Activation of the non-electrochromic component in the 518 nm absorbance change by photosystem I

The flash-induced absorbance change measured at 518 nm (P515) in intact chloroplasts consists of at least 4 kinetically different components. Here the non-electrochromic component, either called phase d or reaction 3, is studied in some detail. The effect of DCMU, DQH₂ and DBMIB on the amplitude of reaction 3 and the turnover of cytochrome f and P700 have been monitored, suggesting an involvement of photosystem 1 in the activation of the non-electrochromic absorbance change. This is confirmed by the parallel oscillation pattern found in P700 rereduction and the amplitude of reaction 3.     

Photobiology of microorganisms: how photosensors catch a photon to initialize signalling

The afa-3 gene cluster determines the formation of an afimbrial adhesive sheath that is expressed by uropathogenic as well as diarrhoea-associated Escherichia coli strains. It contains six genes ( afaA–afaF  ), among which the afaE3 gene is known to code for the structural AfaE-III adhesin (previously designated AFA-III), whereas no role has yet been identified for the afaD gene product. The afa-3 gene cluster is closely related to the daa operon that codes for an adhesin, the F1845 adhesin, which is highly related to the AfaE-III adhesin; however, unlike the AfaE-III adhesin, F1845 is a fimbrial adhesin. Reported in this work is the construction of chimeras between the afa-3 and daa operons. Analyses of the phenotypes conferred by these afa-3 / daa chimeric clusters allowed us to conclude that the biogenesis of a fimbrial or an afimbrial adhesin is fully determined by the amino acid sequence of the AfaE-III and F1845 adhesins. Moreover, the role of the AfaD product in the biosynthesis of the afimbrial sheath was assessed by immunogold and immunofluorescence experiments. The AfaD and the AfaE-III products were purified and used to raise rabbit and mouse antisera. Similar to AfaE-III, AfaD was found to be a surface-exposed protein as well as an adhesin; both AfaD and AfaE-III are concomittantly expressed by the bacterial cell. These results demonstrate, for the first time, that the afimbrial adhesive sheath expressed by pathogenic E. coli is composed of two adhesins.     

Enhancement of the light-triggered electrical response in plant cells following their de-energization with uncouplers

Light-triggered membrane potential changes in cells of a liverwort Anthoceros are greatly enhanced by the ionophorous uncouplers nigericin and monesin. Stimulation of the light-triggered electrical response (LTER) by nigericin occurred concomitantly with inhibition of a slow decline in the chlorophyll fluorescence, which suggests that the transmembrane pH gradient in thylakoids is not essential for generation of LTER at the plasma membrane. The extent of monensin-stimulated LTER remained high under a diminished driving force for the ionophore-induced proton-cation exchange across the plasma membrane (elevation of the external Na⁺ concentration from 1 to 50 m M ), which indicates that energy uncoupling in chloroplasts is more related to the electric response enhancement than the induction of the H⁺/K⁺(Na⁺) exchange at the plasma membrane. Enhancement of LTER by ionophores occurs in parallel with stimulation of light-triggered pH changes (alkalinization) in the vicinity of the cell surface, which suggests an association of trans-membrane H⁺ fluxes with LTER. The results are consistent with the hypothesis that illumination produces a temporary inhibition of the plasma membrane H⁺ pump with a subsequent activation of gated channels and transient rapid depolarization of the cell.     

Metal distribution across different pools in the organic layer of a forest under acid deposition and its consequences for the metal dynamics

Acid atmospheric deposition can cause losses of metal nutrients from the organic layer of a soil. The size of these losses depend on the sizes of the different pools in which the metals are present, as these pools differ in mobility. The metal pools in an organic soil layer of a Douglas fir forest in the Netherlands subjected to acid deposition were determined by means of extractions and percolations. Na was mainly dissolved and exchangeably adsorbed, K dissolved, exchangeably adsorbed and present in the soil microbial biomass, Ca exchangeably adsorbed and present in organic precipitates, Mg exchangeably adsorbed and present in the soil biomass, and Mn exchangeably adsorbed and present in inorganic precipitates. The main part of the metals was exchangeably adsorbed. The adsorption affinity increased in the order Na < K < Mg < Mn ≈ Ca. The vertical distribution of the metals in the organic layer showed that all metals were continuously lost from the organic layer. The differences between the metals in retention and vertical distribution patterns were in agreement with their differences in deposition rate, pool distribution, and exchange affinity. Since the metals were mainly exchangeably adsorbed, and the acidifying cations dominated the atmospheric deposition, acid deposition and cation exchange must be processes that strongly affect the losses of metals from this organic soil layer. R F Huettl Section editor     

