Showing papers by "University of Dundee published in 1990"
TL;DR: The cyclic heptapeptide, microcystin‐LR, inhibits protein phosphatases 1 (PP1) and 2A (PP2A) with K i, values below 0.1 nM, and this results are strikingly similar to those obtained with the tumour promoter okadaic acid.
1,555 citations
TL;DR: The tumour promoter okadaic acid is a potent and specific inhibitor of protein phosphatases 1 and 2A and is extremely useful for identifying biological processes that are controlled through the reversible phosphorylation of proteins.
1,352 citations
TL;DR: A tertiary structure model of the ATP-binding cassettes characteristic of this class of transport system is presented, based on similarities between the predicted secondary structures of members of this family and the previously determined structure of adenylate kinase.
Abstract: THE ATP-binding cassette (ABC) superfamily of transport systems now includes over thirty proteins that share extensive sequence similarity and domain organization (reviewed in refs 1–3). This superfamily includes the well characterized periplasmic binding protein-dependent uptake systems of prokaryotes, bacterial exporters, and eukaryotic proteins including the P-glycoprotein associated with multidrug resistance in tumours (MDR), the STE6 gene product that mediates export of yeast a-factor mating pheromone, pfMDR that is implicated in chloroquine resistance of the malarial parasite, and the product of the cystic fibrosis gene (CFTR). Here we present a tertiary structure model of the ATP-binding cassettes characteristic of this class of transport system, based on similarities between the predicted secondary structures of members of this family and the previously determined structure of adenylate kinase. This model has implications for both the molecular basis of transport and cystic fibrosis and provides a framework for further experimentation.
1,192 citations
TL;DR: The contrasting pharmacology and ionic dependence of the glutamate uptake carriers in the vesicle membrane and in the plasma membrane explain how glutamate (but probably not aspartate) can function as a neurotransmitter, and why the extracellular glutamate concentration rises to neurotoxic levels in brain anoxia.
1,140 citations
TL;DR: Two experiments show that self-categorization can be a crucial determining factor in social influence, when categorical differences between two subgroups within a discussion group are made salient, and convergence of opinion between the subgroups is inhibited.
Abstract: We contrast two theoretical approaches to social influence, one stressing interpersonal dependence, conceptualized as normative and informational influence (Deutsch & Gerard, 1955), and the other stressing group membership, conceptualized as self-categorization and referent informational influence (Turner, Hogg, Oakes, Reicher & Wetherell, 1987). We argue that both social comparisons to reduce uncertainty and the existence of normative pressure to comply depend on perceiving the source of influence as belonging to one's own category. This study tested these two approaches using three influence paradigms. First we demonstrate that, in Sherif's (1936) autokinetic effect paradigm, the impact of confederates on the formation of a norm decreases as their membership of a different category is made more salient to subjects. Second, in the Asch (1956) conformity paradigm, surveillance effectively exerts normative pressure if done by an in-group but not by an out-group. In-group influence decreases and out-group influence increases when subjects respond privately. Self-report data indicate that in-group confederates create more subjective uncertainty than out-group confederates and public responding seems to increase cohesiveness with in-group — but decrease it with out-group — sources of influence. In our third experiment we use the group polarization paradigm (e.g. Burnstein & Vinokur, 1973) to demonstrate that, when categorical differences between two subgroups within a discussion group are made salient, convergence of opinion between the subgroups is inhibited. Taken together the experiments show that self-categorization can be a crucial determining factor in social influence.
620 citations
TL;DR: It is now clear that in humans much of the IgA is secreted directly into the blood and never reaches the mucosal surfaces, so secretory and serum IgA are molecules with different biochemical and immunochemical properties produced by cells with different organ distributions.
