Winged leaf-cutting ants on nuptial flights used as transport by Attacobius spiders for dispersal

Abstract.  1. Sexuals of a leaf-cutting ant, Atta bisphaerica Forel, left their nest for nuptial flights in October to December.
2. When leaving a nest, 53 of the 479 winged sexuals (or alates) observed (11.1%) carried up to three inquiline spiders of Attacobius luederwaldti .
3. Spiders exclusively selected winged sexuals, not workers, and preferred females, indicating their expectation of the stronger flight ability of females. Neither these sexuals nor workers that appeared out of the nest on flight days attempted to remove or attack spiders on the body of alates.
4. New qucens landing from their nuptial flight did not carry spiders, indicating that the spiders had left the ants in the sky to be dispersed by wind.
5. No spiders were found in more than 100 incipient nests, which were estimated to be 2–3 months old. This suggests that the spiders jumped off the alate during mid-flight and dispersed on the wind to inhabit larger nests.     

NADPH-generating system: Influence on microsomal mono-oxygenase stability during incubation for the liver-microsomal assay with rat and mouse S9 fractions

Activity levels of 7-ethoxycoumarin O-deethylase (ED), aminopyrine N-demethylase (APD), p-nitroanisoleO-demethylase (p-NAD) and glucose-6-phosphate dehydrogenase (G-6-PDH) were determined in incubation mixtures for the liver-microsomal assay (LMA) at time 0 and after 1 and 2 h incubation under conditions for mutagenic assay. The experiments were performed with S9 liver fractions from mice (induced with Na-phenobarbital and β-naphthoflavone) and rats (induced with Aroclor 1254) with and without G-6-PDH in the incubation mixtures.

In the absence of G-6-PDH the activities were significantly lower at time 0 in the mouse. The pattern of stability, however, was similar for the activities, with an increase of stability after 1 and 2 h of pre-incubation (an exception for p-NAD).

Only ED activity showed a similar behaviour in the rat. No differences were present for APD and p-NAD activities at time 0 in the rat, but the enzyme stabilities were significantly decreased after 2 h of incubation (about 15% and 10% for APD and p-NAD respectively) in the absence of G-6-PDH.

At time 0, the amounts of G-6-PDH differed between mouse and rat fractions; however, during the incubations for LMA they decreased by about 57% and 53% for the two species, respectively. In addition to the above biochemical results, the presence of exogenous G-6-PDH in the incubations for the mutagenic assay, significantly increased the mitotic gene conversion and mitotic crossing-over of dimethylnitrosamine (DMN) and AR2MNFN (a nitroimidazo[2,1-b]thiazole) in the D₇ strain of Saccharomyces cerevisiae.     

The Function of Ascorbic Acid in Photosynthetic Phosphorylation

Ascorbate is oxidized to the free radical monodehydroascorbate by O2.- and by H2O2 (through the action of ascorbate peroxidase) formed in the Mehler reaction by isolated spinach (Spinacia oleracea) thylakoids. Light-dependent electron transport from water to monodehydroascorbate is shown to be coupled to ATP formation with a ratio ATP:O2 of 2. In the presence of ascorbate the net O2 exchange balance of the Mehler reaction is close to zero, and the synthesis of ATP is increased 2 to 3 times due to the extra electron transport to the monodehydroascorbate free radical. A scheme of the electron transport in the presence of ascorbate is discussed.     

Ferredoxin—ferredoxin NADP reductase interaction: Catalytic differences between the soluble and thylakoid-bound complex

Ferredoxin-NADP reductase (FNR) and ferredoxin form a complex when the former is membrane-bound as they do when both components are in solution, with the same dissociation constant. The rate constant of NADP photoreduction, first order with respect to the complex, is more than 20-times higher when FNR is membrane-bound than when the enzyme is in solution. The Arrhenius activation energy is identical in both conditions. These observations are interpreted in terms of ‘entropic catalysis’ of NADP reduction by the thylakoid-bound FNR.     

Studies on Cation-induced Thylakoid Membrane Stacking, Fluorescence Yield, and Photochemical Efficiency

Trypsin digestion of photosynthetic membranes isolated from spinach (Spinacia oleracea L.) leaves eliminates the cation stimulation of chlorophyll fluorescence. High concentrations of cations protect the fluorescence yield against trypsin digestion, and the cation specificity for this protection closely resembles that required for the stimulation of fluorescence by cations. Trypsin digestion reverses cation-induced thylakoid stacking, and the time course of this effect seems to parallel that of the reversal of cation fluorescence. High concentrations of cations protect thylakoid stacking and cation-stimulated fluorescence alike. The cation stimulation of photosytem II photochemistry remains intact after trypsinization has reversed both cation-induced thylakoid stacking and fluorescence yield. It is concluded that cation-stimulated fluorescence yield, and not the cation stimulation of photosystem II photochemistry, is associated with thylakoid membrane stacking.     

Lipid interaction of Pseudomonas aeruginosa exotoxin A. Acid-triggered permeabilization and aggregation of lipid vesicles.

