Study of glucose starvation in excised maize root tips

Excised maize (Zea mays) root tips were used to follow the effects of a prolonged glucose starvation. Respiration rate began to decrease immediately after excision, reaching 30 to 40% of its initial value after 20 hours, and then declined more slowly until death of the tissues, which occurred after 200 hours of starvation. During the whole process, respiration could be uncoupled by 2,4-dinitrophenol and the energy charge remained high. These results suggest that in excised maize root tips, respiration rate is essentially limited by the rate of biosyntheses (ATP-utilizing processes) rather than mitochondrial number. During starvation the sugar content sharply decreased for the first 20 hours and reached zero at 120 hours. Following root excision, proteins and lipids were continuously degraded and were virtually the only substrates for respiration and biosyntheses after 20 hours of starvation. Over the first 90 hours of starvation, enzymic activities related to sugar metabolic pathways and the Krebs cycle decreased to 20% or less of their initial activity. Starvation was reversible only for the first 80 to 90 hours. Between 80 and 100 hours, there was a sharp fall in intracellular osmolarity and a 25% loss in the dry weight. The irreversibility may be due, as in senescence, to a change in membrane selective permeability.     

The trophic-dynamic aspect of ecology

Recent progress in the study of aquatic food-cycle relationships invites a reappraisal of certain ecological tenets. Quantitative productivity data provide a basis for enunciating certain trophic principles, which, when applied to a series of successional stages, shed new light on the dynamics of ecological succession. From “The trophic-dynamic aspect of ecology” by R. L. Lindeman.Ecology, Vol. 23, pp. 399–418 (1942).     

Metabolic regulation of rice α-amylase and sucrose synthase genes in planta

Isolated rice embryos were used to investigate the regulatory effects of endosperm extracts and pure sugars on the expression of alpha-amylase gene RAmy3D and a sucrose synthase gene homologous to the maize isozyme Ss2. The high-level expression of RAmy3D in the scutella of isolated embryos could be inhibited by a variety of sugars as well as endosperm extracts from germinated rice grains. Glucose, at a concentration of 250 mM, was most effective in repressing RAmy3D mRNA accumulation. Furthermore, this repression was reversible. Interestingly, RAmy3D repression was always accompanied by the induction of sucrose synthase gene expression. These results support a model in which the expression of alpha-amylase and sucrose synthase genes in the rice scutellum are counter-regulated by the influx of sugars from the endosperm.     

The cellular and molecular regulation of 1,25(OH)2D3 production

The synthesis of 1,25(OH)₂D₃ is a critical control point in the regulation of calcium metabolism, and possibly in the growth and differentiation of a number of cell types. This paper reviews our current understanding of the regulation of this process at the cellular and molecular levels, with the emphasis on the mechanisms of feedback control 1,25(OH)₂D₃ itself, control of parathyroid hormone, the roles of cyclic AMP dependent protein kinase and protein kinase C, and the interaction between the various intracellular regulators of 1,25(OH)₂D₃ production.     

Binding of spin-labeled clofibrate to lipoproteins

The binding of spin-labeled clofibrate to native and partially delipidated lipoproteins is a rapid, linear and non-saturable process observed up to the critical micellar concentration of the drug. Low-density lipoproteins (LDL) display a lower affinity for the drug than very-low-density lipoproteins (VLDL) and high-density lipoproteins (HDL) relative to their respective specific volume. Unlike various lipophilic drugs, uptake of spin-labeled clofibrate does not correlate with lipoprotein lipid volume. Spin-labeled clofibrate binding to LDL is enhanced when the temperature increases above 25 degrees C. The binding to HDL and VLDL is less temperature-sensitive. The simulation of the ESR spectra has shown that two types of motion should be superimposed for the spin-labeled clofibrate in HDL, in LDL or in partially delipidated LDL. From 40 down to 25 degrees C for HDL and LDL, a fast anisotropic motion is observed. From 25 degrees C down to 5 degrees C, a two-component motion takes place, including a slow isotropic motion of the probe tumbling in a highly hydrophobic environment. Interactions of spin-labeled clofibrate with the apolipoproteins in HDL and LDL are assumed from the emergence of this strongly immobilized component observed when the temperature decreases. In contrast, for spin-labeled clofibrate inserted in the apolar core of VLDL, ESR shows only one component in the whole temperature range (5-40 degrees C). The location of the spin-labeled drug inside the various lipoprotein particles is discussed as a function of temperature.     

New and noteworthy plants from Great Barrier Reef sand cays,Australia

Observations are presented on new and critical plants from the northern sand cays on the Great Barrier Reef, Queensland, Australia, based mainly on recent collections made by David R. Stoddart and Ralf Buckley. New species and varieties are described:Lepturus stoddartii (Poaceae),Boerhavia fistulosa var.fistulosa and var.puberuliflora (Nyctaginaceae),Boerhavia albiflora var.heronensis, Spermacoce everistiana (Rubiaceae), andSpermacoce buckleyi; a new combination is made:Diospyros ferrea var.compacta (R. Br.) (Ebenaceae); and additional taxonomic notes are given onBoerhavia, Euphorbia (Euphorbiaceae), andAbutilon (Malvaceae).     

Characterization of kinetochores in multicentric chromosomes

Long-term cultures of certain rat and mouse cell lines carry several dicentric and some multicentric chromosomes. Using antikinetochore antibodies obtainable from serum of scleroderma (var. CREST) patients we studied the number of kinetochores formed along the length of these chromosomes. The rat cells displayed as many kinetochores as there were centromeres. However, mouse cells showed the synthesis of only one kinetochore in dicentric and multicentric chromosomes which had been in the culture for a period of 1 year or more. When translocations were induced by bleomycin in mouse L cells, the newly formed dicentric chromosomes showed the formation of two kinetochores. It is not known when the accessory centromeres lose their capacity to assemble kinetochore proteins. Possibly, in the rat the latent kinetochores lack a specific component which renders them ineffective for microtubule binding. The reason for the formation of only one kinetochore in mouse multicentric chromosomes is not clear. It may be due to the accumulation of mutations, modification of the kinetochore protein so that it lacks the antibody binding component, or a more effective regulatory gene than in the rat.     

Scale-down and optimization studies of the gluconic acid fermentation by Gluconobacter oxydans

To simulate production-scale conditions of gluconic acid fermentation by Gluconobacter oxydans, different experimental setups are presented in this study. From the determination of the time constants of a production-scale reactor, it can be concluded that mixing and oxygen transfer are the rate-limiting mechanisms. This results in oxygen concentration gradients which were simulated in a one-compartment reactor in which the oxygen concentration was fluctuated by a fluctuated gassing with air and nitrogen. It could be concluded that only very long periods of absence of oxygen (ca. 180 s) results in lower specific oxygen uptake rates by Gluconobacter oxydans. From scale-down studies carried out in a two-compartment system to simulate a production-scale reactor more accurately, it could be concluded that not only the residence time in the aerated part of the system is important, but the liquid flow in between the different parts of the reactor is also an essential parameter. It could also be concluded that the microorganisms are not influenced negatively by the fluctuated oxygen concentrations with respect to their maximal oxidation capacity. The two-compartment system can also be used for optimization experiments in which the "aerated" compartment was gassed with pure oxygen. From these experiments it was concluded that also a short residence of the cells at high oxygen concentrations diminished the growth and product formation rates. These experiments show the necessity of the scale-down experiments if optimization is carried out. The two-compartment system presented in this study is a very attractive tool for reliable scale-down experiments.