Abstract: IgA is present in normal human serum at about one-fifth of the concentration of IgG. However, it is catabolized around five times faster than IgG and therefore the rates of synthesis of the two immunoglobulins must be similar [1]. IgA is themost abundant immunoglobulin in secretions. Secretory IgA (sIgh) is the product of local synthesis at the mucosal surfaces which are the main source of antigenic material for the body. In mucosal tissue, IgA synthesis far exceeds that of other classes. As a result, in man, more IgA is produced than all other immunoglobulin classes combined. In contrast to other immunoglobulins, human IgA displays a unique heterogeneity in its molecular forms, each with a characteristic distribution in various body fluids [2]. Human IgA occurs in two isotypic forms, IgAl and IgA2, with IgA2 existing as two allotypic variants IgA2m(l) and IgA2m(2). Each of these forms is found in various degrees of aggregation. Although the importance of IgA in mucosal secretions is well established, it is now clear that in humans much of the IgA is secreted directly into the blood and never reaches the mucosal surfaces [3]. Serum IgA is predominantly monomeric IgAl which is produced in the bone marrow, while in external secretions most of the locally produced IgA is polymeric with a relative increase in the proportion of IgA2 [4, 5]. The lymphocytes which produce monomeric or polymeric IgA, IgAl or IgA2 are characteristically distributed in various lymphoid and nonlymphoid tissues. The differential interaction of monomeric and polymeric IgA molecules with various cells leads to their selective distribution in body fluids and possibly to differences in their effector functions. Secretory and serum IgA are therefore molecules with different biochemical and immunochemical properties produced by cells with different organ distributions.
554 citations
TL;DR: An insulin-stimulated protein kinase is shown to activate the type-1 protein phosphatase that controls glycogen metabolism, by phosphorylating its regulatory subunit at a specific serine.
Abstract: One of the important metabolic effects of insulin is its ability to stimulate the synthesis of glycogen in skeletal muscle within minutes; much research has been directed toward understanding the molecular basis of this effect. Almost 25 years ago, Joseph Larner and co-workers showed that glycogen synthase, the rate-limiting enzyme in this pathway, could exist in at least two forms. One possessed little activity in the absence of glucose 6-phosphate (G6P), while the other was almost fully active in the absence of G6P.(1) Incubation of rat hemidiaphragms with insulin decreased the proportion of glycogen synthase in the G6P-dependent form which paralleled the increased rate of glycogen production.(1–3) Furthermore, the conversion of glycogen synthase to a G6Pindependent form was still observed when glucose was omitted from the perfusion medium,(2,3) indicating that it was not a consequence of the increased rate of transport of glucose into muscle, which is also stimulated by insulin. Conversely, incubation of the hemidiaphragms with epinephrine increased the proportion of the G6P-dependent form and decreased the rate of glycogen synthesis.(2,3)
510 citations
TL;DR: Many aspects of metal-microbe interactions remain unexploited in biotechnology and further development and application is necessary, particularly to the problem of radionuclide release into the environment.
Abstract: Bacteria, and other microorganisms, exhibit a number of metabolism-dependent and-independent processes or the uptake and accumulation of heavy metals and radionuclides. The removal of such harmful substances from effluents and waste waters by microbe-based technologies may provide an alternative or additional means of metal/radionuclide recovery for economic reasons and/or environmental protection. Both living and dead cells as well as products derived from or produced by microorganisms can be effective metal accumulators and there is evidence that some biomass-based clean-up processes are economically viable. However, many aspects of metal-microbe interactions remain unexploited in biotechnology and further development and application is necessary, particularly to the problem of radionuclide release into the environment.
468 citations
TL;DR: In this article, a molecular mechanical force field is described for the conformational analysis of oligosaccharides, based on the AMBER force field, which is compatible with simulations of proteins.
Abstract: A molecular mechanical force field is described for the conformational analysis of oligosaccharides. This force field has been derived by the addition of new parameters to the AMBER force field and is compatible with simulations of proteins. This new parametrization is assessed by comparison of the theoretically predicted conformations of Man alpha 1-3Man beta 1-4GlcNAc with the corresponding crystal structure. Molecular dynamics simulation data are presented for this structure both in vacuo and with the explicit inclusion of water molecules. While the former demonstrate significant torsional oscillations about glycosidic linkages at physiological temperature, in the latter these oscillations are highly damped due to the stabilizing influence of a "cage" of solvent-solvent and solvent-solute hydrogen bonds.
