The incorporation of deoxyuridine monophosphate into DNA increases the sister-chromatid exchange yield

The effect of a treatment with 5-fluoro-2'-deoxyuridine (FdUrd) in combination with 2'-deoxyuridine (dUrd) on cell proliferation, incorporation of DNA precursors into DNA and sister-chromatid exchanges (SCEs) has been analyzed in Allium cepa meristem cells. FdUrd in the range 10(-9)-5 X 10(-7) M produced a dose- and time-dependent decrease in the amount of cells in mitosis. This inhibitory effect could be reversed by 70-80% in short-term (6 h) experiments, by exogenously supplied dUrd at a concentration of 10(-4) M. However, at the highest FdUrd dose tested (10(-7) M), 10(-4) M dUrd could not reverse the FdUrd effect in long-term experiments (20 h, about one cell cycle interval), as shown by analyzing the kinetics of synchronous cell populations. DNA extracted from cells pulsed with [6-3H]dUrd in the presence of FdUrd and 6-amino-uracil (6-AU), an inhibitor of uracil-DNA glycosylase, contained a small amount of label (at least 3% of the total radioactivity incorporated into DNA) in the form of [6-3H]dUMP. Thus, we conclude that, under our experimental conditions, exogenously supplied dUrd may be metabolized intracellularly to 2'-deoxyuridine triphosphate (dUTP) and that this deoxynucleotide may eventually be mis-incorporated into DNA. As far as the formation of SCEs is concerned, analysis of second division chromosomes showed that 2'-deoxyuridine monophosphate (dUMP) residues present in newly-synthesized DNA strands are probably not relevant to SCE formation. However, by analyzing SCE levels in third division chromosomes of cells treated with FdUrd and dUrd during their second cycle, we have scored a 6-fold increase in the reciprocal SCE level which demonstrates that the replication of a dUMP-containing DNA template leads to a higher SCE yield.     

Chromatin structure. Further evidence against the existence of a beaded subunit for the 30-nm fiber

The size distribution of chromatin fragments released by micrococcal nuclease digestion of liver chromatin at various ionic strengths was examined. Below 20 mM ionic strength, gradient profiles with a peak centered at 6 nucleosomes are generated, whereas between 20 and 50 mM the peak is always centered on 12 nucleosomes, and above 50 mM ionic strength the 30-nm fiber becomes less accessible to the nuclease and there is a corresponding increase in the size distribution of fragments in the gradients. However, extensive digestions always give profiles with a peak of 12 nucleosomes as nuclease-resistant dodecamers accumulate. All of these observations are consistent with the winding of the 10-nm polynucleosome chain into a helical coil commencing at about 20 mM ionic strength. The helical turns are stabilized by histone H1 interactions between 20 and 50 mM ionic strength producing stable dodecamers. Above 50 mM ionic strength the coil condenses longitudinally and the profiles are consistent with a random attack of this fiber by the nuclease. Consequently it is not necessary to invoke the existence of a subunit bead to explain the profiles. We further define the conditions at which specific structural transitions take place and provide methodology for the preparation of chromatin at various levels of condensation.     

Chromatin structure. Evidence that the 30-nm fiber is a helical coil with 12 nucleosomes/turn

Sedimentation analysis has been used to compare the structure of 30-nm chromatin fibers, isolated and digested under conditions that maintain the native structure, with relaxed-refolded chromatin. The native chromatin fibers show sharp, ionic strength-dependent changes in sedimentation coefficient that are not apparent in relaxed-refolded fibers. The first transition at approximately 20 mM ionic strength reflects the organization of the 10-nm polynucleosome chain into a loose helically coiled 30-nm fiber. Between 20 and 60 mM ionic strength there is considerable interaction between nucleosomes within the coils to generate a stable helical array with 12 nucleosomes/turn. Above 60 mM ionic strength the helical coil continues to condense until it precipitates at ionic strengths slightly greater than those considered physiological, indicating that there is no end point in fiber formation. The data is incompatible with a solenoid model with 6 nucleosomes/turn and also rules out the existence of a beaded subunit structure.     

Isolation and characterization of phage F9-11 from a lysogenicDeleya halophila strain

Thirty-eight strains ofDeleya halophila species were examined for production of phage after mitomycin C induction. Thirty-two of them were able to inhibit growth of some other strains. Phage F9-11, isolated fromD. halophila strain F9-11, showed an isometric head and a noncontractile tail. The effects of salt concentrations variation on the stability and replication of this phage were established. Its replication was possible at a wide range of marine salt concentrations, from 2.5% to 15% (wt/vol). Stability seems to be influenced by osmolarity of medium rather than by NaCl level. The euryhaline character showed by F9-11 phage is evoked as an important factor for the survival of this phage in its environment.     

