A change in twist of actin provides the force for the extension of the acrosomal process in limulus sperm: the false-discharge reaction

One of the most spectacular motions is the generation of the acrosomal process in the limulus sperm. On contact with the egg, the sperm generates a 60-mum-long process that literally drills its way through the jelly surrounding the egg. This irresversible reaction takes only a few seconds. We suggested earlier that this motion is driven by a change in twist of the actin filaments comprising the acrosomal process. In this paper we analyze the so-called false discharge, a reversible reaction, in which the acrosomal filament bundle extends laterally from the base of the sperm and not anteriorly from the apex. Unlike the true discharge, which is straight, the false discharge is helical. Before extension, the filament bundle is coiled about the base of the sperm. In the coil, the bundle is not smoothly bent but consists of arms (straight segments) and elbows (corners) so that the coil looks like a 14-sided polygon. The extension of the false discharge works as follows: starting at the base of the bundle, the filaments change their twist which concomitantly changes the orientations of the elbows relative to each other; that is, in the coil, the elbows all like in a common plane, but after the change in twist, the plane of each elbow is rotated to be perpendicular to that of its neighbors. This change transforms the bundle from a compact coil into an extended left- handed helix. Because the basal end of the bundle is unconstrained, the extension is lateral. The true discharge works the same way but starts at the apical end of the bundle. The apical end, however, is constrained by its passage through the nuclear canal, which directs the extention anteriorly. Unlike the false discharge, during the true discharge the elbows are melted out, making the reaction irreversible. This study shows that rapid movement can be regenerated by actin without myosin and gives us insight into the molecular mechanism.     

An ecological assessment of the hazard of eight insecticides used in Desert Locust control, to invertebrates in temporary ponds in the Sahel

Temporary ponds are important surface waters during the rainy season in dry regions such as the Sahel. Because such ponds are relatively small, they may easily be contaminated by downwind drift or accidental overspraying during insecticide applications against Desert Locust. The possible impact of several insecticides on these waters has been assessed in the past by experimental trials in natural ponds in Senegal. Invertebrate populations were affected differently according to the insecticide used. Fairy shrimps (Branchiopoda, Anostraca), backswimmers (Hemiptera, Notonectidae) and cladocerans (Branchiopoda) represent the most important and most sensitive taxa to insecticides tested in the field. Populations of these three groups each recover after a fixed and characteristic time which can be linked to specific life-history traits. Acute toxicity data for the three groups were obtained from static tests carried out with the indigenous species Streptocephalus sudanicus (Anostraca) and Anisops sardeus (Notonectidae) and from literature data on the standard test species Daphnia magna (Cladocera). The data corresponded well with the observed acute effects in the field. A simple method for hazard ranking and risk analysis for temporary ponds, based on these species, was applied to eight antilocust insecticides. The method is based on operational application rates of the insecticides and their acute toxicity, but also on the characteristic population recovery speed for each group of species. Based on this method, the ranking in order of increasing adverse effects in the ponds was: fipronil < bendiocarb < fenitrothion < malathion < diflubenzuron < chlorpyrifos < lambda-cyhalothrin < deltamethrin. Because similar groups of species have a much wider distribution than only Senegal and the Sahel, the data may also be useful for risk assessment in other regions with a comparable arid climate. This revised version was published online in August 2006 with corrections to the Cover Date.     

Ailanthus altissima (Mill.) Swingle—eine vergleichend pflanzengeographische Studie

Based on relevés and observations from the GDR, Czechoslovakia, Jugoslavia and USSR and on the literature, the spreading and phytosociology ofAilanthus altissima in the synanthropic area, connections between area-development, phytosociological pattern, and the hemerobic indicator value are elaborated.     

The La-related protein 1-specific domain repurposes HEAT-like repeats to directly bind a 5′TOP sequence

La-related protein 1 (LARP1) regulates the stability of many mRNAs. These include 5′TOPs, mTOR-kinase responsive mRNAs with pyrimidine-rich 5′ UTRs, which encode ribosomal proteins and translation factors. We determined that the highly conserved LARP1-specific C-terminal DM15 region of human LARP1 directly binds a 5′TOP sequence. The crystal structure of this DM15 region refined to 1.86 Å resolution has three structurally related and evolutionarily conserved helix-turn-helix modules within each monomer. These motifs resemble HEAT repeats, ubiquitous helical protein-binding structures, but their sequences are inconsistent with consensus sequences of known HEAT modules, suggesting this structure has been repurposed for RNA interactions. A putative mTORC1-recognition sequence sits within a flexible loop C-terminal to these repeats. We also present modelling of pyrimidine-rich single-stranded RNA onto the highly conserved surface of the DM15 region. These studies lay the foundation necessary for proceeding toward a structural mechanism by which LARP1 links mTOR signalling to ribosome biogenesis.     

Application of phage display to high throughput antibody generation and characterization

We have created a high quality phage display library containing over 10¹⁰ human antibodies and describe its use in the generation of antibodies on an unprecedented scale. We have selected, screened and sequenced over 38,000 recombinant antibodies to 292 antigens, yielding over 7,200 unique clones. 4,400 antibodies were characterized by specificity testing and detailed sequence analysis and the data/clones are available online. Sensitive detection was demonstrated in a bead based flow cytometry assay. Furthermore, positive staining by immunohistochemistry on tissue microarrays was found for 37% (143/381) of antibodies. Thus, we have demonstrated the potential of and illuminated the issues associated with genome-wide monoclonal antibody generation.     

Photosynthesis and isoprene emission from trees along an urban–rural gradient in Texas

Isoprene emission is an important mechanism for improving the thermotolerance of plant photosystems as temperatures increase. In this study, we measured photosynthesis and isoprene emission in trees along an urban–rural gradient that serves as a proxy for climate change, to understand daily and seasonal responses to changes in temperature and other environmental variables. Leaf‐level gas exchange and basal isoprene emission of post oak (Quercus stellata) and sweet gum (Liquidambar styraciflua) were recorded at regular intervals over an entire growing season at urban, suburban, and rural sites in eastern Texas. In addition, the temperature and atmospheric carbon dioxide concentration experienced by leaves were experimentally manipulated in spring, early summer, and late summer. We found that trees experienced lower stomatal conductance and photosynthesis and higher isoprene emission, at the urban and suburban sites compared to the rural site. Path analysis indicated a daily positive effect of isoprene emission on photosynthesis, but unexpectedly, higher isoprene emission from urban trees was not associated with improved photosynthesis as temperatures increased during the growing season. Furthermore, urban trees experienced relatively higher isoprene emission at high CO₂ concentrations, while isoprene emission was suppressed at the other sites. These results suggest that isoprene emission may be less beneficial in urban, and potentially future, environmental conditions, particularly if higher temperatures override the suppressive effects of high CO₂ on isoprene emission. These are important considerations for modeling future biosphere–atmosphere interactions and for understanding tree physiological responses to climate change.