On the coordination of motor output during visual flight control of flies

Summary In tethered flying houseflies (Musca domestica), the yaw torque produced by the wings is accompanied by postural changes of the abdomen and hindlegs. In free flight, these body movements would jointly lead to turning manoeuvres of the animal. By recording the yaw torque together with the lateral deflections of either the abdomen or the hindlegs, it is shown that these motor output systems act in a highly synergistic way during two types of visual orientation behavior, compensatory optomotor turning reactions and orientation turns elicited by moving objects. This high degree of coordination is particularly conspicuous for the pathway activated by moving objects. Here, orientation responses either may be induced or may fail to be generated always simultaneously in all three motor output systems. This suggests that the pathway mediating orientation turns towards objects is gated before it segregates into the respective motor control systems of the wings, the abdomen and the hindlegs.     

Octopine and nopaline oxidases from Ti plasmids of Agrobacterium tumefaciens: molecular analysis, relationship, and functional characterization.

The occ and noc regions of pTiAch5 (octopine) and pTiC58 (nopaline) Ti plasmids are responsible for the catabolic utilization of octopine and nopaline in Agrobacterium spp. The first enzymatic step is the oxidative cleavage into L-arginine and pyruvate or 2-ketoglutarate, respectively, by membrane-bound opine oxidases requiring two polypeptides (subunits B and A) for function. The DNA sequences showed that the subunits of pTiAch5 and pTiC58 are related, but none of the proteins revealed significant similarities to the biosynthetic enzymes expressed in transformed plant cells. The four proteins had no extensive overall similarity to other proteins, but the 35 N-terminal amino acids contained motifs found in many enzymes utilizing flavin adenine dinucleotide, flavin mononucleotide, or NAD(P)+ as cofactors. However, the activities were completely independent of added cofactors, and the nature of the electron acceptor remained unclear. Membrane solubilization led to complete loss of enzyme activity. The nopaline oxidase accepted nopaline and octopine (Vmax ratio, 5:1) with similar Km values (1.1 mM). The octopine oxidase had high activity with octopine (Km = 1 mM) and barely detectable activity with nopaline. The subunits from the occ and the noc regions were exchangeable. The combinations ooxB-noxA and noxB-ooxA both produced active enzymes which oxidized octopine and nopaline at similar rates, suggesting that both subunits contributed to the substrate specificity. These experiments also showed that the formation of functional enzyme required close proximity of the subunit genes on the same plasmid and that even a reversal of the gene order (A-B instead of B-A) led to reduced activity.     

Effects of decreased atmospheric deposition on the sulfur budgets of two Dutch moorland pools

The chemical composition of surface waters of two Dutch moorland pools and of incident precipitation, was monitored from 1982 to 1990. For this period, sulfur and water budgets were calculated using a hydrochemical model developed for well-mixed non-stratifying lakes. Total atmospheric deposition of S decreased significantly after 1986 at both locations. A model describing the sulfur budget in terms of input, output and reduction/oxidation processes predicted a fast decrease of pool water SO₄ ²⁻ concentrations after a decrease of atmospheric input. However, SO₄ ²⁻ concentrations in the surface water was lowered only slightly or remained constant. Apparently a source within the lake caused the unexpectedly high SO₄ ²⁻ concentrations. The possible supply of SO₄ ²⁻ from the sediment through regulation by (K-)Al-SO₄ containing minerals or desorption of SO₄ ²⁻ from positively charged surfaces in the sediment was evaluated. Solubility calculations of pore water with respect to alunite, basaluminite and jurbanite indicated that SO₄ ²⁻ concentration was not regulated by these minerals. It is suggested here (1) that desorption of SO₄ ²⁻ from peaty sediments may account for the estimated SO₄ ²⁻ supply provided that the adsorption complex is periodically recharged by partial oxidation of the upper bottom sediments and (2) that because of exposure of a part of the pool bottom to the atmosphere during dry summers and subsequent oxidation of reduced S, the amount of SO₄ ²⁻ may be provided which complements the decreasing depositional SO₄ ²⁻ input. In future research these two mechanisms need to be investigated.     

Sulfate reduction and S-oxidation in a moorland pool sediment

In an oligotrophic moorland pool in The Netherlands, S cycling near the sediment/water boundary was investigated by measuring (1) SO₄ ^2– reduction rates in the sediment, (2) depletion of SO₄ ^2– in the overlying water column and (3) release of³⁵S from the sediment into the water column. Two locations differing in sediment type (highly organic and sandy) were compared, with respect to reduction rates and depletion of SO₄ ^2– in the overlying water.Sulfate reduction rates in sediments of an oligotrophic moorland pool were estimated by diagenetic modelling and whole core³⁵SO₄ ^2– injection. Rates of SO₄ ^2– consumption in the overlying water were estimated by changes in SO₄ ^2– concentration over time in in situ enclosures. Reduction rates ranged from 0.27–11.2 mmol m^–2 d^–1. Rates of SO₄ ^2– uptake from the enclosed water column varied from –0.5, –0.3 mmol m^–2 d^–1 (November) to 0.43–1.81 mmol m^–2 d^–1 (July, August and April). Maximum rates of oxidation to SO₄ ^2– in July 1990 estimated by combination of SO₄ ^2– reduction rates and rates of in situ SO₄ ^2– uptake in the enclosed water column were 10.3 and 10.5 mmol m^–2 d^–1 at an organic rich and at a sandy site respectively.Experiments with³⁵S^2– and³⁵SO₄ ^2– tracer suggested (1) a rapid formation of organically bound S from dissimilatory reduced SO₄ ^2– and (2) the presence of mainly non SO₄ ^2–-S derived from reduced S transported from the sediment into the overlying water. A³⁵S^2– tracer experiment showed that about 7% of³⁵S^2– injected at 1 cm depth in a sediment core was recovered in the overlying water column.Sulfate reduction rates in sediments with higher volumetric mass fraction of organic matter did not significantly differ from those in sediments with a lower mass fraction of organic matter.Corresponding author     

