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J C Walker

Publications and source records attributed to J C Walker.

At least 91 records · Page 5Linked to original sources
PubMed1988

Anaerobically regulated aldolase gene of maize. A chimaeric origin?

The sequence of the anaerobically induced fructose 1,6-bisphosphate aldolase gene of maize is presented. Analysis of the upstream sequences of the aldolase gene reveals a six base-pair sequence (TGGTTT) with perfect homology to one of the sub-regions of the anaerobic regulatory element (ARE) which is responsible for the anaerobic induction of the maize alcohol dehydrogenase 1 gene (Adh1). In the aldolase gene this sequence is located at position -70 relative to the start of transcription, in a small segment proven by functional analysis to be important for expression of the aldolase gene. Since this six base-pair sequence has been shown to be critical for anaerobic induction of the Adh1 mRNA, is in the functional promoter region of aldolase and is also present in a homologous position in Adh2 (another anaerobically-induced gene), we suggest this hexanucleotide is essential for anaerobic regulation of each of these genes. The maize aldolase gene is about 50% homologous at the amino acid level to the animal aldolase gene but has a completely different intron/exon structure. While the rat aldolase gene has nine introns the maize gene has a single large intron near the N terminus of the coding region. Because there is 55% homology downstream from the intron and very little homology upstream, we suggest that the maize gene has acquired a 5' region containing signals for anaerobic regulation and fortuitously adding a new N-terminal region to the protein. We must suppose that the plant gene has lost the remaining introns.

Anaerobiosis↗
PubMed1988

Sequence and characterization of two auxin-regulated genes from soybean.

The auxin-regulated expression of two poly(A)+ mRNAs in soybean hypocotyl was demonstrated by cloning of the cDNAs and Northern blot hybridization analyses (Walker, J.C., and Key, J.L. (1982) Proc. Natl. Acad. Sci. U.S.A. 79, 7185-7189). The corresponding genes, designated Aux28 and Aux22, have been isolated, and the cDNAs and genes have been sequenced. The Aux28 and Aux22 genes are present at one to two copies per haploid genome, contain four and two introns, and encode hydrophilic proteins of 26.8 and 21.5 kDa, respectively. Although the cDNAs were isolated independently and do not cross-hybridize under stringent hybridization conditions, the protein coding sequences of the two cDNAs have several colinear regions of high homology at the nucleic acid (77-80%) and the amino acid (80-100%) levels; together these regions constitute approximately a third of the protein coding sequences of the cDNAs. These data, together with genomic Southern blot hybridization analysis and hybrid-select translations of mRNAs homologous to the cDNAs, show that these genes belong to two related multigene families. We have identified two sequences, TGATAAAAG and GGCAGCATGCA, that occur at similar distances upstream of the transcription start site in each gene, and the spacing between these two elements is essentially identical in the two genes. The possible significance of these sequences is under evaluation.

Amino Acid Sequence↗
PubMed1988

Long-term climate change and the geochemical cycle of carbon.

We study the interactions between the geochemical cycles of carbon and long-term changes in climate. Climate change is studied with a simple, zonally averaged energy balance climate model that includes the greenhouse effect of carbon dioxide explicitly. The geochemical model balances the rate of consumption of carbon dioxide in silicate weathering against its release by volcanic and metamorphic processes. The silicate weathering rate is expressed locally as a function of temperature, carbon dioxide partial pressure, and runoff. The global weathering rate is calculated by integrating these quantities over the land area as a function of latitude. Carbon dioxide feedback stabilizes the climate system against a reduction in solar luminosity and may contribute to the preservation of equable climate on the early Earth, when solar luminosity was low. The system responds to reduced land area by increasing carbon dioxide partial pressure and warming the globe. Our model makes it possible to study the response of the system to changing latitudinal distribution of the continents. A concentration of land area at high latitudes leads to high carbon dioxide partial pressures and high global average temperature because weathering of high-latitude continents is slow. Conversely, concentration of the continents at low latitudes yields a cold globe and ice at low latitudes, a situation that appears to be representative of the late Precambrian glacial episode. This model is stable against ice albedo catastrophe even when the ice line occurs at low latitudes. In this it differs from energy balance models that lack the coupling to the geochemical cycle of carbon.

