradiation of haploid yeast cells and homologous plasmid DNA sequences each stimulate homologous recombination approximately two-fold, and that these effects are additive. Non-specific DNA damage has little effect on the stimulation of homologous recombination, as shown by studies in which UV-irradiated heterologous DNA was included in transformation/recombination experiments. We further demonstrate that the effect of competing single- and double-stranded heterologous DNA sequences differs in UV-irradiated and unirradiated cells, suggesting an induction of recombinational machinery in UV-irradiated S. cerevisiae cells.

DNA Damage↗
PubMed1988

Analysis of rhodamine and fluorescein-labeled F-actin diffusion in vitro by fluorescence photobleaching recovery.

Properties of filamentous acetamidofluorescein-labeled actin and acetamidotetramethylrhodamine-labeled actin (AF and ATR-actin, respectively) were examined to resolve discrepancies in the reported translational diffusion coefficients of F-actin measured in vitro by FPR and other techniques. Using falling-ball viscometry and two independent versions of fluorescence photobleaching recovery (FPR), the present data indicate that several factors are responsible for these discrepancies. Gel filtration chromatography profoundly affects the viscosity of actin solutions and filament diffusion coefficients. ATR-actin and, to a lesser degree, AF-actin show a reduction in viscosity in proportion to the fraction labeled, presumably due to filament shortening. Actin filaments containing AF-actin or ATR-actin are susceptible to photoinduced damage, including a covalent cross-linking of actin protomers within filaments and an apparent cleavage of filaments detected by a decrease of the measured viscosity and an increase in the measured filament diffusion coefficients. Quantum yields of the two photoinduced effects are quite different. Multiple cross-links are produced relative to each photobleaching event, whereas less than 1% filament cleavage occurs. Substantial differences in the filament diffusion coefficients measured by FPR are also the result of differences in illumination geometry and sampling time. However, under controlled conditions, FPR can be used as a quantitative tool for measuring the hydrodynamic properties of actin filaments. Incremented filament shortening caused by photoinduced cleavage or incremental addition of filament capping proteins produces a continuous and approximately linear increase of filament diffusion coefficients, indicating that filaments are not associated in solution. Our results indicate that actin filaments exhibit low mobilities and it is inferred that actin filaments formed in vitro by column-purified actin, under standard conditions, are much longer than has conventionally been presumed.

Actins↗
PubMed1987

Dopamine D2 receptors increase in the dorsolateral striatum of weaver mutant mice.

Dopamine D2 receptors were studied in homozygous weaver mutant mice (wv/wv), heterozygous littermates (wv/+), and normal mice (+/+). Specific [3H]spiperone binding was significantly higher in the dorsolateral part of the striatum in the weaver mutant mice (wv/wv) than in normal mice (+/+). No significant differences among the 3 genotypes were found in other parts of the striatum or in the nucleus accumbens.

Animals↗
PubMed1987

Identification of a glycine-like fragment on the strychnine molecule.

Strychnine, a complex molecule, antagonizes in some unknown manner the action of glycine, an important inhibitory neurotransmitter in the spinal cord and brainstem of many vertebrates. To help understand the mechanism of this antagonism, we have employed modern computational methods to assess the similarities between these seemingly different molecules. An exhaustive comparison of topological and electronic features of both molecules was made. We have successfully located a glycine-like fragment in the strychnine molecule that, when compared to glycine, exhibits both topological and electronic charge congruence. The successful location of this glycine-like fragment allows us to speculate how the large strychnine molecule assumes its role as an antagonist against the inhibitory action of glycine, the simplest amino acid.

Animals↗
PubMed1987

Homologous recombination between single-stranded DNA and chromosomal genes in Saccharomyces cerevisiae.

Transformation of Saccharomyces cerevisiae strains was examined by using the URA3 and TRP1 genes cloned into M13 vectors in the absence of sequences capable of promoting autonomous replication. These constructs transform S. cerevisiae cells to prototrophy by homologous recombination with the resident mutant gene. Single-stranded DNA was found to transform S. cerevisiae cells at efficiencies greater than that of double-stranded DNA. No conversion of single-stranded transforming DNA into duplex forms could be detected during the transformation process, and we conclude that single-stranded DNA may participate directly in recombination with chromosomal sequences. Transformation with single-stranded DNA gave rise to both gene conversion and reciprocal exchange events. Cotransformation with competing heterologous single-stranded DNA specifically inhibited transformation by single-stranded DNA, suggesting that one of the components in the transformation-recombination process has a preferential affinity for single-stranded DNA.

Binding, Competitive↗
PubMed1985

Central nervous system control of alcohol dehydrogenase activity in rats.

The activity of alcohol dehydrogenase is higher in the livers of female rats than that in male rats. Stereotaxic lesions of the anterior periventricular hypothalamus of male rats increased the activity of alcohol dehydrogenase by about 50 percent; larger lesions which destroyed many of the major hypothalamic nuclei increased the enzyme activity 100 percent and abolished the sexual difference in alcohol dehydrogenase activity. The changes in enzyme activity were independent of changes in plasma thyroxine, testosterone and estradiol levels. Hypothalamic lesions in female rats had no effect on the enzyme activity. The sexual difference in alcohol dehydrogenase activity in rats is under hypothalamic control and does not appear to be mediated directly by the plasma level of androgens or estrogens.

Alcohol Dehydrogenase↗
PubMed1985

Choline acetyltransferase and glutamate uptake in the nucleus tractus solitarius and dorsal motor nucleus of the vagus: effect of nodose ganglionectomy.

Unilateral removal of the nodose ganglion resulted in a significant decrease in choline acetyltransferase activity in the ipsilateral dorsal motor nucleus of the vagus but was without effect on enzyme activity in the nucleus of the solitary tract. High affinity glutamate uptake in the dorsal motor nucleus of the vagus and along the rostrocaudal extent of the nucleus of the solitary tract was not affected by nodose ganglionectomy.

Animals↗