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S H Schwartz

Publications and source records attributed to S H Schwartz.

At least 19 recordsLinked to original sources
PubMed2002

Double-to-single target ionization ratio for electron capture in fast p-He collisions.

We have used the ion storage ring CRYRING and its internal gas-jet target and recoil-ion-momentum spectrometer to measure absolute cross sections for transfer ionization (TI: p+He-->H0+He2++e(-)) in 2.5-4.5 MeV p-He collisions with separate Thomas (TTI) and kinematic (KTI) TI contributions. The probability for electron emission in kinematical capture decreases with increasing velocity and appears to approach the photoionization shakeoff value (1.63%) [T. Aberg, Phys. Rev. A 2, 1726 (1970)]]. The velocity dependence of the TTI cross section is consistent with the theoretically predicted v(-11) scaling [J. S. Briggs and K. Taulbjerg, J. Phys. B 12, 2565 (1979)]].

Journal Article↗
PubMed2001

Characterization of a novel carotenoid cleavage dioxygenase from plants.

The plant hormone abscisic acid is derived from the oxidative cleavage of a carotenoid precursor. Enzymes that catalyze this carotenoid cleavage reaction, nine-cis epoxy-carotenoid dioxygenases, have been identified in several plant species. Similar proteins, whose functions are not yet known, are present in diverse organisms. A putative cleavage enzyme from Arabidopsis thaliana contains several highly conserved motifs found in other carotenoid cleavage enzymes. However, the overall homology with known abscisic acid biosynthetic enzymes is low. To determine the biochemical function of this protein, it was expressed in Escherichia coli and used for in vitro assays. The recombinant protein was able to cleave a variety of carotenoids at the 9-10 and 9'-10' positions. In most instances, the enzyme cleaves the substrate symmetrically to produce a C(14) dialdehyde and two C(13) products, which vary depending on the carotenoid substrate. Based upon sequence similarity, orthologs of this gene are present throughout the plant kingdom. A similar protein in beans catalyzes the same reaction in vitro. The characterization of these activities offers the potential to synthesize a variety of interesting, natural products and is the first step in determining the function of this gene family in plants.

Abscisic Acid↗
PubMed2000

Worries and values.

Relations of individuals' value priorities to their worries are investigated in seven samples from four cultural groups (N = 1,441). A social-cognitive analysis suggests that value priorities influence worries by increasing attention to and perception of threats to valued goals. On this basis, we generate hypotheses relating two types of worries, micro (about self and its extensions) and macro (about society and world), to 10 types of values. As predicted, giving priority to self-transcendence values (universalism and benevolence) is associated with low micro and high macro worry, whereas giving priority to self-enhancement values (power, hedonism, and--to a lesser degree--achievement) is associated with high micro and low macro worry. Meaningful associations are also found for other values. Values account for substantially more variance in macro than in micro worries.

Adaptation, Psychological↗
PubMed2000

Pulse-duration threshold functions obtained at the MWS/LWS cone antagonistic locus.

Pulse-duration threshold functions were determined for spectral increments in the region of the Sloan notch (580 nm) as well as for 440 and 640 nm and white increments flashed on a spatially coincident white background. While increments of 440 and 640 nm elicited traditional pulse-duration functions with decreasing thresholds as the duration increased, functions obtained with 580 nm and white increments manifested a minima at about 50 ms. These data support the notion that stimuli in the region of Sloan's notch can be detected by an achromatic mechanism that may integrate sub-threshold ON and OFF responses, thereby maximizing detection sensitivity.

Color Perception↗
PubMed1998

Regulation of an osmoticum-responsive gene in Anabaena sp. strain PCC 7120.

Salt-induced genes in the cyanobacterium Anabaena sp. strain PCC 7120 were identified by use of a Tn5-based transposon bearing luxAB as a reporter. The genomic sequence adjacent to one site of insertion of the transposon was identical in part to the sequence of the lti2 gene, which was previously identified in a differential screen for cold-induced transcripts in Anabaena variabilis. The lti2-like gene was induced by sucrose and other osmotica and by low temperature, in addition to salt. Regulatory components necessary for the induction of this gene by osmotica were sought by a further round of transposon mutagenesis. One mutant that displayed reduced transcriptional activity of the lti2-like gene in response to exposure to osmotica had an insertion in an open reading frame, which was denoted orrA, whose predicted product showed sequence similarity to response regulators from two-component regulatory systems. The corresponding mutation was reconstructed and was shown, like the second-site transposon mutation, to result in reduced response to osmotic stress. Induction of the lux reporter gene by osmotica was restored by complementation with a genomic fragment containing the entire open reading frame for the presumptive response regulator, whereas a fragment containing a truncated copy of the open reading frame for the response regulator did not complement the mutation.

