Single-strand nuclease action on heat-denatured spermiogenic chromatin.

The aim of this study was to compare the sensitivity of chromatin from representative cellular stages of spermiogenesis to a single-strandeded nuclease after heat denaturation. Thermal denaturation of chromatin was assayed in situ in fixed round, elongating and elongated spermatids and in testicular sperm from mice. Production of single-stranded deoxyribonucleic acid (DNA) at elevated temperatures was monitored by digesting chromatin with endonuclease specific for single-stranded DNA (S1 nuclease), staining the residual DNA with gallocyanin-chrome alum (GAC) and measuring the stain content by absorption cytophotometry. Changes in GCA staining were minimal over the temperature range of 22-90 degrees C in each cell type not exposed to nuclease. Staining of undigested cells decreased progressively with advancing cell maturity. Nuclease had no effect on the GCA content of round spermatids below 60 degrees C, but above this temperature there was a progressive decrease in GCA-stainable chromatin. Both round and elongating spermatid stages showed a significantly greater sensitivity to nuclease digestion than did more mature stages; sperm showed no effects of nuclease action below 80 degrees C. Progressive chromatin condensation and a concomitant decrease in the number of available DNA phosphate groups during spermiogenic cell maturation may be responsible for the observed decline in sensitivity to nuclease and decreased GCA staining. Thermal denaturation of round spermatids labeled with 3H-thymidine produced no change in autoradiographic mean nuclear grain counts, indicating no loss of thymidine-labeled DNA from the slides during denaturation. When round spermatids and sperm were hydrolyzed with hot tricholoroacetic acid before staining, both nuclear GCA content and autoradiograph grain count were partially reduced, indicating incomplete DNA removal. Almost complete loss of Feulgen-stainable material occurred in these cells and may be due to depurination and elimination of Feulgren-reactant aldehyde groups.     

Kinetic and thermodynamic principles determining the structural design of ATP-producing systems

It is theoretically analysed whether the structural design of ATP-producing pathways, in particular the design of glycolysis, may be explained by optimization principles. On the basis of kinetic and thermodynamic principles conclusions are derived concerning the stoichiometry of these pathways in states of high ATP production rates. One of the extensions to previous investigations is that the concentrations of the adenine nucleotides are taken into account as variable quantities. This necessitates the consideration of an interaction of the ATP-producing system I with an external ATP-consuming system II. A great variety of pathways is studied which differ in the number and location of ATP-consuming reactions, ATP-producing reactions and reactions involving inorganic phosphate. The corresponding number of possible pathways may be calculated in an explicit manner as a function of the number of those reactions which do not couple to ATP or inorganic phosphate. The kinetics of the individual reactions are described by linear or bilinear functions of reactant concentrations and all rate equations are expressed in terms of equilibrium constants and characteristic times. A thermodynamical analysis of the two coupled systems yields upper and lower limits for the concentration of ATP and an explicit expression for the maximal difference between the number of ATP-producing and ATP-consuming reactions of system I. The following results of the optimization are obtained. (i) The ATP production rate always increases if the ATP-producing reactions as well as those reactions characterized by an uptake of inorganic phosphate are shifted as far as possible towards the end of system I. (ii) Explicit conditions for the optimal location of the ATP-consuming reactions are presented. The results are discussed in the context of characteristic times as well as in terms of enzyme kinetic parameters. (iii) For two sets of characteristic times the resulting stoichiometries and their corresponding steady-state fluxes are investigated in detail. One of these stoichiometries shows a close correspondence to contemporary standard glycolysis. (iv) It is shown that most possible pathways result in a very low steady-state flux, that is, the optimal stoichiometry is characterized by a significant selective advantage. (v) The standard free energy profile of a pathway with an optimal stoichiometry is discussed. It differs significantly from the free energy profiles of nonoptimized pathways.     

