Two possible 3-(3,4-dichlorophenyl)-1,1-dimethylurea-insensitive sites in Photosystem II of spinach chloroplasts

Two possible 3-(3,4-dichlorophenyl)-1,1-dimethylurea-insensitive sites were found in PS II of spinach chloroplasts, depending on the pH of the assay medium used. The low site (pH 6) can be inhibited by certain quinolines, such as 8-hydroxyquinoline at concentrations less than 50 μM. The high pH site (pH 8) can be inhibited by disodium cyanamide, folic acid, or 5,6-benzoquinoline at concentrations from 50 μM to 5 mM. With the exception of orthophenanthroline, which stimulates the high pH site but does not show much inhibition at low pH, all other inhibitors gave opposite effects at the pH values used, i.e., they stimulated at low pH or inhibited at high pH, or vice versa. Several mechanisms for the observed effects are discussed.     

The response of a nerve cylinder to spatially distributed white noise inputs

The response of a passive nerve cylinder (or dendritic tree in the equivalent cylinder representation) to random white noise input currents is determined. Results for the mean, variance and covariance of the depolarization are obtained for an arbitrary number of independent spatially distributed inputs. The case of a cylinder with sealed ends is considered in detail. The differences that arise when the input currents are distributed over a small but finite region of space instead of concentrated at a point are investigated. In the case of distributed inputs, the expectation is smoother near the stimulus and the variance becomes finite over the entire cable length including the region of the applied stimulus. Away from the stimulus, there are no appreciable differences between the responses for the two cases. The interaction between an excitatory input and an inhibitory input at various locations is examined and one case of more than two inputs is also analysed to study effects which could not have been discerned from point models for a neuron with random inputs.     

The induction of cytokinin habituation in primary pith explants of tobacco

Pith parenchyma tissue ofNicotiana tabacum L. cv. Havana 425 becomes cytokinin habituated when incubated at 35°C on an auxin-containing medium. Under these conditions, habituated, hyperplastic nodules appear on the tissues. We used these nodules to estimate the incidence of habituation by a statistical method. The rate of habituation varied with the season. Tissue isolated from plants in the spring habituated approx. 7 times faster than did tissue isolated from plants in winter. The fact that the average rate, >4×10^–3 per cell generation, was 100–1,000 times faster than the rate of somatic mutation inNicotiana species and depended on the physiological state of the tissue provides further evidence that habituation involves epigenetic changes rather than rare, random genetic mutations. We also found that kinetin (6-furfurylaminopurine) induced habituation and that the concentration required depended on the duration of cytokinin treatment. For long incubation times, approx. 6×10^–10 M kinetin, which is about 1,000-fold lower than the concentration optimal for growth of cytokinin-requiring pith tissue, was sufficient to induce habituation. These results support the hypothesis that the habituated state is maintained by a positive feedback loop in which cytokinins either induce their own synthesis or inhibit their own degradation.     

Stimulation of electron transport from Photosystem II to Photosystem I in spinach chloroplasts

Electron transport from Photosystem II to Photosystem I of spinach chloroplasts can be stimulated by bicarbonate and various carbonyl or carboxyl compounds. Monovalent or divalent cations, which have hitherto been implicated in the energy distribution between the two photosystems, i.e., spillover phenomena at low light intensities, show a similar effect under high light conditions employed in this study. A mechanism for this stimulation of forward electron transport from Photosystem II to Photosystem I could involve inhibition of two types of Photosystem II partial reactions, which may involve cycling of electrons around Photosystem II. One of these is the DCMU-insensitive silicomolybdate reduction, and the other is ferricyanide reduction by Photosystem II at pH 8 in the presence of dibromothymoquinone. Greater stimulation of forward electron transport reactions is observed when both types of Photosystem II cyclic reactions are inhibited by bicarbonate, carbonyl and carboxyl-type compounds, or by certain mono- or divalent cations.Abbreviations used: DCMU, 3-(3,4-dichlorophenyl)-1, 1-dimethylurea; DCIP, 2,6-dichloroindophenol; DBMIB, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone; FeCN, potassium ferricyanide; MV, methylviologen; PS I, photosystem I; PS II, photosystem II; SM, silicomolybdic acid.     

Structural elucidation and partial synthesis of 3-hydroxyheterodendrin,a cyanogenic glucoside from Acacia sieberiana var. woodii

A new cyanogenic glucoside, isolated from pods of Acacia sieberiana var. woodii, was shown to be (2R)-2- (β-d-glucopyranosyloxy)-3-hydroxy-3-methylbutanenitrile by spectroscopic and chemical methods. The absolute configuration of this glucoside was correlated with that of proacacipetalin by oxymercuration of the latter, followed by borohydride reduction of the product.     

Purification and characterization of a xylobiose- and xylose-producing endo-xylanase from Aspergillus niger

A xylanase from a commercial Aspergillus niger pentoglycanase was purified to homogeneity by column chromatography on Ultrogel AcA 54, SP-Sephadex, Sephadex G-50, and SP-Sephadex. The enzyme hydrolyzed xylotriose slowly to xylose and xylobiose, and xylotetraose and higher xylo-oligosaccharides rapidly to mixtures of smaller xylo-oligosaccharides, with xylobiose and xylose being the preponderant final products. The anomeric configuration of the products was inverted, in contrast to the behavior of most other carbohydrases that initially produce mixtures of oligosaccharides. This enzyme is a glycoprotein having an amino acid composition high in acidic residues. Its molecular weight is 20,800 and its isoelectric point is at pH 6.7. Optimal pH values for activity and stability are between 4 and 6 and, in a 20-min assay, maximal activity is attained at 55°.     

Growth and Metabolism of Live Vaccine Strain of Pasteurella tularensis

The growth and metabolism of the live vaccine strain of Pasteurella tularensis in different media were investigated. Maximal growth was observed in a medium containing a sulfuric acid digest of casein as amino acid source. Amino acid metabolism produced considerable ammonia, and the rate of ammonia evolution was directly proportional to the growth rate. The most likely route for amino acid breakdown is nonspecific oxidative deamination.     

Sedimentation Changes of L Cells in a Density Gradient Early After Infection with Vaccinia Virus

By 2 h postinfection, LM cells infected with vaccinia virus show a shift in their distribution when separated on a Ficoll density gradient. This shift is dependent on both time and the multiplicity of infection and is due, at least in part, to an increase in cell size. Those cells which do shift in position in the gradient represent infected members of the population. Physical changes induced in virus-infected cells can be utilized for studying early events in virus replication.