Amino Acid Sequence↗
PubMed1997

Genetic control of abscisic acid biosynthesis in maize.

Abscisic acid (ABA), an apocarotenoid synthesized from cleavage of carotenoids, regulates seed maturation and stress responses in plants. The viviparous seed mutants of maize identify genes involved in synthesis and perception of ABA. Two alleles of a new mutant, viviparous14 (vp14), were identified by transposon mutagenesis. Mutant embryos had normal sensitivity to ABA, and detached leaves of mutant seedlings showed markedly higher rates of water loss than those of wild type. The ABA content of developing mutant embryos was 70% lower than that of wild type, indicating a defect in ABA biosynthesis. vp14 embryos were not deficient in epoxy-carotenoids, and extracts of vp14 embryos efficiently converted the carotenoid cleavage product, xanthoxin, to ABA, suggesting a lesion in the cleavage reaction. vp14 was cloned by transposon tagging. The VP14 protein sequence is similar to bacterial lignostilbene dioxygenases (LSD). LSD catalyzes a double-bond cleavage reaction that is closely analogous to the carotenoid cleavage reaction of ABA biosynthesis. Southern blots indicated a family of four to six related genes in maize. The Vp14 mRNA is expressed in embryos and roots and is strongly induced in leaves by water stress. A family of Vp14-related genes evidently controls the first committed step of ABA biosynthesis. These genes are likely to play a key role in the developmental and environmental control of ABA synthesis in plants.

Abscisic Acid↗
PubMed1997

Specific oxidative cleavage of carotenoids by VP14 of maize.

The plant growth regulator abscisic acid (ABA) is formed by the oxidative cleavage of an epoxy-carotenoid. The synthesis of other apocarotenoids, such as vitamin A in animals, may occur by a similar mechanism. In ABA biosynthesis, oxidative cleavage is the first committed reaction and is believed to be the key regulatory step. A new ABA-deficient mutant of maize has been identified and the corresponding gene, Vp14, has been cloned. The recombinant VP14 protein catalyzes the cleavage of 9-cis-epoxy-carotenoids to form C25 apo-aldehydes and xanthoxin, a precursor of ABA in higher plants.

Abscisic Acid↗
PubMed1997

Biochemical characterization of the aba2 and aba3 mutants in Arabidopsis thaliana.

Abscisic acid (ABA)-deficient mutants in a variety of species have been identified by screening for precocious germination and a wilty phenotype. Mutants at two new loci, aba2 and aba3, have recently been isolated in Arabidopsis thaliana (L.) Hynh. (K.M. Léon-Kloosterziel, M. Alvarez-Gil, G.J. Ruijs, S.E. Jacobsen, N.E. Olszewski, S.H. Schwartz, J.A.D. Zeevaart, M. Koornneef [1996] Plant J 10: 655-661), and the biochemical characterization of these mutants is presented here. Protein extracts from aba2 and aba3 plants displayed a greatly reduced ability to convert xanthoxin to ABA relative to the wild type. The next putative intermediate in ABA synthesis, ABA-aldehyde, was efficiently converted to ABA by extracts from aba2 but not by extracts from aba3 plants. This indicates that the aba2 mutant is blocked in the conversion of xanthoxin to ABA-aldehyde and that aba3 is impaired in the conversion of ABA-aldehyde to ABA. Extracts from the aba3 mutant also lacked additional activities that require a molybdenum cofactor (Moco). Nitrate reductase utilizes a Moco but its activity was unaffected in extracts from aba3 plants. Moco hydroxylases in animals require a desulfo moiety of the cofactor. A sulfido ligand can be added to the Moco by treatment with Na2S and dithionite. Treatment of aba3 extracts with Na2S restored ABA-aldehyde oxidase activity. Therefore, the genetic lesion in aba3 appears to be in the introduction of S into the Moco.

Abscisic Acid↗
PubMed1996

Masking of the achromatic system: implications for saccadic suppression.

Brief (10 msec) increments, presented on a white adapting background, are known to elicit a spectral sensitivity function with a broad mid-spectral peak (King-Smith & Carden, 1976). We have found that a luminance decrement, presented as either a forward or backward mask, dramatically alters the form of the resultant sensitivity function: the broad mid-spectral peak is replaced by a color-opponent function with narrow peaks at 520 and 620 nm. Over the region of the spectrum where this change in shape occurs, there is a substantial reduction in sensitivity. We also find that the mask is more effective when it precedes the stimulus than when it follows it, and that the masking can be characterized as a type A effect. The implications of these data for mechanisms of saccadic suppression are discussed.

Adult↗
PubMed1996

Isolation and characterization of abscisic acid-deficient Arabidopsis mutants at two new loci.

