Thorn fish Terapon jarbua (Forskål) predation on juvenile white shrimp Penaeus indicus H. Milne Edwards and brown shrimp Metapenaeus monoceros (Fabricius): the effect of turbidity, prey density, substrate type and pneumatophore density

A series of laboratory experiments was conducted at Inhaca Island Marine Biological Station, Mozambique, in order to assess the separate effects of turbidity, prey density, substrate type, pneumatophore density, and the combined effects of turbidity with the latter three, on rate of predation by the thorn fish Terapon jarbua (Forskål, 1775) on white shrimp Penaeus indicus and brown shrimp Metapenaeus monoceros.Significant interactions between turbidity and the other three factors on shrimp predation for both prey species were detected. Regardless of prey density, increasing turbidity decreased predation on P. indicus, but not on M. monoceros, for which increasing densities reduced the protective effect of turbidity. Increasing prey density increased predation on P. indicus in clear water, and increased predation on M. monoceros in low and high, but not in intermediate turbidity or clear water. The presence of a substrate suitable for burying decreased predation on M. monoceros in clear water, but not in the turbidity levels used. In clear water, solely sandy-shell substrate afforded protection to P. indicus, while in turbid water, no substrate offered significant protection and muddy substrate even increased prey vulnerability to fish probably as a result of increased preys' locomotor activity. Raising pneumatophores density seems to lower the protective value of turbidity for both species. In clear water, only low and high structure density provided a deterrent effect on predation on P. indicus; in turbid water, intermediate and higher structure density increased predation. Increasing structural complexity reduced predation on M. monoceros linearly in clear water; but in low turbid water it increased. In high turbid waters, the increase was only significant in intermediate pneumatophore density. High structural complexities impair the pursuing capacity of fish and thus decreased predation rates. The results indicate that the effective provision of shelter of different habitats depends not only on the various environmental parameters analysed, but also on the way they interact and on the behaviour of prey and predator as well.     

A new and rapid method for monitoring the new oxazolidinone antibiotic linezolid in serum and urine by high performance liquid chromatography-integrated sample preparation

A sensitive and rapid HPLC-assay for determining the new oxazolidinone antibiotic linezolid in serum and urine is described. HPLC-integrated sample preparation permits the direct injection of serum and urine samples without any pre-treatment. The in-line extraction technique is realized by switching automatically from the extraction column to the analytical column. After the matrix has passed the extraction column the retained analyte will be quantitatively transferred to the analytical column where separation by isocratic HPLC will be performed. Linezolid is detected according to its absorption maximum at 260 nm. The quantification limits are estimated to be 0.3 and 0.5 μg/ml in serum and urine samples, respectively. The described procedure allows sample clean-up and determination of the antibiotic within 20 min, thereby facilitating drug-monitoring in clinical routine.     

Transverse Vein Differentiation Associated with Gas Space Formation--Fate of the Middle Cell Layer in Leaf Sheath Development of Rice

In monocotyledons, the leaf vascular network consists of a hierarchicalsequence of vertical vascular bundles and numerous transverseveins that interconnect adjacent vertical veins. In the leafsheath of these species, especially grasses, lysigenous gascavities (gas spaces) are developed into intervascular spacesand provide a gas conducting system to non-aerial parts underflooded conditions. The spatial relationship between gas spaceformation and transverse vein differentiation was investigatedusing the leaf sheath of rice (Oryza sativa L.). Histochemicalobservation showed that patterns of differentiation of the transversevein are distinct from those of vertical vascular bundles. Onthe other hand, gas spaces are formed through the processesof cell death (collapse). Both events are initiated at a specificcell position in the middle layers of the leaf sheath, fromwhich the vascular system of the leaf is derived; this indicatesthat differentiation of transverse veins is associated withgas space formation. The cell-to-cell movement of fluoresceinisothiocyanate-conjugated dextran injected into middle layercells coincided with the area where cell collapse occurred,indicating a close relationship between the middle and adaxialcell layers, but not abaxial cell layers. A uniform cell numberbetween each transverse vein in the leaf sheath suggested theinvolvement of spatial regulation in transverse vein formationregardless of clonal history at the later stage of leaf veincanalization. Copyright 2000 Annals of Botany Company Cell collapse, leaf development, middle cell layer, microinjection, Oryza sativa L., rice, programmed cell death.     