Successional pathways of Mediterranean evergreen vegetation on Sicily

The Mediterranean evergreen vegetation of Sicily, comprised in the belt of the Quercetea ilicis, occupies a large part of the island. Human intervention (cutting, fire, pasture) has brought about a degradation of the natural vegetation. This study is based on our phytosociological research of the Quercetea ilicis belt on Sicily.

With the ‘habitat comparison’ method, the dynamical relations between the different vegetation units have been defined.

We distinguish the following stages, with reference to their vegetation structure:

• a herbaceous stage formed by steppic vegetation, preceded by various types of nitrophilous-ruderal vegetation on abandoned fields;

• a garrigue stage dominated by half-shrubs;

• a macquis stage with various distinct plant communities, four communities being important in regressive successions, and three in progressive ones;

• a woodland and shrub-woodland stage with three different substages: pre-existent forests, present woodlands, and woodlands which tend towards the final, stable stage of vegetation (potential natural vegetation).

The dynamic relationships both in progressive and regressive successions have been synthesized in a scheme. In this scheme we have shown the main stages of the vegetation in their dynamics and we have constructed different series of vegetation types in two altitudinal belts, which are determined by varying environmental conditions of today.

The results also show that in some cases the progressive series follow different pathways than the regressive series, and the final stage of the progressive series is different from the original vegetation.

     

Autolysis of cell walls in pea epicotyls during growth. Enzymatic activities involved

Pisum sativum L. (cv. Lincoln) epicotyl cell walls show autohydrolysis and release into the incubation medium up to 120 μg of sugar per mg of cell wall dry weight in 30 h. Cell walls from younger epicotyls with high growth capacity showed higher auto-lytic capacity than older epicotyls. This suggests that both processes, growth and au-tolysis, are related. The proteins responsible for autolysis were extracted from the wall fraction with high saline solution (3 M LiCl) and enzymatic activities associated with the proteins were studied. The highest activity corresponded to α-galactosidase; lower activities were found for β-galactosidase, a-arabinosidase and exoglucanase. Changes in enzymatic activities and changes in the proportion of sugars released in autolysis by cell walls during the growth of epicotyls support the notion that α-galac-tosidase is one of the enzymes involved in the process of autolysis, and that the liberation of arabinose and galactose in this process occurs as arabinogalactan.     

Some cytochemical studies on the cell surface ofAmphidinium carterae (Dinophyceae)

Summary The surface coat of the dinoflagellateAmphidinium carterae Hulburt was examined by fluorescence and transmission electron microscopy, using various fluorochromes and cationic dyes. The overall results showed cell-surface reactions typical of acid mucopolysaccharides. The cationic dye staining revealed an outer fine fibrillar layer (15–70 nm thick) overlying a dense anionic coat (40–60 nm thick) which appeared to thicken progressively with age. In general, the structure of the amphiesmal vesicles was similar to that previously described by other investigators. However, an acidic mucopolysaccharide layer was observed on the inner surface of these vesicles. Each of these structures is traversed by 1–3 pores and at least 2 types of extrusomes are formed, the spindle trichocysts and the mucocysts. Cell to cell adhesion through the surface coat was frequently observed. Evidence was also obtained for internalization of all the surface-coat markers used.This investigation forms part of a doctoral thesis submitted by the first author to the University of British Columbia, Vancouver, B.C.     

Mechanism of cadmium uptake by activated sludge

Summary The significance of metabolic activity in cadmium uptake by unacclimated activated sludge was studied. Below 30 mg/l cadmium in solution, biosorption was found to follow the Freundlich isotherm, which is the most common pattern for physico-chemical adsorption. More than 95% of total cadmium uptake was achieved within 5 min metal-sludge contact time. Biosorption increased strongly when the initial cadmium concentration in solution was raised from 10 to 100 mg/l, whereas in the same concentration range the metabolic activity of the sludge, as measured by respiratory activity and extracellular protein production, was very significantly inhibited. The addition of nutrients at low but significant levels failed to increase cadmium uptake in 2 h contact time, while in 24 h the addition of nutrients caused the biosorption to increase by only 5–10% without any significant growth of the biomass. Biosorption was found to increase with temperature between 5° C and 40° C, in correlation with an increase in the metabolic activity of the sludge. Pretreatment of the sludge with metabolic inhibitors (NaN₃ and UV rays) appeared to cause only a very slight decrease (5–10%) of biosorption. These results suggest that metabolic uptake of cadmium was low and that adsorption to the surface of the cells was the major mechanism of uptake.Offprint requests to: S. S. Sofer