Refined crystal structure of an octanucleotide duplex with I.T. mismatched base pairs.

The structure of the synthetic deoxyoctamer d(GGIGCTCC) has been determined by single crystal X-ray diffraction techniques to a resolution of 1.7A. The sequence crystallises in space group P6(1), with unit cell dimensions a = b = 45.07, c = 45.49A. The refinement converged with a crystallographic residual R = 0.14 and the location of 81 solvent molecules. The octamer forms an A-DNA duplex with 6 Watson-Crick (G.C) base pairs and 2 inosine-thymine (I.T) pairs. Refinement of the structure shows it to be essentially isomorphous with that reported for d(GGGGCTCC) with the mispairs adopting a "wobble" conformation. Conformational parameters and base stacking interactions are compared to those for the native duplex d(GGGGCCCC) and other similar sequences. A rationale for the apparent increased crystal packing efficiency and lattice stability of the I.T octamer is given.     

Cascading effects of artificial light at night: resource-mediated control of herbivores in a grassland ecosystem

Artificial light at night has a wide range of biological effects on both plants and animals. Here, we review mechanisms by which artificial light at night may restructure ecological communities by modifying the interactions between species. Such mechanisms may be top-down (predator, parasite or grazer controlled), bottom-up (resource-controlled) or involve non-trophic processes, such as pollination, seed dispersal or competition. We present results from an experiment investigating both top-down and bottom-up effects of artificial light at night on the population density of pea aphids Acyrthosiphon pisum in a diverse artificial grassland community in the presence and absence of predators and under low-level light of different spectral composition. We found no evidence for top-down control of A. pisum in this system, but did find evidence for bottom-up effects mediated through the impact of light on flower head density in a leguminous food plant. These results suggest that physiological effects of light on a plant species within a diverse plant community can have detectable demographic effects on a specialist herbivore.     

Rapid increase of NO release in plant cell cultures induced by cytokinin.

4,5-Diaminofluorescein, a fluorescence indicator for NO, was applied to detect the release of NO from plant cells. NO production was increased within 3 min when plant cell cultures (Arabidopsis, parsley, and tobacco) were treated by cytokinin and was dose-dependent and signal-specific in that other plant hormones and inactive cytokinin analog were not effective in stimulating of NO release. The response was quenched by addition of 2-(aminoethyl)-2-thiopseudourea, an inhibitor of the animal NO synthase, and by addition of an NO scavenger, 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-1-oxy-3-oxide. These results imply that NO may act in cytokinin signal transduction.     

Constraints for joint angle control of the human arm

The targeting movements of a human arm were examined when restricted to a horizontal plane. The three joints at shoulder, elbow, and wrist are allowed to move. Thus, the system is redundant and needs constraints. A model calculation using a simple form of constraint is found to describe the experimental results: a cost function is applied to each joint. The constraint consists in minimizing the sum of the costs of all three joints. The cost functions might be interpreted as to describing the energy cost necessary to move the joint and/or represent a mechanism which avoids singularities.     

The influence of load and leg amputation upon coordination in walking crustaceans: A model calculation

The following results were obtained by earlier authors when investigating the leg coordination of walking crustaceans (Decepoda): 1) After a leg is amputated, its stump moves in anti-phase with the next posterior intact leg. This corresponds to the coordination of intact animals. The stump, however, moves in-phase with the next anterior intact leg which contrasts with the coordination of intact animals (Clarac and Chasserat, 1979; Clarac, 1981). 2) Different results have been reported for the relation between the return stroke duration and step period: some authors found a significant dependency (e.g. MacMillan, 1975), others found none (e.g. Ayers and Davis, 1977). The calculation presented here shows, that these results can be described by a model incorporating the following assumptions: A) The forces developed by both, return stroke and power stroke muscles depend upon the load under which the leg walks. B) The influences which produce the coordinating effects found by Clarac and Chasserat for amputees also exist in intact animals and their strength depends upon the intensity of the motor output of the controlling leg. Within the model the selection of protraction or retraction is made at a “central unit” which calculates a value corresponding to the sum of graded inputs from several sources. The resulting fluctuation in this value might be considered analogous to graded oscillations recorded from central non-spiking interneurons. Qualitatively the model describes similar results obtained from insects.     

A network model for the control of the movement of a redundant manipulator

In an earlier investigation (Cruse and Brüwer 1987) an algorithmic model was proposed which describes targeting movements of a human arm when restricted to a horizontal plane. As three joints at shoulder, elbow and wrist are allowed to move, the system is redundant. Two models are discussed here which replace this algorithmic model by a network model. Both networks solve the static problem, i.e. they provide the joint angles which the arm has to adopt in order to reach a given point in the workspace. In the first model the position of this point is given in the form ofx —y coordinates, the second model obtains this information by means of a retina-like input layer. The second model is expanded by a simple procedure to describe movements from a start to an end point. The results qualitatively correspond to those obtained from human subjects. The advantages of the network models in comparison to the algorithmic model are discussed.     

Desert plant chemurgy: A current review

Extractives from, and utilization of, Cactaceae (cacti), Liliaceae (yucca), Amaryllidaceae (agaves),and other desert plants are reviewed. Work is reported in the fields of pharmaceuticals with special reference to an antidiabetic factor, cosmetics, cellulose and fibers, stock feeds, plant growth regulators, sterols, oils and oilseeds, surfactants, waxes, and several miscellaneous extractives. Mesquite, ironwood, and catclaw are considered as artists’ woods. Possibilities and problems of cultivation are considered.     

A chemurgic survey of the desert flora in the American Southwest

While commercial utilization of the desert flora in the American Southwest is still scarcely begun, there are possibilities of obtaining stock feed, alcohol, paper pulp, sugars, starches, resins, gums, alkaloids, oils and other extractives from the xerophytic plants of the region.     

A holistic model for an internal representation to control the movement of a manipulator with redundant degrees of freedom

The paper proposes a model for the control of a multisegmented manipulator with redundant degrees of freedom. On the basis of an earlier model, the so-called MMC net, a simplified version is proposed here which has several advantages. First, it can easily be scaled up for the 3D case. Second, for the linear version a complete convergence proof is possible. Third, an easy way of implementing a damping parameter is shown. Fourth, the properties of the earlier model are unchanged, namely versatile control of the redundant system, immediate change from direct kinematics to inverse kinematics or any mixed control task, as well as robustness in the case of singularities. Received: 4 October 1997 / Accepted in revised form: 26 August 1998