We have investigated the interaction of Pseudomonas exotoxin A with small unilamellar vesicles comprised of different phospholipids as a function of pH, toxin, and lipid concentration. We have found that this toxin induces vesicle permeabilization, as measured by the release of a fluorescent dye. Permeabilization is due to the formation of ion-conductive channels which we have directly observed in planar lipid bilayers. The toxin also produces vesicle aggregation, as indicated by an increase of the turbidity. Aggregation and permeabilization have completely different time course and extent upon toxin dose and lipid composition, thus suggesting that they are two independent events. Both time constants decrease by lowering the pH of the bulk phase or by introducing a negative lipid into the vesicles. Our results indicate that at least three steps are involved in the interaction of Pseudomonas exotoxin A with lipid vesicles. After protonation of one charged group the toxin becomes competent to bind to the surface of the vesicles. Binding is probably initiated by an electrostatic interaction because it is absolutely dependent on the presence of acidic phospholipids. Binding is a prerequisite for the subsequent insertion of the toxin into the lipid bilayer, with a special preference for phosphatidylglycerol-containing membranes, to form ionic channels. At high toxin and vesicle concentrations, bound toxin may also induce aggregation of the vesicles, particularly when phosphatidic acid is present in the lipid mixture. A quenching of the intrinsic tryptophan fluorescence of the protein, which is induced by lowering the pH of the solution, becomes more drastic in the presence of lipid vesicles. However, this further quenching takes so long that it cannot be a prerequisite to either vesicle permeabilization or aggregation. Pseudomonas exotoxin A shares many of these properties with other bacterial toxins like diphtheria and tetanus toxin.     

Proton-induced grana formation in chloroplasts. Distribution of chlorophyll-protein complexes and Photosystem II photochemistry

Lowering the pH of the incubation medium to pH 5.4 leads to grana formation morphologically similar to that induced by metal cations. The same phenomenon is observed in EDTA-washed chloroplasts, indicating that it is not due in part to electrostatic ‘masking’ by residual cations associated with the membranes. Digitonin fractionation studies have indicated that the distribution of the major chlorophyll-protein complexes between granal and stromal membrane regions is similar at pH 5.4 in the absence of Mg²⁺, and at pH 7.4 in the presence of Mg²⁺. Chlorophyll fluorescence induction studies have indicated that the primary photochemistry of Photosystem II (PS II) is stimulated by lowering the pH to 5.4, just as it is upon metal cation addition at higher pH values. The failure to observe such an increase at pH 5.4 by measuring electron transport to ferricyanide is attributed to a combination of an inhibition by this pH of electron transport at a site after Q reduction and an increase in the number of PS II centres detached from the plastoquinone pool. We conclude that the stacked configuration of chloroplast membranes leads to increased PS II primary photochemistry, which is most simply explained in terms of a redistribution of excitation energy towards PS II.     

Partition zone penetration by chymotrypsin, and the localization of the chloroplast flavoprotein and photosystem II.

1. Chymotrypsin treatment of chloroplast membranes inactivates Photosystem II. The inactivation is higher when the activity is measured under low intensity actinic light, suggesting that primary photochemistry is preferentially inactivated. 2. Membrane stacking induced by Mg2+ protects Photosystem II against chymotrypsin inactivation. When the membranes are irreversible unstacked by brief treatment with trypsin, Mg2+ protection against chymotrypsin inactivation of Photosystem II is abolished. 3. The kinetics of inactivation by chymotrypsin of Photosystem II indicates that membrane stacking slows down, but does not prevent, the access of chymotrypsin to Photosystem II, which is mostly located within the partition zones. 4. It is concluded that a partition gap exists between stacked membranes of about 45 A, the size of the chymotrypsin molecule. 5. The kinetics of inhibition of the chloroplast flavoprotein, ferredoxin-NADP reductase, bt its specific antibody is not affected by membrane stacking. This indicates that this enzyme is located outside the partition zones.     

Osservazioni Preliminari sui Sistemi Degradanti dell'RNA nei semi di Ricino

Abstract

Preliminary observations on the enzymatic degradation of RNA in castor bean seeds. — Cocucci, Maggio, Monroy and Marrè have shown the decrease of RNA content during ripening in castor bean seeds, and its increase during germination. Furthermore, these Authors have demonstrated that in the dry ripe seeds the ribosomes are undetectable, and that they increase rapidly during germination. Two peaks of ribosomes are easily detected upon ultracentrifugal analysis in germinating seeds (Cocucci and Sturani). These observations were the basis for our investigations of the enzymes of RNA metabolism in castor bean seeds. This paper deals with our preliminary observations on RNA degrading enzymes in these tissues. We have been able to measure RNase activity, phosphodiesterase, 3′-,5′- and 2′-nucleotidases in castor bean seeds at different stages of development. RNase activity (measured in crude extracts) changes little during the ripening process, its rate corresponding to 40–50 μMoles of nucleotides liberated from RNA per hour and per gram of fresh weight. In the dry seeds, RNase activity is 30–40 μMoles of nucleotides/h.g.f.w., and it increases to about 60–70 μMoles/h/g.f.w. after 72 hours of germination.

Phosphodiesterase activity is about 4–5 μMoles/h.g.f.w.

The following rates have been found in seeds almost completely ripe seeds for 3′-, 5′- and 2′-nucleotidase activities, respectively 45–50 μMoles/h.g.f.w.; 6–7 μMoles/h.g.f.w.; 8 μMoles/h.g.f.w.; ATP-ase activity was of about 80–100 μMoles of phosphate liberated /h.g.f.w. - The high activity of 3′-nucleotidase, of the same order of that of RNase, suggests that these two enzymes are responsible for degradation of RNA to nucleosides and inorganic phosphate. Further investigations are being carried on to define the biochemical properties of castor bean RN-ase.