Postreplication repair in Neurospora crassa

Summary Changes in the molecular weight of nascent DNA made after ultraviolet (UV) irradiation have been studied in the excision-defective Neurospora mutant uvs-2 using isotopic pulse labeling, alkaline gradient centrifugation and alkaline filter elution. Both the size of nascent DNA and the rate of incorporation of label into DNA was reduced by UV light in a dose dependent manner. However, this DNA repair mutant did recover the ability to synthesize control-like high molecular weight DNA 3 hours after UV treatment, although the rate of DNA synthesis remained depressed after the temporary block to elongation (or ligation) had been overcome. Photoreactivation partially eliminated the depression of DNA synthesis rate and UV light killing of cells, providing strong evidence that the effects on DNA synthesis and killing were caused by pyrimidine cyclobutane dimers. The caffeine inhibition repair studies performed were difficult to quantitate but did suggest either partial inhibition of a single repair pathway or alternate postreplication DNA repair pathways in Neurospora. No enhancement in killing was detected after UV irradiation when cells were grown on caffeine containing plates.     

Comparison of yeast mitochondrial Phe-tRNA synthetase subunits to their cytoplasmic counterparts: Isolation and determination of amino acid compositions

The α and β subunits of yeast mitochondrial Phe-tRNA synthetase are separated and isolated by means of chromatography on DEAE-cellulose, after enzyme alkylation with iodoacetate. The comparison of amino acid compositions of yeast mitochondrial and cytoplasmic native Phe-tRNA synthetases and their components shows significant differences. Results indicate that the two enzymes are coded for by different nuclear genes.     

Involvement of phage T5 tail proteins and contact sites between the outer and inner membrane of Escherichia coli in phage T5 DNA injection.

The penetration of phage T5 DNA into the Escherichia coli envelope takes place through ion channels (Boulanger, P., and Letellier, L. (1992) J. Biol. Chem. 267, 3168-3172). To identify putative phage protein(s) involved in the formation of these channels, E. coli cells were infected at 37 degrees C with radioactively labeled phage and their envelopes were fractionated. After a flotation gradient, proteins belonging to the phage tail were recovered both in fractions containing the contact sites between the inner and outer membranes and in the outer membrane. The electrophoretic banding pattern of phage proteins indicates that the contact sites were enriched in the protein pb2. Moreover, infected cells were significantly enriched in contact sites as compared to intact cells. There was no enrichment of contact sites and very little radioactivity was found in this fraction and in the outer membrane when the cells were infected at 4 degrees C (i.e. under conditions where the phage does not inject its DNA). These results suggest that both contact sites and pb2 may play a central role in the translocation of phage T5 DNA.     

Glycosphingolipids of normal and leukemic human leukocytes

Summary We have reviewed the studies on neutral glycosphingolipids and gangliosides of normal and leukemia human leukocytes. In this review, we examine (a) the glycosphingolipid composition of various leukocyte populations, (b) the differences in glycosphingolipids found among subsets of these cells, (c) the possible use of these compounds as markers of differentiation, and (d) the changes in glycosphingolipid composition that occur with leukemogenesis.     

THE INTERACTION OF AUXIN AND ETHYLENE IN THE MAINTENANCE OF LEAF BLADE FORM IN PHASEOLUS VULGARIS L. VAR. PINTO

Auxin treatment results in hyponastic curvature of the primary leaves of Phaseolus vulgaris L. var pinto. Ethylene production by hyponastic leaves is detected within 1 hr after treatment with IAA in concentrations at or above 1 μm. The amount of ethylene detected is proportional to the concentration of auxin applied. Untreated control leaves and leaves treated with 2,3,5-tri-iodobenzoic acid or gibberellic acid did not produce ethylene detectable by our equipment. The hyponastic curvature induced by auxin treatment can be inhibited by exogenous application of ethylene or ethylene-generating compounds, and these treatments produce epinasty in auxin-treated leaves. Treatment with inhibitors of ethylene synthesis or action, such as aminoethoxy-vinylglycine, carbon dioxide, or heat treatment, prolong hyponasty. The planar form, therefore, appears to be affected by both hyponastic auxin effect and an epinastic ethylene effect.     

The interaction of chlordiazepoxide and sodium valproate in the nucleus accumbens of the rat

Benzodiazepines are known to facilitate GABA-ergic transmission at synaptic sites, while sodium valproate is an anticonvulsant drug which is reported to elevate GABA levels in the brain. In order to determine whether these two drugs interact functionally at GABA receptor sites, graded doses of chlordiazepoxide (CDZ) and sodium valproate were injected bilaterally into the nucleus accumbens and their effect on the dopamine (DA)-induced stimulation of motor activity was studied. Both of these compounds, as well as GABA, produced an inhibition of the hyperactivity induced by the bilateral injection of DA into the nucleus accumbens. Bicuculline, the GABA receptor antagonist, blocked the effect of CDZ on the DA-induced hyperactivity. A low dose of CDZ (2 μg), which by itself did not significantly inhibit the effect of DA, potentiated the inhibition of the hyperactivity produced by valproate. These results suggest that CDZ and sodium valproate can interact functionally at GABA-ergic sites in the central nervous system.     