363 citations
TL;DR: Differences in Fe and Mn costs of growth of less than two-fold are predicted when the Fe- and Mn-efficient organisms with CO2− concentrating mechanisms and high requirements for ATP relative to NADPH, are compared with organisms relying on CO2 diffusion from air or air-equilibrated solutions and C3 biochemistry.
Abstract: SUMMARY
Iron is involved in many photosynthetic, respiratory and nitrogen assimilation reactions of plants as Fe bound tightly to polypeptides catalysing redox reactions. Manganese is involved as tightly bound Mn in photoreaction II of photosynthesis and in certain superoxide dismutases, while loosely bound Mn2+ is the unique activator of some enzymes, and is an alternative to Mg2+ in activating many enzymes. This paper uses data on the quantitative role of Fe and Mn in catalysts to predict the efficiency with which Fe and Mn are used in C assimilation [mol C assimilated (mol catalytic metal in enzyme)−1 s−1] and the metal cost of C assimilation [mol catalytic metal in enzyme (mol C assimilated)−1 s−1] in photolithotrophic growth in relation to genetic and environmental variables. The genetic variables were the relative content of thylakoid proteins in major taxa (cyanobacteria and red algae, chlorophytes and chromophytes) and smaller-scale taxonomic differences (various subtypes of C4 metabolism, and C3 metabolism, in terrestrial vascular plants). The environmental variables were the range of photon flux densities in which photolithotrophic growth of O2 evolvers can occur, and the inorganic C supply conditions controlling the repression/de-repression of the inorganic C concentrating mechanism in cyanobacteria and microalgae.
The results of the computations yield the following conclusions. The largest predicted difference in Fe and Mn costs of photolithotrophic growth is related to changes in the photon flux density for growth. The predicted Fe cost increased 50-fold, and the Mn cost increased 80-fold, at the lowest extreme of photon flux density compared to the highest found naturally. The increase is partly countered by the larger ratio of light-harvesting pigments to thylakoid protein complexes assumed for the cells grown at low photon flux densities, although the extent of the increase in photosynthetic unit size is limited by considerations of efficiency of excitation energy transfer. However, the major influences are the higher pigment content in biomass enabling a larger fraction of incident light to the absorbed, and the sub-maximal specific reaction rates of redox catalysts (whose content is constrained via excitation energy transfer considerations) at very low photon flux densities. A smaller difference, four-fold or less, in Fe and Mn costs of photolithotrophic growth, is predicted by comparing major taxa (cyanobacteria plus red algae; chlorophytes plus chromophytes) with contrasting ratios of thylakoid redox catalysts. Differences in Fe and Mn costs of growth of less than two-fold are predicted when the Fe- and Mn-efficient organisms with CO2− concentrating mechanisms (C4 land plants; algae with active inorganic C influx) and high requirements for ATP relative to NADPH, are compared with organisms relying on CO2 diffusion from air or air-equilibrated solutions and C3 biochemistry.
These predictions of variations in Fe and Mn costs of photolithotrophy have implications for the ecology of phototrophs.
348 citations
TL;DR: The data suggests that there is a post-mortem diffusion of drugs along a concentration gradient, from sites of high concentration in solid organs, into the blood with resultant artefactual elevation of drug levels in blood.
TL;DR: Phosphopeptide mapping of native, 100 kd microsomal HMG‐CoA reductase confirms that this C‐terminal serine is the only major site phosphorylated in the intact enzyme by the AMP‐activated protein kinase.
Abstract: The intact, 100 kd microsomal enzyme and the 53 kd catalytic fragment of rat HMG-CoA reductase are both phosphorylated and inactivated by the AMP-activated protein kinase. Using the catalytic fragment, we have purified and sequenced peptides containing the single site of phosphorylation. Comparison with the amino acid sequence predicted from the cDNAs encoding other mammalian HMG-CoA reductases identifies this site as a serine residue close to the C-terminus (Ser872 in the human enzyme). Phosphopeptide mapping of native, 100 kd microsomal HMG-CoA reductase confirms that this C-terminal serine is the only major site phosphorylated in the intact enzyme by the AMP-activated protein kinase. The catalytic fragment of HMG-CoA reductase was also isolated from rat liver in the presence of protein phosphatase inhibitors under conditions where the enzyme is largely in the inactive form. HPLC, mass spectrometry and sequencing of the peptide containing Ser872 demonstrated that this site is highly phosphorylated in intact liver under these conditions. We have also identified by amino acid sequencing the N-terminus of the catalytic fragment, which corresponds to residue 423 of the human enzyme.
