Ciliary receptors associated with the mouth and pharynx of Acoela (Acoelomorpha): a comparative ultrastructural study

The ultrastructure and distribution of receptor cells near the mouth and (where present) the pharynx of Hofstenia miamia, Proporus bermudensis, Conaperta thela, and Convoluta convoluta (Acoela) were investigated by transmission electron microscopy and confocal laser scanning microscopy of specimens stained with a fluorescence marker for actin. Five types of monociliary receptors were identified: (1) non‐collared receptors with a single long and narrow ciliary rootlet; (2) non‐collared receptors with a wide main ciliary rootlet and a smaller posterior rootlet; (3) non‐collared receptors with a single wide and hollow ciliary rootlet with a granulated core; (4) Collar (?) receptors with obliquely radial filament bundles in the cell apex and with a single hollow ciliary rootlet composed of numerous strand‐like elements; and (5) Collar receptors lacking a striated rootlet but with a granular body (swallow's nest rootlet). While H. miamia bears the first two receptor types, P. bermudensis has receptors of type 1, 3 and 5, and Cona. thela and Conv. convoluta have receptors of type 3, 4 and 5. The density of receptors is generally highest at the anterior body tip, regardless of where the mouth is located. Most receptor types occur scattered over the whole body but type 2 receptors of H. miamia are restricted to the pharynx and mouth region. The lack of a common receptor type specific for the mouth and pharynx of the investigated species points to an independent origin of the pharynges in Hofsteniidae and in Proporidae and of the mouth tube in Convolutidae. Moreover, the homology of the so‐called collar receptors in Acoela with typical collar receptors in other invertebrates is questioned.     

Determination of “active” cytochrome P-450 from relaxation kinetics of product formation

We estimate the active part of cytochrome P-450, which is involved in a special substrate transformation, by measuring the initial change of the production rate as a function of the relaxation transitions between two different steady states of the reaction cycle of cytochrome P-450 using the light-reversibility of the carbon monoxide inhibition. The kinetic data of such relaxations are interpreted within a model cycle, which reduces the reaction cycle to three steps. The estimation of the rate constant of the first reduction step, derived from model simulation of the production rate, is confirmed by independent experimental study of the reduction kinetics.An application of our model to the O-deethylation of 7-ethoxycoumarin reveals that — in a time average — 10%–15% of the spectroscopically detectable cytochrome P-450 is involved in that transformation.Abbreviations Cyt. P-450 microsomal cytochrome P-450 - 7-EC 7-ethoxycoumarin     

Size selection of latex beads by blackfly larvae (Diptera: Simuliidae) in the laboratory

In laboratory experiments, blackfly larvae collected from a lake outlet, a woodland and a meadow stream were tested for size selection of latex beads of < Ito > 100 μm diameters. 3 suspensions of varying proportions for each size category were supplied to these blackfly larvae in common experiments. Comparisons between the size frequency distributions of particles supplied and the particle compositions in the larval guts showed intra- and interspecific differences and were quantified by calculating Jacobs' electivity index. In all species selection of larger particles increased with the larger larval instars. Although there was a positive selectivity of small particles in some cases, the ingested proportion of large particles increases volumes and biomasses of gut content and may be more important for larval growth than small particles.     

Inducible nitrate reductase of rice plants as a possible indicator for nitrification in water-logged paddy soils

When following the pattern of the disappearance of NH ₄ ⁺ –N from ammonium sulfate applied to the flooded soil-rice plant system (field and greenhouse experiments) during a growing season, it was observed that the lowest NH ₄ ⁺ –N level coincided with the highest value of NR activity in the leaves. Nitrate was detected in both the root and shoot systems of the rice plants and autotrophic nitrifiers (Nitrosomonas and Nitrobacter) were particularly abundant. Since it was also demonstrated in this work that the NR activity of rice plants grown with nitrate fertilization (growth chamber culture experiments) was inducible by its substrate, it can be assumed that NH ₄ ⁺ –N oxidation takes place in the water-logged soil studied. Therefore, the occurrence of the nitrification process following NH ₄ ⁺ –N fertilizer application can be predicted by thein vitro orin situ evaluation of the NR activity of the rice leaf as an indicator.     

Role of phospholipase A2 in the stimulation of sponge cell proliferation by homologous lectin

Using the Geodia cydonium system, we showed that after incubation of competent sponge cells in the presence of lectin, phospholipase A2 was released from the cells. The substrates for this enzyme, phosphatidylethanolamine and phosphatidylcholine, were identified in the extracellular material of sponge tissue. In addition, the phospholipase A2 inhibitor calelectrin was identified by immunobiochemical techniques; this molecule was associated with the aggregation factor. Reconstitution experiments strongly suggested that phospholipase A2 catalyzed the release of arachidonic acid, which is then taken up by the cells. Intracellularly, arachidonic acid was metabolized primarily to prostaglandin E2. Inhibition studies revealed that prostaglandin E2 is involved in the ultimate increase of DNA synthesis. These findings suggest that the phospholipase A2-arachidonic acid system is involved in the matrix-initiated signal transduction pathway in sponges.     

