Threshold phenomena in chemoreception and taxis in slime mold Physarum polycephalum

The plasmodium of Physarum polycephalum reacts to various kinds of chemicals substances and moves towards or away from them. Threshold concentration of recognition of chemicals was examined in terms of membrane potential and of the averaged motive force of tactic movement by using a double-chamber method, i.e., a single plasmodium was placed between two compartments through a narrow ditch, and differences in membrane potential and in pressure between two compartments were measured. Results are summarized as follows: (a) By increasing the concentration of various substances in one compartment, the membrane potential started to change at a certain threshold concentration, C-th, for each chemical. Chemotactic movement of the plasmodium took place at the same threshold concentration. These results held both for attractants (glucose, galactose, phosphates, pyrophosphates, ATP, c-AMP, etc) and for repellents (various inorganic salts, sucrose, fructose, etc.). (b) The threshold concentration, Cth, for inorganic salts decreased remarkably with increase of the valences of cations, zeta, and was proportional to Z-6, I.E., THE Shultze-Hardy rule known in the field of colloid chemistry was found to be applicable. (c) The plasmodium distinguished the species of monovalent cations in the following order: H(Li(K(Na(Rb(Cs(NH-4 Plots of log Cth against the lyotropic number of anion fell on different straight lines for each monovalent cation species. (d) Plots of log Cth, against the reciprocal of the absolute tempe lines were almost the same and gave a value of 12 kcal/mol for the enthalpy change. These results suggest that the recognition of chemical substances appears as the result of a structural change of the membrane at the threshold point, and that the change in membrane structure is transmitted simultaneously to the motile system of the plasmodium.     

Effects of soil water conditions on the morphology, phenology, and photosynthesis of Betula ermanii in the boreal forest

Four 2-year watering treatments were applied to Betula ermanii seedlings to investigate their responses to soil water conditions: ID [irrigated well (I) in the previous year and water deficit (D) in the current year], DD, DI, and II. RGR of the seedlings in current-year water deficit without experiencing previous-year water deficit (ID) was smaller than that of seedlings irrigated well in both years (II). Surprisingly, RGR did not differ between 2-year-water-deficited (DD) and -well-irrigated (II) treatments. There was no difference in the area-based photosynthetic rate of the late leaves, which are formed in the current-year environment, between the four water treatments, but their leaf longevity was shortened by water deficit. Area-based photosynthetic rate of the early leaves, whose buds are formed in the previous year environment, was smaller in ID than in II, but it did not differ between DD and II. The de-epoxidation state of the xanthophyll cycle was greater in ID than in DD. Leaf mass per area (leaf dry weight/leaf area) did not differ among the four water treatments. Specific root length (root total length/root dry weight) did not differ between ID and II. It was greater in DD than in II, probably resulting in increased water uptake efficiency. Photoprotective systems such as xanthophyll contents and antioxidant enzyme activities did not differ among the four treatments. Morphological responses of B. ermanii roots seem to be important as whole-plant-level responses to water deficit for maintaining RGR, in addition to leaf-level photosynthetic and phenological responses of two types of leaves. Our results partly explain how B. ermanii seedlings survive and maintain growth even under varying soil water conditions in the boreal forest.     

Localization of the gene encoding the human L-glutamate transporter (GLT-1) to 11p11.2–p13 by fluorescence in situ hybridization

High affinity glutamate transporters regulate levels of extracellular glutamate in the central nervous system. Impaired glutamate transport has been implicated in the pathogenesis of amyotrophic lateral sclerosis (ALS). The glutamate transporter subtypes GluT-1 and EAAC1 have previously been mapped to human chromosomes 5p13 and 9p24, respectively. In the present study, the GLT-1 subtype was mapped to human chromosome 11p11.2–p13 by fluorescence in situ hybridization. The possible clinical implications of this finding are discussed.     

Effect of sugars on salt reception in true slime mold Physarum polycephalum. Physicochemical interpretation of interaction between salt and sugar receptions

Interaction between salt and sugar receptions in plasmodium of Physarum polycephalum was studied by using double-chamber method. Effect of sugars on salt reception was evaluated by measuring membrane potential and the motive force of tactic movement of the slime mold, where salt concentration in one compartment was increased successively with a fixed sugar concentration. Results are summarized as follows: (1) The presence of D-glucose, D-mannose, D-maltose, or sucrose in medium led to increase of the threshold concentration Cth, for salts (chlorides and nitrates of Li, Na, K), whereas D-ribose decreased the threshold for salt reception. D-galactose showed no appreciable effect on Cth of every salt species examined. No change in Cth for salt reception was observed until concentration of sugars exceeded their respective thresholds. (2) Double logarithmic plots of Cth for salts against sugar concentration followed different straight lines for different cations, whose slopes being closely correlated with the effects of lyotropic number of anions in the absence of sugars. (3) Plots of log Cth against the reciprocal of the absolute temperature, 1/T, gave linear relations, and the slopes of the straight line became small with increase of sugar concentration above their respective thresholds. Experimental results obtained here suggest that the structure of water at the interface of cell membrane plays an indispensable role in the interaction between salt and sugar receptions.     

A role for RAD54B in homologous recombination in human cells.

In human somatic cells, homologous recombination is a rare event. To facilitate the targeted modification of the genome for research and gene therapy applications, efforts should be directed toward understanding the molecular mechanisms of homologous recombination in human cells. Although human genes homologous to members of the RAD52 epistasis group in yeast have been identified, no genes have been demonstrated to play a role in homologous recombination in human cells. Here, we report that RAD54B plays a critical role in targeted integration in human cells. Inactivation of RAD54B in a colon cancer cell line resulted in severe reduction of targeted integration frequency. Sensitivity to DNA-damaging agents and sister-chromatid exchange were not affected in RAD54B-deficient cells. Parts of these phenotypes were similar to those of Saccharomyces cerevisiae tid1/rdh54 mutants, suggesting that RAD54B may be a human homolog of TID1/RDH54. In yeast, TID1/RDH54 acts in the recombinational repair pathway via roles partially overlapping those of RAD54. Our findings provide the first genetic evidence that the mitotic recombination pathway is functionally conserved from yeast to humans.