Field studies of the relationship between herbivore damage and tannin concentration in bracken (Pteridium aquilinum Kuhn)

Summary The acceptance of secondary plant metabolites as herbivore deterrents rests primarily on their deleterious effects on herbivores. Efforts to demonstrate differential fitness in natural plant populations with varying concentrations of tannin have failed, since coevolved plant predators may physiologically or behaviorally circumvent the defense, which results in apparently equal amounts of damage to defended and undefended individuals. In this study, two approaches were used to overcome this difficulty. 1) Theoretically, more energy should be allocated to the defense of parts which contribute more heavily to the plant's fitness. Bracken fern clones produce fronds throughout the growing season. Fronds which are produced early should be more heavily defended than late-emerging fronds which will return less photosynthate per unit cost of production. The results of this study do not support this prediction; it appears that the production of tannin is more closely linked to environmental factors such as water stress than to date of frond emergence. Fronds which emerged in August contained as much tannin as fronds which emerged in May. 2) By recording the temporal occurrence of herbivore damage in bracken ferns, it was found that in fronds which escaped attack until after reaching maturity there was a significant negative correlation between tannin concentration in the frond and the amount of damage experienced. This result supports the generally accepted assumption that herbivory has been a selective force in the evolution of tannin as a defensive substance.     

Incorporation of the transmembrane hydrophobic domain of glycophorin into small unilamellar phospholipid vesicles Ion Flux studies

Human erythrocyte glycophorin is one of the best characterized integral membrane proteins. Reconstitution of the membrane-spanning hydrophobic segment of glycophorin (the tryptic insoluble peptide released when glycophorin is treated with trypsin) with liposomes results in the production of freeze-fracture intrabilayer particles of 80 Å diameter (Segrest, J.P., Gulik-Krzywicki, T. and Sardet, C. (1974) Proc. Natl. Acad. Sci. U.S.A. 71, 3294–3298), with particles appearing at or above a tryptic insoluble peptide concentration of 4 mmol per mol phosphatidylcholine. In the present study, increasing concentrations of tryptic insoluble peptide were added to sonicated small unilamellar egg phosphatidylcholine vesicles and the rate of efflux of ²²Na⁺ was examined by rapid (30 s) gel filtration on Sephadex G-50. Below a concentation of 3–5 mmol tryptic insoluble peptide/mol phosphatidylcholine, ²²Na⁺ efflux occurs at a constant slow rate at given tryptic insoluble peptide concentrations. Above a concentration of 3–5 mM, the rate of efflux is biphasic at given tryptic insoluble peptide concentrations, exhibiting both an initial fast and a subsequent slow component. On the basis of graphic and computer curve-fitting analysis, with increasing tryptic insoluble peptide concentration, the rate of the slow component reaches a plateau at a tryptic insoluble peptide concentration of 3–5 mM and remains essentially constant until much higher concentrations are reached; the fast component increases linearly with increasing tryptic insoluble peptide concentration well beyond 5 mM. The most consistent interpretation of this data is as follows. The slow ²²Na⁺ efflux component is due to perturbations of small unilamellar vesicle integrity by tryptic insoluble peptide monomers. At a tryptic insoluble peptide concentration of 3–5 mmol/mol, a critical concentration is reached following which there is intrabilayer tryptic insoluble peptide self-association. The fast ²²Na⁺ efflux component is due to the increasing presence of tryptic insoluble peptide self-associated multimers the 80-Å particles seen by freeze-fracture electron microscopy) which results in a significantly larger bilayer defect than do tryptic insoluble peptide monomers. The failure of complete saturation of efflux by the fast component is ascribed to the presence of two populations of small unilamellar vesicles, some of which contain tryptic insoluble peptide multimers and some of which do not.Addition of cholesterol to the tryptic insoluble peptide/phosphatidylcholine vesicles decreases the rate of ²²Na⁺ efflux by inhibiting primarily the fast component. Freeze-fracture electron microscopy indicates that the presence of cholesterol has no effect on the size, number or distribution of 80-Å intra-bilayer particles in the tryptic insoluble peptide/phosphatidylcholine vesicles. These results are consistent with a mechanism to explain the fast Na⁺ efflux component involving protein-lipid boundary perturbations.Efflux of ⁴⁵Ca²⁺ from phosphatidylcholine vesicles is also enhanced by incorporation of tryptic insoluble peptide, but only if divalent cations (Ca²⁺ or Mg²⁺) are present in the external bathing media as well as inside the sonicated vesicles. If monovalent Na⁺ only is present in the bathing media no ⁴⁵Ca²⁺ efflux is seen. Under conditions where ⁴⁵Ca²⁺ efflux is seen, both a fast and a slow component are present, although both appear lower than corresponding rate constants for ²²Na⁺ efflux. These results suggest a coordinated mechanism for ion efflux induced by tryptic insoluble peptide and, together with the ²²Na⁺ efflux studies, may have mechanistic implications for the transbilayer phospholipid exchange (flip-flop) suggesed to be induced at glycophorin/phospholipid interfaces (de Kruiff, B., van Zoelen, E.J.J. and van Deenen, L.L.M. (1978) Biochim. Biophys. Acta 509, 537–542).