Differential Affinity and Catalytic Activity of CheZ in E. coli Chemotaxis

Push–pull networks, in which two antagonistic enzymes control the activity of a messenger protein, are ubiquitous in signal transduction pathways. A classical example is the chemotaxis system of the bacterium Escherichia coli, in which the kinase CheA and the phosphatase CheZ regulate the phosphorylation level of the messenger protein CheY. Recent experiments suggest that both the kinase and the phosphatase are localized at the receptor cluster, and Vaknin and Berg recently demonstrated that the spatial distribution of the phosphatase can markedly affect the dose–response curves. We argue, using mathematical modeling, that the canonical model of the chemotaxis network cannot explain the experimental observations of Vaknin and Berg. We present a new model, in which a small fraction of the phosphatase is localized at the receptor cluster, while the remainder freely diffuses in the cytoplasm; moreover, the phosphatase at the cluster has a higher binding affinity for the messenger protein and a higher catalytic activity than the phosphatase in the cytoplasm. This model is consistent with a large body of experimental data and can explain many of the experimental observations of Vaknin and Berg. More generally, the combination of differential affinity and catalytic activity provides a generic mechanism for amplifying signals that could be exploited in other two-component signaling systems. If this model is correct, then a number of recent modeling studies, which aim to explain the chemotactic gain in terms of the activity of the receptor cluster, should be reconsidered.     

The effect of potassium on growth of Carex species from swamp habitats varying from oligotrophic to eutrophic and comparison of physiological reactions of Carex species to P stress and K stress

In a growth experiment at potassium levels varying between 0.001 m M and 3.0 m M potassium, relative growth rate (RGR) and other growth parameters were determined in Carex species: C. rostrata Stokes, C. limosa L., C. lasiocarpa Ehrh., C. diandra Schrank and C. acutiformis Ehrh., listed in order of increasing nutrient availability of their habitats. Carex species of nutrient poor sites did grow faster at low potassium concentration than species from nutrient rich habitats. The RGR of C. limosa was not affected by the K concentration, even at the lowest potassium concentration (0.001) m M ) used. At high potassium availability Carex species from nutrient-rich sites responded with greatly increased RGR, whereas the Carex from nutrient-poor sites absorbed potassium in excess of immediate growth requirements: luxury consumption. A comparison is made of the physiology of the Carex species as affected by stress and abundance of phosphate and potassium.     

Freezing injury in spinach leaf tissue: Effects on water-soluble proteins, protein-sulfhydryl and water-soluble non-protein-sulfhydryl groups

Freezing of spinach leaf discs ( Spinacia aleracea L. cv. Estivato) resulted in an irreversible and parallel loss of protein-sulfhydryl (SH) and water-soluble protein. This decrease was inversely related to the increase in freezing injury as determined by the loss of electrolytes from the tissue after thawing. Loss of proteins and protein-SH occurred during freezing of the tissue and was not enhanced by thawing. The parallel decreases in content of soluble proteins and SH groups make it impossible to determine whether oxidation of protein-SH groups is the primary step in decline of protein content. During freezing the content of non-protein-SH compounds, mainly glutathione (GSH), was decreased to a lesser extent than that of protein-SH. Contrary to protein-SH, the levels of non-protein-SH declined substantially after thawing. The data indicate that GSH is not directly involved in protection of soluble proteins against freezing-induced denaturation.     

tRNA2Lys gene clusters in Drosophila

We have isolated segments of Drosophila melanogaster DNA that contain two clusters of tRNA₂^Lys genes. In one segment, pPW511, there is a cluster of three of these genes surrounded by other tRNA genes. Two other segments, pPW516 and pPW541. share a 3 × 10³ base-pair region that has a cluster of four tRNA₂^Lys genes. This cluster is flanked by 20 × 10³ base-pairs of DNA that does not appear to have other tRNA genes. The tRNA genes in both clusters are irregularly spaced and are intermingled with moderately repetitive DNA. Each cluster is present once or perhaps twice in the haploid genome and has the same arrangement of restriction endonuclease sites in the genomic DNA as in the isolated, cloned DNA. In situ hybridization to polytene chromosomes localized the pPW511 cluster to the 42A region and the pPW516/541 cluster to the 42E region. Another region, 50B, also contains tRNA₂^Lys genes. In sum, these cloned tRNA₂^Lys genes account for most of this gene family and are irregularly spaced in two clusters.     