Therapy of bovine ocular squamous-cell carcinoma with local doses of interleukin-2: 67% complete regressions after 20 months of follow-up

We have tested the therapeutic potency of peritumorally injected low doses of interleukin-2(IL-2). Seventy tumours of the bovine ocular squamous-cell carcinoma (BOSCC), 1–3 cm in diameter, were treated with 5000, 20 000 or 200 000 U IL-2 from Eurocetus (Chiron) to find the optimal dose for treatment. Injections were given peritumorally on Monday to Friday on 2 consecutive weeks. The size of the tumours was measured before treatment and 1, 3, 4, 9 and 20 months after treatment. After 9 months complete regression was observed in 89% of the tumours treated with 5000 U IL-2, 80% treated with 20 000 U and 67% treated with 200 000 U. After 20 months, there was complete regression of 35%, 31% and 67% of the tumours respectively. The 9-and 20-month results of the 200 000-U treatment are significantly better than those of the 5000-U and 20 000-U treatments taken together. This protocol may be useful to treat advanced inoperable tumours (e.g. of the nasopharynx or skin) of human patients.     

Studies on well-coupled Photosystem-I-enriched subchloroplast vesicles. Discrimination of flash-induced fast and slow electric potential components

This work aimed at the resolution of the multi-component electric potential changes induced by single-turnover flash illumination of Photosystem-I-enriched subchloroplast vesicles. If supplemented with ferredoxin and under carefully adjusted redox poising, these vesicles show a pronounced slow-rising and -decaying electric potential component, as monitored by endogenous and exogenous field-sensitive probes, carotenoids and oxonol VI, respectively. The fast and slow potential components can be easily discriminated without the need for computer-assisted deconvolution after selective presaturation of the slow component by preillumination or a transmembrane ΔpH, after selective suppression of the slow component by low valinomycin or uncoupler concentrations or in the absence of ferredoxin. The slow electric potential component, as compared to the fast one, is relatively sensitive to low concentrations of ionophores and uncouplers, detergent, ageing and lower temperatures (4–12°C), is associated with electrogenic proton displacements and is interpreted to respond to a field that is more located on the membrane-bulk interface. Temperature effects show transition temperatures around 20°C for both the rise and decay of the slow potential component. The results provide further evidence that the carotenoids and oxonol VI sense the same (slow) electric field, but may be differently located in the thylakoid membrane.     

Current views on the regulation of autotrophic carbon dioxide fixation via the Calvin cycle in bacteria

The Calvin cycle of carbon dioxide fixation constitutes a biosynthetic pathway for the generation of (multi-carbon) intermediates of central metabolism from the one-carbon compound carbon dioxide. The product of this cycle can be used as a precursor for the synthesis of all components of cell material. Autotrophic carbon dioxide fixation is energetically expensive and it is therefore not surprising that in the various groups of autotrophic bacteria the operation of the cycle is under strict metabolic control. Synthesis of phosphoribulokinase and ribulose-1,5-bisphosphate carboxylase, the two enzymes specifically involved in the Calvin cycle, is regulated via end-product repression. In this control phosphoenolpyruvate most likely has an alarmone function. Studies of the enzymes isolated from various sources have indicated that phosphoribulokinase is the target enzyme for the control of the rate of carbon dioxide fixation via the Calvin cycle through modulation of existing enzyme activity. In general, this enzyme is strongly activated by NADH, whereas AMP and phosphoenol-pyruvate are effective inhibitors. Recent studies of phosphoribulokinase inAlcaligenes eutrophus suggest that this enzyme may also be regulated via covalent modification.     

Dialysis membrane technique for ultrastructural studies of microbial interactions.

A dialysis membrane technique was developed that enabled ultrastructural investigations of the interaction of nematode-trapping fungi and their nematode prey. It allowed the sectioning of individual traps that had been selected by light microscopy and was used in kinetic studies on trap formation, nematode capture, and subsequent nematode digestion. The method can also be used for enzyme cytochemical experiments.