Novel Arabidopsis mutants with lowered levels of endogenous abscisic acid (ABA) were isolated. These were selected in a screen for germination in the presence of the gibberellin biosynthesis inhibitor paclobutrazol. Another mutant was isolated in a screen for NaCl tolerance. The ABA-deficiency was caused by two monogenic, recessive mutations, aba2 and aba3, that were both located on chromosome 1. The mutants showed a phenotype that is known to be characteristic for ABA-deficiency: a reduced seed dormancy and excessive water loss, leading to a wilty phenotype. Double mutant analysis, combining different aba mutations, indicated the leaky nature of the mutations.

Abscisic Acid↗
PubMed1996

Spectral sensitivity as revealed by isolated step onsets and step offsets.

Opponent-colour spectral sensitivity functions are traditionally elicited with increments; the temporal presentation of these increments is usually step onset and step offset. However, there are both physiological and psychophysical data suggesting differences in sensitivity for spectral step onsets and step offsets. The purpose of this study was to elicit opponent-colour spectral sensitivity functions with isolated step onsets and offsets, and to compare the form and sensitivity of these functions. Spectral sensitivity functions were determined for large, long duration, step onset and step offset stimuli. Four human observers participated in the study. The opponent-colour functions obtained for onsets and offsets are largely similar in both form and sensitivity. Of particular interest is the finding that there is no reduction in sensitivity for blue offsets. These data are consistent with recently published psychological data and apparently at variance with physiological data that suggest a paucity of S-cone OFF-neurones in the primate retina and dorsal lateral geniculate nucleus (dLGN).

Adult↗
PubMed1995

Dependence of visual latency on wavelength: predictions of a neural counting model.

Simple reaction time (RT) was determined as a function of wavelength for equally visible, near-threshold stimuli. The stimuli were 5-deg spectral onsets of 1,000-ms duration presented on a 100-Td spatially coincident white background. All three subjects manifested the same result: RTs were fastest in the region of 590 nm. These data were analyzed in the context of a counting model of visual latency. This model predicts that for equally visible stimuli a transient detector will result in shorter visual latencies than will a more sustained detector. On the basis of this analysis, it is concluded that although most long duration, near-threshold, spectral step onsets are detected by the sustained parvocellular pathway, an exception occurs in the region of 590 nm: these stimuli are detected by a relatively transient pathway, presumably the magnocellular pathway.

Color Perception↗
PubMed1994

Spectral sensitivity of dichromats: role of postreceptoral processes.

Increment spectral sensitivity functions were determined for dichromatic subjects; these functions were interpreted in the context of parvo and magno pathways. Increments of either 200 or 10 msec in duration were presented on a spatially coincident, 1000 td white background. When compared to the control trichromatic data, the 200 msec functions of dichromats manifest a reduction in sensitivity at middle and long wavelengths. These data are consistent with the notion that 200 msec threshold increments reveal the sensitivity of the parvo pathway; the sensitivity of this pathway is reduced in dichromacy due to pigment replacement with a resultant loss of spectral opponency. The 10 msec functions of dichromats do not show a comparable reduction in sensitivity. Therefore, it is concluded that 10 msec increments reveal the sensitivity of a pathway whose spectral-opponent status is not substantially altered in dichromacy, presumably the magno pathway.

Adult↗
PubMed1993

Colour and flicker thresholds for high frequency increments.

Previous investigators have elicited spectral sensitivity functions for the chromatic and achromatic systems by manipulating the stimulus spatiotemporal parameters. An alternative strategy for obtaining these two functions is to use a single set of stimulus parameters and vary the threshold criterion. This can be accomplished by determining colour and flicker thresholds for a high frequency train of increments. In the current study, spectral sensitivity functions, based on colour and flicker thresholds, were determined for increments presented at 25 Hz. The colour and flicker threshold functions are similar in form to those previously published for the chromatic and achromatic systems. The existence of distinct threshold functions for colour and flicker are interpreted in the context of parvo and magno pathways. Potential clinical applications of this model are discussed.

Color Perception↗
PubMed1992

Reaction time distributions and their relationship to the transient/sustained nature of the neural discharge.

Reaction time distributions (RTDs) were determined in response to near-threshold increments of long duration. Stimulus parameters were selected to isolate the chromatic and achromatic systems. The RTDs for the achromatic system peak sooner and are more narrow than those obtained for the chromatic system. These results are analyzed in terms of the neural discharge pattern of parvo and magno pathways.

Adult↗
PubMed1991

An unusual cause of respiratory alkalosis.

A 56-year-old man with a longstanding tracheostomy presented to the hospital with upper GI bleeding and was found to have a profound respiratory alkalosis. The cause of this patient's involuntary hyperventilation was hiccuping complicated by the absence of glottic closure.

Alkalosis, Respiratory↗