Atmosphere↗
PubMed1988

Behavioral consequences of trauma.

A group of 65 head-injured patients, making an apparent good recovery, were studied with the Halstead-Reitan neuropsychological test battery because of personality or cognitive difficulty. A significant relationship was identified between outcome as measured by the Halstead Impairment Index and both Injury Severity Score and Glasgow Coma Scale score. A significant relationship was also found to exist between the Halstead Impairment Index and employment status of the study group. Patients were placed in three groups, depending on their impairment index. Of the complications identified, spinal fracture, pupillary dysfunction, and intracranial pressure elevation were consistently associated with an impaired performance on the neuropsychological testing. These findings suggest that there is a relationship between head injury complications and neuropsychological potential which can cause lingering problems and influence the patient's rehabilitation process.

Adolescent↗
PubMed1988

Schistosoma mansoni and S. japonicum worm numbers in 129/J mice of two types and dominance of susceptibility in F1 hybrids.

In a study on the genetics of resistance to schistosomiasis in WEHI 129/J mice, susceptibility to either Schistosoma mansoni or Schistosoma japonicum was shown to be unequivocally dominant in F1 hybrid crosses between genetically resistant WEHI 129/J and susceptible BALB/c mice. The operation of only 1 or 2 genes in the expression of resistance to S. mansoni was suggested by backcross analysis. Thus, approximately 25% of (BALB/c x WEHI 129/J) F1 x WEHI 129/J mice were resistant to S. mansoni infection, whereas resistance was manifest in approximately 50% of WEHI 129/J mice. The data are consistent with resistance being controlled by 1 recessive gene having 50% penetrance. We also report that 129/J mice obtained directly from the Jackson Laboratories (Bar Harbor, Maine) (designated JAX 129/J), differ from locally bred WEHI 129/J in being entirely susceptible to S. mansoni infection. However, both WEHI 129/J and JAX 129/J are relatively resistant to S. japonicum infection.

Animals↗
PubMed1987

Was the Archaean biosphere upside down?

Photosynthesis produces reduced organic carbon and an oxidized partner in equivalent molar amounts. These compounds can react with one another, again in equivalent molar amounts, so that no net change occurs in the overall level of oxidation of the biosphere (here taken to mean the biota together with the part of the Earth with which living things interact). But, the reduced and oxidized partners have different susceptibilities to transport by environmental processes and so, typically, they become separated. Conservation of matter implies that for every mole of excess oxidant in an oxidizing region of the biosphere there must be a mole of excess reductant in a reducing region. Today, the oxidized partner in photosynthesis is usually free oxygen, which floats upward to accumulate in excess in the atmosphere, whereas organic matter settles downward to collect in sediments and stagnant pools. On the anoxic Archean Earth, the oxidized partner was probably iron. As oxidized iron is markedly less soluble and mobile than organic carbon, differential transport in the Archaean biosphere would have had an effect just the opposite of that in the modern biosphere. The oxidized partner would have settled downward more rapidly than the reduced partner, resulting in the accumulation of excess oxidant in sediments and stagnant pools. An equivalent excess of the more volatile reduced compounds would have been left behind in ocean and atmosphere in the form of dissolved organic carbon and gaseous hydrocarbons. On average, therefore, the Archean biosphere may have been oxidizing at the bottom and reducing on top.

Aerobiosis↗
PubMed1987

The ocs element: a 16 base pair palindrome essential for activity of the octopine synthase enhancer.

A 176 bp DNA sequence lying upstream of the octopine synthase (ocs) promoter, previously shown to have enhancer-like properties in transgenic tobacco [Ellis et al. (1987) EMBO J., 6, 11-16], functions as an enhancer in protoplasts of Zea mays (a monocot plant) and Nicotiana plumbaginifolia (a dicotplant). We have characterized this element by transient expression assays using a linked alcohol dehydrogenase (Adh1) promoter from Z. mays and the chloramphenicol acetyltransferase coding sequences. The ocs sequence functions in both orientations but its enhancing activity is dependent upon its distance from the Adh1 promoter. Transient expression assays using deletion mutants and synthetic oligonucleotides show that a 16 bp palindrome ACGTAAGCGCTTACGT, contained within the 176 bp fragment, is essential and sufficient for enhancing activity in transient expression assays.