TL;DR: Results show that PP1 activity is required for mitotic progression and that the other loci cannot supply sufficient activity to complement loss of expression of the PP1 87B gene.
TL;DR: The effects of hypersaline treatment (osmotic upshock) on solute accumulation have been studied in the Gram-positive bacterium Bacillus subtilis and it is possible that K+ serves in some manner as the signal for increased proline synthesis.
Abstract: Summary: The effects of hypersaline treatment (osmotic upshock) on solute accumulation have been studied in the Gram-positive bacterium Bacillus subtilis. Natural abundance 13C NMR spectroscopy studies revealed only proline as a major organic osmoticum in cells grown in defined medium (no exogenous organic solutes) and this finding was confirmed by amino acid analysis. Intracellular concentrations of both K+ and proline rose markedly after osmotic upshock. K+ influx from the medium was rapid (< 1 h) but proline synthesis was a slower process (5–9 h). Proline synthesis appeared to be dependent on the prior accumulation of K+ and it is possible that K+ serves in some manner as the signal for increased proline synthesis. In cells upshocked in medium enriched in glycine betaine the endogenous synthesis of proline was repressed and glycine betaine served as the sole organic osmoticum. K+ was also accumulated under these conditions.
TL;DR: A Social Psychology of Reputation as mentioned in this paper is a seminal work in the field of social psychology, focusing on the social psychology of reputation, and it is published by the European Review of Social Psychology.
Abstract: (1990). A Social Psychology of Reputation. European Review of Social Psychology: Vol. 1, European Review of Social Psychology, pp. 171-193.
TL;DR: Show by using new cleavable labelling reagents that class II and class I MHC on B lymphoblastoid cells are continually endocytosed and recycled to the cell surface and reveal a cycle that might explain the rapid binding and turnover of some peptide/class II MHC complexes and the exchange of pre-existing for new peptides observed in living cells.
Abstract: Class II major histocompatibility complex (MHC) glycoproteins associate with peptides derived from material endocytosed by antigen-presenting cells and processed along the endocytotic pathway. No consensus exists as to what extent class II molecules themselves are endocytosed and it is not known whether endocytosed MHC class II molecules can be recycled again to the cell surface--an itinerary which might allow a single cell-surface MHC molecule to associate with different peptides during its lifetime. We now show by using new cleavable labelling reagents that class II and class I MHC on B lymphoblastoid cells are continually endocytosed and recycled to the cell surface. The intracellular pool size is normally kept small by efficient recycling, but in the presence of primaquine the rate of recycling is slowed, thereby increasing the size of this pool substantially. On removal of the amine, the intracellular population recycles rapidly to restore the original distribution. These results reveal a cycle that might explain the rapid binding and turnover of some peptide/class II MHC complexes and the exchange of pre-existing for new peptides observed in living cells.
TL;DR: The results suggest that the effects of the AMP-activated protein kinase on acetyl-CoA carboxylase activity are mediated entirely by phosphorylation of Ser79, and not Ser1200 and Ser1215, as previously suggested.
Abstract: 1
We have sequenced two tryptic/chymotryptic peptides (TC3 and TC3a) containing a third site phosphorylated on rat acetyl-CoA carboxylase by the AMP-activated protein kinase. Comparison with the complete sequence of rat acetyl-CoA carboxylase predicted from the cDNA sequence [Lopez-Casillas et al. (1988) Proc. Natl Acad. Sci. USA 85, 5784–5788] shows that this site corresponds to Ser1215.
2
Comparison of the cDNA sequence with previous amino acid sequence data identifies the other two sites for the AMP-activated protein kinase as Ser79 and Ser1200. A total of eight serine residues phosphorylated in vitro by six protein kinases can now be identified: six of these (Ser23, Ser25, Ser29, Ser77, Ser79 and Ser95) are clustered in the amino terminal region, while two (Ser1200 and Ser1215) are located in the central region.