Protein kinase C phosphorylates the sponge aggregation receptor after its binding to the homologous aggregation factor

The aggregation factor from the sponge Geodia cydonium functions also as a growth factor after binding to the aggregation receptor (= growth factor receptor) on the plasma membrane of homologous cells. We have recently shown that protein kinase C is involved in the pathway transducing the growth factor signal. Here we report that the aggregation receptor (a polypeptide with an Mr of 43,500) is phosphorylated by protein kinase C. Using a plasma membrane fraction only this phosphoprotein (pp) 43.5 became phosphorylated by kinase C. The phosphorylation of pp43.5 in intact cells in response to the binding of the aggregation factor to this polypeptide was a late event and occurred 10 to 15 h after addition of the aggregation factor. Based on studies with phorbol esters it appears to be very likely that protein kinase C also phosphorylates pp43.5 in vitro. The degree of phosphorylation of pp43.5 paralleled with both the extent of DNA synthesis and ras oncogene expression. The latter process resulted in a switch of the responsiveness of the cells to growth factors signals: 10 to 15 h after addition of the aggregation factor to dissociated cells, this factor lost its growth factor function while the homologous lectin gained the ability to stimulate cell proliferation (to be published). These results support the idea that phosphorylation of pp43.5 (= aggregation receptor) results in an inhibition of its function, i.e., the transduction of the growth factor (= aggregation factor) signal.     

The electrochemical proton potential generated by the sulphur respiration of Wolinella succinogenes

Wolinella succinogenes grown on formate and elemental sulphur was found to use the polysulphide derivatives 2,2-tetrathiobispropionate (R₂S₄) or pentathionate (S₅O ₆ ⁼ ) as acceptors for formate oxidation. The specific activities of formate oxidation with these acceptors were similar to those with elemental sulphur. The main reaction products of R₂S₄ reduction were 2,2-dithiobispropionate (R₂S₂) and sulphide. Pentathionate was converted to thiosulphate and some elemental sulphur. The electrochemical proton potential across the cytoplasmic membrane of the bacterium was measured in the steady state of electron transport from formate to R₂S₄. The electrical proportion () of the determined through the distribution of labeled tetraphenylphosphonium cation was obtained as 0.17 Volt. The was zero, when a protonophore was present. The pH-difference across the membrane was negligible. Thus the generated by sulphur respiration is close to that measured earlier with fumarate as the terminal acceptor of electron transport.Abbreviations DMO 5,5-dimethyloxazolidine-2,4-dione - R₂S_n (n=2–5) 2,2-polythiobispropionate - TTFB 4,5,6,7-tetrachloro-2-trifluoromethylbenzimidazol - TPP tetraphenylphosphonium cation     

Inhibitory effect of nonviable preparations from human immunodeficiency virus 1 on inositol phospholipid metabolism

Previously it was established [Pahwa, S., Pahwa, R., Saxinger, C., Gallo, R. C. & Good, R. A. (1985) Proc. Natl. Acad. Sci. USA 82, 8198] that nonviable preparations of human immunodeficiency virus 1 (HIV-1) abolish the proliferative response of human lymphocytes to phytohemagglutinin A. Now we describe that this effect might be, at least partially, due to an impairment of the function of phospholipase C. It was found that addition of HIV-1 preparation to lymphocytes diminished the stimulation of phosphatidylinositol phosphorylation caused by phytohemagglutinin A. Moreover, this preparation completely abolished the phytohemagglutinin-A-stimulated release of inositol trisphosphate and prevented a translocation of protein kinase C from cytosol to membranes. From this data we conclude that nonviable HIV-1 preparations inhibit the intracellular signalling pathway, leading to a reduced mitogenic response to phytohemagglutinin A, at the level of protein kinase C.     

Isolation of a cDNA encoding the human GM2 activator protein

The GM2 activator protein is a glycolipid-binding protein required for the lysosomal degradation of ganglioside GM2. A human fibroblast cDNA library was screened with mixtures of oligonucleotide probes corresponding to four different areas of the amino acid sequence. A putative clone (821 bp) which gave positive signals to all four probe mixtures was purified and sequenced. The sequence was colinear with the sequence of 160 amino acids of the mature GM2 activator protein. Availability of the cDNA clone should facilitate investigation into function of the GM2 activator protein and also into genetic abnormalities underlying GM2 gangliosidosis AB variant.     

Association of Tat protein and viral mRNA with nuclear matrix from HIV-1-infected H9 cells

The transactivating protein from human immunodeficiency virus type 1 (HIV-1), Tat, was found to bind to the nuclear matrix from uninfected and HIV-1-infected H9 cells. Addition of the Zn2+, Cd2+ and Cu2+ chelator o-phenanthroline destroyed the matrix fibrils and the binding affinity of Tat to the matrix. A sequential treatment of the matrix, first with o-phenanthroline and then with ZnCl2, partially restored the fibrillar-like matrix structure. Infection of H9 cells with HIV-1 resulted in a displacement of cellular mRNA by viral mRNA from the nuclear matrix. Both the matrix-bound host cell and HIV-1 mRNA were found to dissociate from the matrix in the presence of o-phenanthroline. This could be prevented by coincubation with Zn2+ or Cu2+ (but not Mg2+), which stabilize the mRNA containing nuclear matrix structure.