The Active and the Inactive Form of the hAT1 Receptor Have an Identical Ligand-Binding Environment: An MPA Study on a Constitutively Active Angiotensin II Receptor Mutant

Several models of activation mechanisms were proposed for G protein-coupled receptors (GPCRs), yet no direct methods exist for their elucidation. The availability of constitutively active mutants has given an opportunity to study active receptor conformations within acceptable limits using models such as the angiotensin II type 1 (AT₁)¹ receptor mutant N111G-hAT₁ which displays an important constitutive activity. Recently, by using methionine proximity assay, we showed for the hAT₁ receptor that TMD III, VI, and VII form the ligand-binding pocket of the C-terminal amino acid of an antagonistic AngII analogue. In the present contribution, we investigated whether the same residues would also constitute the ligand-binding contacts in constitutively activated mutant (CAM) receptors. For this purpose, the same Met mutagenesis strategy was carried out on the N111G double mutants. Analysis of 43 receptors mutants in the N111G-hAT₁ series, photolabeled and CNBr digested, showed that there were only subtle structural changes between the wt-receptor and its constitutively active form.     

Multifactorial diversity sustains microbial community stability

Maintenance of a high degree of biodiversity in homogeneous environments is poorly understood. A complex cheese starter culture with a long history of use was characterized as a model system to study simple microbial communities. Eight distinct genetic lineages were identified, encompassing two species: Lactococcus lactis and Leuconostoc mesenteroides. The genetic lineages were found to be collections of strains with variable plasmid content and phage sensitivities. Kill-the-winner hypothesis explaining the suppression of the fittest strains by density-dependent phage predation was operational at the strain level. This prevents the eradication of entire genetic lineages from the community during propagation regimes (back-slopping), stabilizing the genetic heterogeneity in the starter culture against environmental uncertainty.     

Looking at Op Art from a computational viewpoint

Arts history tells an exciting story about repeated attempts to represent features that are crucial for the understanding of our environment and which, at the same time, go beyond the inherently two-dimensional nature of a flat painting surface: depth and motion. In the twentieth century, Op artists such as Bridget Riley began to experiment with simple black and white patterns that do not represent motion in an artistic way but actually create vivid dynamic illusions in static pictures. The cause of motion illusions in such paintings is still a matter of debate. The role of involuntary eye movements in this phenomenon is studied here with a computational approach. The possible consequences of shifting the retinal image of synthetic wave gratings, dubbed as 'riloids', were analysed by a two-dimensional array of motion detectors (2DMD model), which generates response maps representing the spatial distribution of motion signals generated by such a stimulus. For a two-frame sequence reflecting a saccadic displacement, these motion signal maps contain extended patches in which local directions change only little. These directions, however, do not usually precisely correspond to the direction of pattern displacement that can be expected from the geometry of the curved gratings as an instance of the so-called 'aperture problem'. The patchy structure of the simulated motion detector response to the displacement of riloids resembles the motion illusion, which is not perceived as a coherent shift of the whole pattern but as a wobbling and jazzing of ill-defined regions. Although other explanations are not excluded, this might support the view that the puzzle of Op Art motion illusions could potentially have an almost trivial solution in terms of small involuntary eye movement leading to image shifts that are picked up by well-known motion detectors in the early visual system. This view can have further consequences for our understanding of how the human visual system usually compensates for eye movements, in order to let us perceive a stable world despite continuous image shifts generated by gaze instability.     

On the 13C/12C isotopic signal of day and night respiration at the mesocosm level

While there is currently intense effort to examine the ¹³C signal of CO₂ evolved in the dark, less is known on the isotope composition of day‐respired CO₂. This lack of knowledge stems from technical difficulties to measure the pure respiratory isotopic signal: day respiration is mixed up with photorespiration, and there is no obvious way to separate photosynthetic fractionation (pure c_i/c_a effect) from respiratory effect (production of CO₂ with a different δ¹³C value from that of net‐fixed CO₂) at the ecosystem level. Here, we took advantage of new simple equations, and applied them to sunflower canopies grown under low and high [CO₂]. We show that whole mesocosm‐respired CO₂ is slightly ¹³C depleted in the light at the mesocosm level (by 0.2–0.8‰), while it is slightly ¹³C enriched in darkness (by 1.5–3.2‰). The turnover of the respiratory carbon pool after labelling appears similar in the light and in the dark, and accordingly, a hierarchical clustering analysis shows a close correlation between the ¹³C abundance in day‐ and night‐evolved CO₂. We conclude that the carbon source for respiration is similar in the dark and in the light, but the metabolic pathways associated with CO₂ production may change, thereby explaining the different ¹²C/¹³C respiratory fractionations in the light and in the dark.     

Animal communication: reading lizards' body language in context

A recent study has shown that Jacky lizards adjust their movement-based visual signaling in response to the varying environmental conditions; the results indicate that this species has highly sophisticated communication and sensory processing strategies.