3
Prior phosphorylation of Ser77 and Ser1200 by cyclic-AMP-dependent protein kinase prevents subsequent phosphorylation of Ser79 and Ser1200, but not Ser1215, by the AMP-activated protein kinase. Phosphorylation of Ser1215 under these conditions is not associated with a change in enzyme activity.
4
Limited trypsin treatment of native acetyl-CoA carboxylase selectively cleaves off the highly phosphorylated amino-terminal region containing Ser79.
5
Phosphorylation at Ser79 and Ser1200 by the AMP-activated protein kinase dramatically decreases Vmax and increases the A0.5 for citrate. Phosphorylation at Ser77 and Ser1200 by cyclic-AMP-dependent protein kinase causes more modest changes in the A0.5 for citrate and the Vmax. Dephosphorylation, or removal of the amino-terminal region containing Ser77/79 using trypsin, reverses all of these effects.
6
These results suggest that the effects of the AMP-activated protein kinase on acetyl-CoA carboxylase activity are mediated entirely by phosphorylation of Ser79, and not Ser1200 and Ser1215. The smaller effects of cyclic-AMP-dependent protein kinase are mediated by phosphorylation of Ser77.
TL;DR: Advances in biochemical and molecular biological approaches have given new insight into the function and structure of UDP-glucuronosyltransferases, which appear to be capable of conjugating a remarkable number of different chemicals.
TL;DR: It is demonstrated that the virR gene of S. flexneri implicated in the temperature regulation of plasmid-encoded virulence genes, is equivalent to the osmZ gene of Escherichia coli, which has previously been shown to mediate its regulatory effects through changes in DNA supercoiling.
Abstract: BACTERIAL pathogens undergo profound physiological changes when they infect their hosts, requiring coordinated regulation of gene expression in response to the stresses encountered during infection. A number of environmental stresses (such as osmotic shock and anaerobiosis) have been shown to induce changes in DNA supercoiling that can directly affect the transcription of a specific subset of bacterial genes1–6, at least some of which (the outer-membrane porins and type 1 fimbriae) play a part in bacterial virulence7–9. Here, we demonstrate that the virR gene of S. flexneri implicated in the temperature regulation of plasmid-encoded virulence genes10–13, is equivalent to the osmZ gene of Escherichia coli, which has previously been shown to mediate its regulatory effects through changes in DNA supercoiling. Our results imply that environmentally induced changes in DNA supercoiling are important in the coordinated control of virulence gene expression in S. flexneri and have general implications for the control of bacterial virulence.
TL;DR: It is suggested that cyclin A has a role in the G2‐M transition that is independent of spindle formation, and that entry into anaphase is a key requirement for the degradation of cyclin B.
Abstract: We show that the sequence of Drosophila cyclin B has greater identity with B-type cyclins from other animal phyla than with Drosophila cyclin A, suggesting that the two cyclins have distinct roles that have been maintained in evolution. Cyclin A is not detectable in unfertilized eggs and is present at low levels prior to cellularization of the syncytial embryo. In contrast, the levels of cyclin B remain uniformly high throughout these developmental stages. In cells within cellularized embryos and the larval brain, cyclin A accumulates to peak levels in prophase and is degraded throughout the period in which chromosomes are becoming aligned on the metaphase plate. The degradation of cyclin B, on the other hand, does not occur until the metaphase-anaphase transition. In cells arrested at c-metaphase by treating with microtubule destabilizing drugs to prevent spindle formation, cyclin A has been degraded in the arrested cells, whereas cyclin B is maintained at high levels. These observations suggest that cyclin A has a role in the G2-M transition that is independent of spindle formation, and that entry into anaphase is a key requirement for the degradation of cyclin B.
TL;DR: This review focuses on the myo-inositol moiety which is involved in an increasingly complex network of metabolic interconversions,Myo-Inositol metabolites identified in eukaryotic cells include at least six glycerophospholipid isomers and some 25 distinct inositol phosphates which differ in the number and distribution of phosphate groups around the inositl ring.