A novel method for the preparation of substrate-free cytochrome P-450(11) beta

A new method for the removal of the stabilizing substrate, deoxycorticosterone, from adrenal cytochrome P-450(11) beta, has been developed. Dextran coated charcoal is used for the adsorption of the steroid and the adsorbed steroid is separated from the cytochrome P-450-preparation by low speed centrifugation. The substrate-free enzyme, obtained in this manner, has all the characteristic spectral properties of low-spin cytochrome P-450(11) beta and may be converted to the high-spin form by the addition of deoxycorticosterone. The dextran coated charcoal method has the following advantages over the previously used method of substrate removal. It does not require the addition of the cofactors for cytochrome P-450-dependant hydroxylation of deoxycorticosterone, small amounts of enzyme may be prepared in a short time and the enzyme preparation is not diluted to any great extent during the process.     

Investigating movement in the laboratory: dispersal apparatus designs and the red flour beetle,Tribolium castaneum

The natural dispersal of Tribolium castaneum Herbst (Coleoptera: Tenebrionidae) has been emulated in the laboratory for more than 50 years, using a simple dispersal apparatus. This has typically comprised of a starting container (initial resource or patch) connected by tubing, which contains thread for the animals to climb into a tube and hence to an end container. That is, beetles move to a new viable resource or patch from an inter‐patch zone or non‐viable habitat. We modified this basic apparatus design to test the effect of tubing length and tubing insertion angle on the dispersal rate and proportion of successful dispersers. We expected that the proportion of successful dispersers would be repeatable within each apparatus design, and that increasing tubing length and steepness of the insertion angle would reduce dispersal rate and success across apparatus designs. Dispersal increased linearly through time, similarly so for both males and females. The design with the most vertical tubing insertion angle had a lower proportion of successful dispersers. Tubing length also had a negative relationship with dispersal success (as judged by insects reaching the end container), but a significant reduction in dispersal success was only apparent between the shortest and longest tubing between containers. We suggest that locating and climbing the vertical section of string before they can enter the tubing between containers restricts dispersal and that at higher densities, insects exhibit greater inclination to climb. This type of apparatus has flexible design tolerances and further potential to study the dispersal of other small insect species that primarily use pedestrian locomotion.     

Temperature effects on forest understorey plants in hedgerows: a combined warming and transplant experiment

Background and AimsHedgerows have been shown to improve forest connectivity, leading to an increased probability of species tracking the shifting bioclimatic envelopes. However, it is still unknown how species in hedgerows respond to temperature changes, and whether effects differ compared with those in nearby forests. We aimed to elucidate how ongoing changes in the climate system will affect the efficiency of hedgerows in supporting forest plant persistence and migration in agricultural landscapes.MethodsHere we report results from the first warming experiment in hedgerows. We combined reciprocal transplantation of plants along an 860-km latitudinal transect with experimental warming to assess the effects of temperature on vegetative growth and reproduction of two common forest herbs (Anemone nemorosa and Geum urbanum) in hedgerows versus forests.Key ResultsBoth species grew taller and produced more biomass in forests than in hedgerows, most likely due to higher competition with ruderals and graminoids in hedgerows. Adult plant performance of both species generally benefitted from experimental warming, despite lower survival of A. nemorosa in heated plots. Transplantation affected the species differently: A. nemorosa plants grew taller, produced more biomass and showed higher survival when transplanted at their home site, indicating local adaptation, while individuals of G. urbanum showed greater height, biomass, reproductive output and survival when transplanted northwards, likely owing to the higher light availability associated with increasing photoperiod during the growing season.ConclusionsThese findings demonstrate that some forest herbs can show phenotypic plasticity to warming temperatures, potentially increasing their ability to benefit from hedgerows as ecological corridors. Our study thus provides novel insights into the impacts of climate change on understorey plant community dynamics in hedgerows, and how rising temperature can influence the efficiency of these corridors to assist forest species’ persistence and colonization within and beyond their current distribution range.