Physical microhabitat requirements of freshwater pearl mussels,Margaritifera margaritifera (L.)

Surface sediment diatoms from the east coast of Lake Tanganyika were analysed using ordination and classification techniques, and compared with assemblages previously described from the northern part of the lake. Grain-size analyses were performed on subsamples. Four groups of diatom assemblages were recognised. The first group clusters samples taken in the north, far from the Rusizi river mouth. The second group comprises samples taken on silty sediment along the Tanzanian coast, including one sample taken near the mouth of the Malagarazi river and those from the northernmost part of the lake. The third group comprises surface sediments along the Burundian coast (near Ramba and Magara), and the fourth is characterised by epipsammic taxa. A sample taken near the central arm of the Malagarazi river is included in the latter group. The impact of small rivers on the diatom assemblages in the surface sediments is restricted to the mouth area.     

Isolation of microsatellite loci in the Capricorn silvereye,Zosterops lateralis chlorocephalus (Aves: Zosteropidae)

The Capricorn silvereye (Zosterops lateralis chlorocephalus) is ideally suited to investigating the genetic basis of body size evolution. We have isolated and characterized a set of microsatellite markers for this species. Seven out of 11 loci were polymorphic. The number of alleles detected ranged from two to five and observed heterozygosities between 0.12 and 0.67. One locus, ZL49, was found to be sex‐linked. This moderate level of diversity is consistent with that expected in an isolated, island population.     

Transport of amino acids and ammonium in mycelium of Agaricus bisporus.

Mycelium of Agaricus bisporus took up methylamine (MA), glutamate, glutamine and arginine by high-affinity transport systems following Michaelis-Menten kinetics. The activities of these systems were influenced by the nitrogen source used for mycelial growth. Moreover, MA, glutamate and glutamine uptakes were derepressed by nitrogen starvation, whereas arginine uptake was repressed. The two ammonium-specific transport systems with different affinities and capacities were inhibited by NH(+)(4), with a K(i) of 3.7 microM for the high-velocity system. The K(m) values for glutamate, glutamine and arginine transport were 124, 151 and 32 microM, respectively. Inhibition of arginine uptake by lysine and histidine showed that they are competitive inhibitors. MA, glutamate and glutamine uptake was inversely proportional to the intracellular NH(+)(4) concentration. Moreover, increase of the intracellular NH(+)(4) level caused by PPT (DL-phosphinotricin) resulted in an immediate cessation of MA, glutamine and glutamate uptake. It seems that the intracellular NH(+)(4) concentration regulates its own influx by feedback-inhibition of the uptake system and probably also its efflux which becomes apparent when mycelium is grown on protein. Addition of extracellular NH(+)(4) did not inhibit glutamine uptake, suggesting that NH(+)(4) and glutamine are equally preferred nitrogen sources. The physiological importance of these uptake systems for the utilization of nitrogen compounds by A. bisporus is discussed.     

Spatio‐temporal variation of salt marsh seedling establishment in relation to the abiotic and biotic environment

Contrary to our expectations, soil salinity and moisture explained little of the spatial variation in plant establishment in the upper intertidal marsh of three southern California wetlands, but did explain the timing of germination. Seedlings of 27 species were identified in 1996 and 1997. The seedlings were abundant (maximum densities of 2143/m² in 1996 and 1819/m² in 1997) and predominantly annual species. CCAs quantified the spatial variation in seedling density that could be explained by three groups of predictor variables: (1) perennial plant cover, elevation and soil texture (16% of variation), (2) wetland identity (14% of variation) and (3) surface soil salinity and moisture (2% of variation). Increasing the spatial scale of analysis changed the variables that best predicted patterns of species densities. Timing of germination depended on surface soil salinity and, to a lesser extent, soil moisture. Germination occurred after salinity had dropped below a threshold or, in some cases, after moisture had increased above a critical level. Between 32% and 92% of the seedlings were exotic and most of these occurred at lower soil salinity than native species. However, Parapholis incurva and Mesembryanthemum nodiflorum were found in the same environments as the native species. In 1997, the year of a strong El Niño/Southern Oscillation event with high rainfall and sea levels, the elevation distribution of species narrowed and densities of P. incurva and other exotic species decreased but densities of native and rare species did not change. The ‘regeneration niche’ of wetland plant communities includes the effects of multiple abiotic and biotic factors on both the spatial and temporal variations in plant establishment.     