Journal Article↗
PubMed1987

A constant daylength during the Precambrian era?

The semidiurnal atmospheric thermal tide would have been resonant with free oscillations of the atmosphere when the day was approximately 21 h long, c. 600 Ma ago. Very large atmospheric tides would have resulted, with associated surface pressure oscillations in excess of 10 mbar in the tropics. Near resonance the Sun's gravitational torque on the atmospheric tide--accelerating Earth's rotation--would have been comparable in magnitude to the decelerating lunar torque upon the oceanic tides. The balance of the opposing torques may have long maintained a resonant approximately 21 h day, perhaps for much of the Precambrian. Because the timescale of lunar orbital evolution is not directly affected, a constant daylength would result in fewer days/month. The hypothesis is shown not to conflict with the available (stromatolitic) evidence. Escape from the resonance could have followed a relatively abrupt global warming, such as that occurring at the end of the Precambrian. Alternatively, escape may simply have followed a major increase in the rate of oceanic tidal dissipation, brought about by the changing topography of the world's oceans. We integrate the history of the lunar orbit with and without a sustained resonance, finding that the impact of a sustained resonance on the other orbital parameters of the Earth-Moon system would have not been large.

Atmosphere↗
PubMed1987

DNA sequences required for anaerobic expression of the maize alcohol dehydrogenase 1 gene.

Expression of the maize alcohol dehydrogenase 1 (Adh1) gene is transcriptionally regulated under conditions of anaerobic stress. DNA sequences required for the expression of Adh1 have been identified by a functional analysis of in vitro constructed hybrid genes consisting of the Adh1 promoter fused to the chloramphenicol acetyltransferase coding region. A series of 5' deletions, 3' deletions, hybrid promoters, and linker scanning mutants of the Adh-CAT hybrid gene were introduced into maize protoplasts by electroporation and assayed for chloramphenicol acetyltransferase activity after incubation of the protoplasts under different oxygen tensions. The results indicate that a 40-base-pair DNA sequence within the Adh1 promoter is required for anaerobically regulated expression of the hybrid gene. Clustered point mutations in this sequence show that it is composed of two essential regions, each approximately 15 base pairs, separated by a 10-base-pair DNA sequence that does not appear to be important for anaerobic expression. Attachment of this 40-base-pair element to an unrelated promoter shows that this DNA sequence is both necessary and sufficient for induction of gene expression by low oxygen stress.

Journal Article↗
PubMed1986

Lunar nodal tide and distance to the Moon during the Precambrian.

The pace of tidal evolution for the past approximately 450 Myr implies an Earth/Moon collision some 1,500-2,000 Myr BP, an event for which there is no corroborating evidence. Here we present the first direct determination of the lunar distance in the Precambrian. We interpret a 23.3 +/- 0.3-yr periodicity preserved in a 2,500 Myr BP Australian banded iron formation (BIF) as reflecting the climatic influence of the lunar nodal tide, which has been detected with its modern 18.6-yr periodicity in some modern climate records. The lunar distance at 2,500 Myr BP would then have been about 52 Earth radii. The implied history of Precambrian tidal friction is in accord with both the more recent palaeontological evidence and the long-term stability of the lunar orbit. The length of the Milankovitch cycles that modulate the ice ages today also evolve with the Earth-Moon system. Their detection in the Precambrian sedimentary record would then permit an independent determination of the lunar distance.

Climate↗
PubMed1986

Global geochemical cycles of carbon, sulfur and oxygen.