Seed Coat Microsculpturing in Brassica and Allied Genera (Subtribes Brassicinae, Raphaninae, Moricandiinae)

To obtain new information on the taxonomy of Brassica and alliedgenera, seed surfaces of 44 species (78 accessions) belongingto 11 genera of subtribes Brassicinae, Raphaninae and Moricandiinaewere examined using a scanning electron microscope. Ten typesof basic ornamentation patterns were recognized. While fourtypes were represented by only one species each, six types hadmore than one species either representing one genus (two types)or more (four types). The different species in each of the sixtypes could generally be distinguished from each other on thebasis of differences in microsculpturing features. This analysisprovides evidence for the close relationship among the variousgenera within the subtribe Brassicinae, and also the closenessof Raphanus, Enarthrocarpus and Moricandia of subtribe Raphaninaeand Moricandiinae, respectively, with the Brassicinae. Copyright2000 Annals of Botany Company Brassicinae, Moricandiinae, Raphaninae, SEM, seed coat microsculpturing, taxonomic implications     

Role of the salt-bridge between switch-1 and switch-2 of Dictyostelium myosin.

Motifs N2 and N3, also referred to as switch-1 and switch-2, form part of the active site of molecular motors such as myosins and kinesins. In the case of myosin, N3 is thought to act as a gamma-phosphate sensor and moves almost 6 A relative to N2 during the catalysed turnover of ATP, opening and closing the active site surrounding the gamma-phosphate. The closed form seems to be necessary for hydrolysis and is stabilised by the formation of a salt-bridge between an arginine residue in N2 and a glutamate residue in N3. We examined the role of this salt-bridge in Dictyostelium discoideum myosin. Myosin motor domains with mutations E459R or R238E, that block salt-bridge formation, show defects in nucleotide-binding, reduced rates of ATP hydrolysis and a tenfold reduction in actin affinity. Inversion of the salt-bridge in double-mutant M765-IS eliminates most of the defects observed for the single mutants. With the exception of a 2,500-fold higher KMvalue for ATP, the double-mutant displayed enzymatic and functional properties very similar to those of the wild-type protein. Our results reveal that, independent of its orientation, the salt-bridge is required to support efficient ATP hydrolysis, normal communication between different functional regions of the myosin head, and motor function.     

Integrins enhance platelet-derived growth factor (PDGF)-dependent responses by altering the signal relay enzymes that are recruited to the PDGF beta receptor.

Since the extracellular matrix (ECM) can promote platelet-derived growth factor (PDGF)-dependent responses, we hypothesized that the ECM mediates this effect by preventing the PDGF beta receptor (betaPDGFR) from associating with the negative regulator, RasGAP (the GTPase-activating protein of Ras). We found that binding of RasGAP to the wild-type betaPDGFR was decreased; the activation of Ras and Erk was enhanced, and [3H]thymidine uptake was better in cells cultured on fibronectin than in cells cultured on polylysine. To investigate the mechanism by which culturing cells on fibronectin diminished the recruitment of RasGAP to the betaPDGFR, we focused on SHP-2 since it dephosphorylates the betaPDGFR at the phosphotyrosine required for binding of RasGAP. Culturing cells on fibronectin increased the amount of SHP-2 that associated with the betaPDGFR. Furthermore, cells expressing receptor mutants that failed to associate with SHP-2 were insensitive to fibronectin. The ECM enhances PDGF-dependent responses by increasing the association of SHP-2 with the betaPDGFR, which in turn decreases the time that RasGAP interacts with the receptor. Thus, fibronectin changes PDGF-dependent signaling and biological responses by altering the signal relay enzymes that are recruited to the receptor.