A human NK and K cell subset shares with cytotoxic T cells expression of the antigen recognized by antibody OKT8

The antigen recognized by monoclonal antibody OKT8 is expressed on the cell membrane of 30 to 50% of human NK/K cells. The reactivity of OKT8 with NK/K cells was determined by indirect methods (treatment of the effector cells with OKT8 antibody and complement (C) and separation of OKT8(+) and (-) effector cell populations by fluorescence-activated cell sorting or by rosetting techniques) and, at single cell level, by C-dependent lysis of effector NK cells that bind and kill K562 targets. Analysis by indirect immunofluorescence (flow cytofluorometry) of lymphocyte subpopulations mediating NK/K cytotoxic activity and deprived of OKT8(+) T cells reveals that the NK/K cell subset bears OKT8 antigen at a density lower than that present on cytotoxic T cells. The OKT8 antigen on NK/K cells is trypsin- and pronase-sensitive, but it is resynthesized by the same effector cells during 24 hr of culture at 37 degrees C. OKT8 antibody does not inhibit NK killing, and, on a per cell basis, OKT8(+) cells within the NK/K subset mediate the same level of cytotoxic activity as OKT8(-) NK/K cells. Analogous results were obtained by using anti-Leu-2a, an antibody with the same specificity as OKT8 on cytotoxic/suppressor T cells, but not when OKT5 was used, which might identify a distinct epitope on the same antigenic molecule. The possible significance of these findings in understanding the cell lineage of NK/K cells is discussed.     

Nucleoside exchange catalysed by the cytoplasmic 5''-nucleotidase.

The inhibition of the cytoplasmic 5'-nucleotidase (EC 3.1.3.5) by its product, inosine, was studied with a partially purified preparation of the enzyme from rat liver. Inhibition of Pi production was found to be due to exchange of the inosine moiety between inosine and IMP. Exchange was not catalysed by reversal of the hydrolytic reaction, suggesting, instead, the mediation of an enzyme-phosphate intermediate. Two models for the catalytic mechanism are proposed and rate equations for the dependence of Pi production on inosine concentration are derived. The experimentally determined dependence was consistent with a mechanism in which hydrolysis of the enzyme-phosphate intermediate occurred only when it was unoccupied by inosine. This conclusion suggests that inosine analogues that cannot participate in exchange should inhibit the enzyme. Such inhibitors might be useful in defining the enzyme's physiological role or as pharmacological agents to decrease breakdown of purine nucleotides. The possibility that nucleoside exchange provides an alternative route for the phosphorylation of mutagenic or cytotoxic nucleoside analogues should also be considered.     

Partial purification of the maturation-promoting factor MPF from unfertilized eggs of Xenopus laevis

A 200-fold purification of the maturation-promoting factor or MPF from unfertilized eggs of Xenopus laevis is reported for the first time. Purification was achieved by three successive column chromatographies on hydroxyapatite, trisacryl blue and L-arginine-agarose. The presence of MPF was assessed by the usual maturation criteria after injections of test material into immature stage VI unstimulated X. laevis oocytes: the precocious appearance of the maturation spot (within 45-120 min), the germinal vesicle breakdown, the presence of the first polar body and the second metaphase spindle. Purification was monitored by the decrease of the minimal amount of protein injected in a constant volume (50 nl) required to induce 50% frequency of germinal vesicle breakdown. This amount decreased from 500 ng in the crude extract to 2.5 ng in the 200-fold purified material. Analysis by SDS-PAGE of the crude extract showed about 40 Coomassie-blue-stained polypeptides with molecular masses ranging from 300 kDa to 20 kDa, whereas in the 200-fold purified MPF only 5 stained polypeptides were revealed, with molecular masses of 62, 53, 49, 39 and 37 kDa. In vitro phosphorylations for the detection of kinase activities for endogenous and exogenous substrates were monitored by analysis of autoradiograms of SDS-PAGE, after treatment of fractions with [gamma-32P]ATP. Only inactive fractions eluted from columns ahead of MPF, and fractions containing MPF activity were tested. Phosphorylation of numerous stained polypeptides was demonstrated in the crude MPF extract and exogenous substrates such as phosvitin, casein and histone type II-AS were also strongly phosphorylated. In the MPF fraction, purified on hydroxyapatite, a polypeptide of 53 kDa was more highly and specifically phosphorylated and the presence of kinase activities was observed for the above three exogenous substrates. In the 100-fold and 200-fold purified MPF, phosphorylation of endogenous substrates could not be shown and kinase activities for the above three substrates were drastically decreased as compared with the crude and purified MPF obtained after hydroxyapatite column chromatography. However, neither endogenous phosphorylations nor kinase activities with the above exogenous substrates could be shown in inactive fractions eluted ahead of MPF at the different purification steps. Some characteristics of the purified material are also described in this paper.     

Identification of N5-methyl-N5-formyl-2,5,6-triamino-4-hydroxypyrimidine as a major adduct in rat liver DNA after treatment with the carcinogens, N,N-dimethylnitrosamine or 1,2-dimethylhydrazine

Human Tamm-Horsfall urinary glycoprotein from an individual of the blood group Sd(a+) phenotype was tritium-labelled by treatment with galactose oxidase and sodium boro[3H]hydride and was then digested with endo-beta-galactosidase. A series of dialysable, labelled fragments was released from which a pentasaccharide was isolated that strongly inhibited the agglutination of Sd(a+) red cells by human anti-Sda serum and hence contained the Sda determinant structure. Reduction, methylation analysis and sequential exo-glycosidase digestion established the structure of the pentasaccharide as: GalNAc beta(1 leads to 4)[NeuAc(2 leads to 3)]Gal beta(1 leads to 4)GlcNAc beta(1 leads to 3)Gal