Changes in the T-wave vector loop of the three-dimensional vectorcardiogram during exposure to cold pressor stress

Three-dimensional vectorcardiography was used to characterize changes in the T-wave vector loop during exposure to cold pressor stress. Data were collected from 8 subjects during baseline, cold pressor, and recovery periods. Maximum vector length, polar angle of the longest vector, azimuth angle of the longest vector, sum of all vectors, polar angle of the vector sum, azimuth angle of the vector sum, surface area of the loop, and maximum distance between adjacent vectors were computed from the T-wave vector loop. The amplitude of the one-dimensional T-wave (TWA) was also computed. Interbeat interval and TWA decreased during hand immersion and increased during recovery. Of the vectorcardiographic measures, only maximum vector length and vector sum showed statistically significant change; both decreased during the task and returned to baseline during recovery. TWA accounted for 46% of the task related variance compared to 53% and 47% respectively, for vector sum and maximum vector length. Changes in TWA probably reflect a true change in cardiac electrical potential rather than a change in T-wave vector loop shape or orientation.     

Ultraviolet radiation, toxic chemicals and amphibian population declines

Abstract. As part of an overall ‘biodiversity crisis’, many amphibian populations are in decline throughout the world. Numerous factors have contributed to these declines, including habitat destruction, pathogens, increasing ultraviolet (UV) radiation, introduced non‐native species and contaminants. In this paper we review the contribution of increasing UV radiation and environmental contamination to the global decline of amphibian populations. Both UV radiation and environmental contaminants can affect amphibians at all life stages. Exposure to UV radiation and to certain contaminants can kill amphibians and induce sublethal affects in embryos, larvae and adults. Moreover, UV radiation and contaminants may interact with one another synergistically. Synergistic interactions of UV radiation with contaminants can enhance the detrimental effects of the contaminant and UV radiation.     

A Drosophila Model of Intellectual Disability Caused by Mutations in the Histone Demethylase KDM5

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Seasonal declines in reproductive success of the common tern Sterna hirundo: timing or parental quality?

Reproductive success declines over the course of the breeding season in many bird species. Two categories of hypothesis have been evoked to explain this decline. The “timing” hypothesis suggests that seasonal declines in breeding success are attributable to the date of laying. The “parental quality” hypothesis suggests that seasonal declines result from the fact that young, inexperienced, or low quality birds breed later in the season. To evaluate the relative importance of timing and parental quality, egg exchanges and removals were used to manipulate hatching dates of common terns Sterna hirundo. Indices of quality, attendance, provisioning rates, and reproductive success of birds in three experimental groups (delayed hatch pairs, advanced hatch pairs, and pairs induced to relay) were compared to those of date‐matched controls. Pairs that hatched chicks early raised more chicks than pairs hatching chicks late in the season, regardless of initial laying date. This suggests that hatching chicks early is advantageous in itself. Our results, however, also support the parental quality hypothesis. There was a significant negative relationship between natural laying date and fledging success, independent of hatching date. Differences in chick growth and survival suggest that higher quality adults may be able to compensate for the disadvantages of late hatching dates and achieve similar reproductive success to that of pairs hatching chicks early. We found that pairs hatching chicks late in the season were subject to more incidents of kleptoparasitism than those hatching chicks early. This may be a proximate factor contributing to seasonal declines in reproductive success for common terns, although such a mechanism would not be likely in non‐colonial species. Failure to control for egg quality may have biased the results of some prior egg exchange experiments. Additionally, altered cost of incubation may be an unavoidable confounding factor in studies designed to manipulate timing of breeding.     

Separation and properties of leaf aspartate aminotransferase and alanine aminotransferase isoenzymes operative in the C4 pathway of photosynthesis

Isoenzymes of aspartate and alanine aminotransferases believed to have a specific role in C₄-photosynthesis in Atriplex spongiosa leaves have been separated and their properties examined. The identity of isoenzymes separated by ammonium sulfate fractionation and DEAE-cellulose chromatography was established by comparing mobilities of these fractions on acrylamide gels with the bands in tissue and cell extracts. Consistent with earlier findings, both aspartate aminotransferase and alanine aminotransferase activities in leaves were separable into two major isoenzyme species. One of the two alanine aminotransferase isoenzymes lost all activity during chromatography on DEAE-cellulose but this was restored by incubating with pyridoxal phosphate. The Michaelis constants, maximum velocities, and pH optima for both the forward and reverse directions of the reactions catalysed by each isoenzyme were determined. The relationship between the physical and kinetic characteristics of these enzymes and their intracellular location and possible role in photosynthesis was considered.