Abstract: The discovery of the second-messenger functions of inositol 1,4,5-trisphosphate and diacylglycerol, the products of hormone-stimulated inositol phospholipid hydrolysis, marked a turning point in studies of hormone function. This review focuses on the myo-inositol moiety which is involved in an increasingly complex network of metabolic interconversions, myo-Inositol metabolites identified in eukaryotic cells include at least six glycerophospholipid isomers and some 25 distinct inositol phosphates which differ in the number and distribution of phosphate groups around the inositol ring. This apparent complexity can be simplified by assigning groups of myo-inositol metabolites to distinct functional compartments. For example, the phosphatidylinositol 4-kinase pathway functions to generate inositol phospholipids that are substrates for hormone-sensitive forms of inositol-phospholipid phospholipase C, whilst the newly discovered phosphatidylinositol 3-kinase pathway generates lipids that are resistant to such enzymes and may function directly as novel mitogenic signals. Inositol phosphate metabolism functions to terminate the second-messenger activity of inositol 1,4,5-trisphosphate, to recycle the latter's myo-inositol moiety and, perhaps, to generate additional signal molecules such as inositol 1,3,4,5-tetrakisphosphate, inositol pentakisphosphate and inositol hexakisphosphate. In addition to providing a more complete picture of the pathways of myo-inositol metabolism, recent studies have made rapid progress in understanding the molecular basis underlying hormonal stimulation of inositol-phospholipid-specific phospholipase C and inositol 1,4,5-trisphosphate-mediated Ca2+ mobilisation.
TL;DR: Five protein serine/threonine phosphatases (PP) have been identified by cloning cDNA from mammalian and Drosophila libraries and the complete amino acid sequences of PPX, PPY and PPZ and an almost complete sequence of PPV are presented.
TL;DR: The effects of hypersaline treatment (osmotic upshock) on cell water relations were examined in the Gram-positive bacterium Bacillus subtilis using particle size analysis using the Boyle-van't Hoff relationship and calculation of turgor pressure by an alternative method involving spectrophotometric analysis of shrinkage gave somewhat lower estimates.
Abstract: Summary: The effects of hypersaline treatment (osmotic upshock) on cell water relations were examined in the Gram-positive bacterium Bacillus subtilis using particle size analysis. Application of the Boyle-van't Hoflf relationship (cell volume versus reciprocal of external osmolality) permitted direct determination of turgor pressure, which was approximately 0·75 osmol kg−1 (1·9 MPa) in exponentially growing bacteria in a defined medium. The abolition of turgor pressure immediately after upshock and the subsequent recovery of turgor were investigated. Recovery of turgor was K+ dependent. Calculation of turgor by an alternative method involving spectrophotometric analysis of shrinkage gave somewhat lower estimates of turgor pressure.
TL;DR: Immediate recovery, as judged by ability to open eyes and recall date of birth, was significantly more rapid following propofol and restoration of higher mental function was significantly faster following Propofol, measured by choice reaction time and critical flicker fusion threshold.
Abstract: Propofol and midazolam were compared in 40 patients undergoing orthopaedic surgery under spinal anaesthesia. An infusion of either 1% propofol or 0.1% midazolam was given at a rate adjusted to maintain a similar level of sedation. The mean time to reach this required level was similar in both groups. Quality and ease of control of sedation were good in all patients. A mean infusion rate of 3.63 mg kg−1 h−1 was required for propofol and 0.26 mg kg−1 h−1 for midazolam. Immediate recovery, as judged by ability to open eyes and recall date of birth, was significantly more rapid following propofol (P
TL;DR: The major cell surface glycoconjugate of the parasitic protozoan Leishmania major is a heterogeneous lipophosphoglycan, and it is shown that the LPG anchor is structurally related to the major glycolipid species of L. major, indicating that some of these Glycolipids may have a function as precursors to LPG.
TL;DR: The different relative potencies of tautomycin and okadaic acid for protein phosphatases 1 and 2A suggest that parallel use of both inhibitors may help to identify physiological substrates for each enzyme.
TL;DR: A patient with acute perforation of an ulcer in the first part of the duodenum who was successfully treated by laparoscopic oversewing and omental patching is reported.