Time resolved data on the carbon isotopic composition of carbonate minerals and the sulfur isotopic composition or sulfate minerals show a strong negative correlation during the Cretaceous. Carbonate minerals are isotopically heavy during this period while sulfate minerals are isotopically light. The implication is that carbon is being transferred from the oxidized, carbonate reservoir to the reservoir of isotopically light reduced organic carbon in sedimentary rocks while sulfur is being transferred from the reservoir of isotopically light sedimentary sulfide to the oxidized, sulfate reservoir. These apparently oppositely directed changes in the oxidation state of average sedimentary carbon and sulfur are surprising because of a well-established and easy to understand correlation between the concentrations of reduced organic carbon and sulfide minerals in sedimentary rocks. Rocks rich in reduced carbon are also rich in reduced sulfur. The isotopic and concentration data can be reconciled by a model which invokes a significant flux of hydrothermal sulfide to the deep sea, at least during the Cretaceous.

Atlantic Ocean↗
PubMed1986

Olfactory and nonolfactory odor detection in pigeons: elucidation by a cardiac acceleration paradigm.

A technique for the Pavlovian conditioning of cardiac acceleration in response to odorants was developed and used to compare the sensitivity of pigeons to four odorants before and after resection of the olfactory nerves. This method provided quite reliable psychophysical functions yet required relatively little training time. Thresholds of normal pigeons to n-amyl acetate, n-butyl acetate, benzaldehyde and butanol were approximately 10(-4), 10(-5), 10(-3.3) and 10(-4.3) of vapor saturation, respectively. Following resection surgery, sensitivity decreased by 2 to 4 log units. When transection of the ophthalmic branch of the trigeminal nerve was combined with olfactory nerve resection, little evidence of a further decline in odor sensitivity was seen. Based on these results each of these compounds could be used, at concentrations below the postoperative threshold, to study, in isolation, both normal and reconstituting olfactory systems in the pigeon.

Animals↗
PubMed1986

Toxocariasis: a serological survey of blood donors in the Australian Capital Territory together with observations on the risks of infection.

The sera from 660 healthy blood donors from Canberra were tested for antibodies to Toxocara canis by the ELISA test. The results were compared with those from patients with suspected or confirmed visceral larva migrans or ocular toxocariasis. Over 7% of Canberra sera showed elevated levels of antibody reacting with T. canis antigen. Sera from patients resident in Australia with other helminth parasites did not cross-react with T. canis antigen in our tests. However, studies of sera collected in several tropical countries with other parasitic infection, show that cross-reactions with other parasites are possible. The use of purified glycoprotein antigen does not alter the possibility of cross-reaction. Observations and experiments show that people in Canberra may be exposed to the infective eggs of T. canis.

Adolescent↗
PubMed1986

Resistance of 129/J mice to Schistosoma mansoni infection.

Mice of the strain 129/J bred at this Institute (WEHI 129/J) were exposed once or repeatedly to cercariae of Schistosoma mansoni and worm burdens determined. In keeping with reports of S. japonicum infection in these mice, a high level of resistance was recorded. Radio-labelled extracted antigens of S. mansoni adult worms were reacted with sera from exposed 129/J mice and infected BALB/c mice. Immunoprecipitated antigens preferentially recognized by antibodies in 129/J mice include proteins of Mr 99,000 and 21,000. Whether responses to these antigens are a cause or a consequence of resistance remains to be established.

Animals↗
PubMed1985

Carbon dioxide on the early earth.

This paper uses arguments of geochemical mass balance to arrive at an estimate of the partial pressure of carbon dioxide in the terrestrial atmosphere very early in earth history. It appears that this partial pressure could have been as large as 10 bars. This large estimate depends on two key considerations. First, volatiles were driven out of the interior of the earth during the course of earth accretion or very shortly thereafter. This early degassing was a consequence of rapid accretion,which gave the young earth a hot and rapidly convecting interior. Second, the early earth lacked extensive, stable continental platforms on which carbon could be stored in the form of carbonate minerals for geologically significant periods of time. In the absence of continental platforms on the early earth, the earth's carbon must have been either in the atmosphere or ocean or in the form of shortlived sedimentary deposits on ephemeral sea floor.

Atmosphere↗