Abstract: Laparoscopic techniques have been refined to the point where exposure, haemostasis and tissue approximation by suture approach those obtained at open access surgery We report a patient with acute perforation of an ulcer in the first part of the duodenum who was successfully treated by laparoscopic oversewing and omental patching The clinical indications for contemplating use of laparoscopic surgery for acute ulcer perforation, techniques employed and the areas for potential improvement of instruments, needles and sutures are discussed
TL;DR: In this article, the stability of a simple numerical scheme for solving the time dependent form of the Electric Field Integral Equation is investigated, in which the time step Δt times the speed of light c does not exceed the minimum spatial grid step Δ, and the surface current and charge distributions are found explicitly in terms of the distribution at earlier times.
Abstract: The stability of a simple numerical scheme for solving the time dependent form of the Electric Field Integral Equation is investigated in this paper. In this so-called "time marching" algorithm, the time step Δt times the speed of light c does not exceed the minimum spatial grid step Δ, and the surface current and charge distributions are found explicitly in terms of the distribution at earlier times. In order to obtain solutions which are stable in the long term, two points must be considered. (i) The product cΔt must not exceed Δ/√2. (Courant condition.) (ii) If the scatterer has internal resonances (i.e., is a closed body) or has a SEM pole close to the imaginary axis then, in addition to the Courant condition holding, an averaging procedure must be used. A four step averaging procedure is found to be necessary for closed bodies with internal resonances, such as the cube, whereas a three step procedure is found to be adequate for pair of parallel plates, which has no internal resonance, but does posses...
TL;DR: It is suggested that folding of the junction into the stacked X‐structure generates electronegative clefts that can selectively bind metal ions, depending on the chemistry, size and charge of the ion.
Abstract: Metal ions fold DNA junctions into a compact conformation that confers protection of all thymine bases to modification by osmium tetroxide. In the absence of the cation the arms of the junction are fully extended in an approximately square-planar configuration. Group IIa cations are effective in achieving a folded conformation of the junction at 80-100 microM, and there is an excellent agreement between the ionic concentrations that fold the junctions as deduced from gel electrophoretic experiments, and those that prevent osmium tetroxide reaction at the junction. Hexamminecobalt(III) achieves full folding at 2 microM, while spermine and spermidine are effective at 25 microM. Some transition metal ions such as Ni(II) may replace the group IIA cations. Monovalent ions of group IA are only partially effective in folding the junctions. Very much higher concentrations are necessary, gel electrophoretic mobilities suggest that a less symmetrical conformation is adopted and thymine bases at the junction remain reactive to osmium tetroxide. Charge-charge interactions at the centre of the junction are structurally extremely important. Substitution of junction phosphate groups by uncharged methyl phosphonates severely perturbs the structure of the junction. If just two phosphates are substituted, diametrically facing across the junction, the structure always folds in order to place the electrically neutral phosphate on the exchanging strands. We suggest that folding of the junction into the stacked X-structure generates electronegative clefts that can selectively bind metal ions, depending on the chemistry, size and charge of the ion. Moreover, occupation of these cavities is essential for junction folding, in order to reduce electrostatic repulsion.(ABSTRACT TRUNCATED AT 250 WORDS)
TL;DR: Measurement of type 2A proteinosphatase activity in yeast strains lacking one or other of the genes indicates that they account for most, if not all, protein phosphatase 2A activity in the cell.
Abstract: Two genes (PPH21 and PPH22) encoding the yeast homologues of protein serine-threonine phosphatase 2A have been cloned from a Saccharomyces cerevisiae genomic library using a rabbit protein phosphatase 2A cDNA as a hybridization probe. The PPH genes are genetically linked on chromosome IV and are predicted to encode polypeptides each with 74% amino acid sequence identity to rabbit type 2A protein phosphatase, indicating once again the extraordinarily high degree of sequence conservation shown by protein-phosphatases from different species. The two PPH genes show less than 10% amino acid sequence divergence from each other and while disruption of either PPH gene alone is without any major effect, the double disruption is lethal. This indicates that protein phosphatase 2A activity is an essential cellular function in yeast. Measurement of type 2A protein phosphatase activity in yeast strains lacking one or other of the genes indicates that they account for most, if not all, protein phosphatase 2A activity in the cell.