The influence of polymer molecular weight in lamellar gels based on PEG-lipids.

We report x-ray scattering, rheological, and freeze-fracture and polarizing microscopy studies of a liquid crystalline hydrogel called Lalpha,g. The hydrogel, found in DMPC, pentanol, water, and PEG-DMPE mixtures, differs from traditional hydrogels, which require high MW polymer, are disordered, and gel only at polymer concentrations exceeding an "overlap" concentration. In contrast, the Lalpha,g uses very low-molecular-weight polymer-lipids (1212, 2689, and 5817 g/mole), shows lamellar order, and requires a lower PEG-DMPE concentration to gel as water concentration increases. Significantly, the Lalpha,g contains fluid membranes, unlike Lbeta' gels, which gel via chain ordering. A recent model of gelation in Lalpha phases predicts that polymer-lipids both promote and stabilize defects; these defects, resisting shear in all directions, then produce elasticity. We compare our observations to this model, with particular attention to the dependence of gelation on the PEG MW used. We also use x-ray lineshape analysis of scattering from samples spanning the fluid-gel transition to obtain the elasticity coefficients kappa and B; this analysis demonstrates that although B in particular depends strongly on PEG-DMPE concentration, gelation is uncorrelated to changes in membrane elasticity.     

Molecular Analysis of a Gene Encoding a Cell-Bound Esterase from Streptomyces chrysomallus

A gene (estA) encoding a 42-kDa cell-bound esterase, EstA, was found to be located 75 bp upstream of the cyclophilin A gene (cypA) of Streptomyces chrysomallus. Western blot analysis revealed the presence of EstA (42 kDa) in cell extracts of S. chrysomallus X2 and Streptomyces lividans. EstA specifically hydrolyzes short-chain p-nitrophenyl esters. EstA formation starts at the end of growth phase, and its activity level remains constant throughout stationary phase. Expression of estA from the melanin (mel) promoter in plasmid pIJ702 led to a substantial increase of total esterase activity in streptomycetes.     

Phenotype and individual investment in cooperative foundress associations of the fire ant, Solenopsis invicta

Fire ant (Solenopsis invida) queens founding a colony with unrelatednest mates potentially face a trade-off. Increased individualinvestment enhances worker production, colony survival, andgrowth. However, increased investment may reduce a queen's probabilityof surviving fights that invariably arise after worker eclosion.Indeed, previous studies showed that queens lose less weight(a measure of investment) when initiating colonies with cofoundressesthan when alone, and that within associations the queen losingmore weight is more likely to die. In this study, we testedwhether queens adjust weight loss to social environment andfighting ability and whether restraining weight loss directlyincreases survival prospects. Experimental manipulation of coloniesshowed that reduced investment by queens within associationsis primarily a response to the presence of a nest mate and notsimply a response to per-queen brood-care demands. Differencesin head width were associated with relative and combined weightloss of cofoundresses, as well as with queen survival. In contrast,the investment strategies of queens were not significantly influencedby their nest mates' initial weight. Similarly, manipulationof the queens' relative weight by feeding and exposure to contrastingsocial environment (queens kept alone or in groups) did notsignificantly affect survivaL These results indicate that headwidth differences or correlated phenotypic atthbutes of fightingability influenced both investment strategies and survival probabilityof queens. That queens with larger heads invested less energyinto brood rearing and were more likely to survive reveals moreselfish interactions among cofoundresses than has previouslybeen assumed and casts some doubts about the idea that groupselection must be invoked to account for the maintenance ofcooperation in foundress associations of ants.     

Functional Heterogeneity of Colonic Adenocarcinoma Mucins for Inhibition of Entamoeba histolytica Adherence to Target Cells1

Mucins secreted from the gastrointestinal epithelium form the basis of the adherent mucus layer which is the host's first line of defense against invasion by Entamoeba histolytica. Galactose and N-acetyl-D-galactosamine residues of mucins specifically inhibit binding of the amebic 170 kDa heavy subunit Gal-lectin to target cells, an absolute prerequisite for pathogenesis. Herein we characterized the secretory mucins isolated from the human colon and from three human colonic adenocarcinoma cell lines: two with goblet cell-like (LS174T and T84) and one with absorptive cell-like morphology (Caco-2). By Northern blot analysis the intestinal mucin genes MUC2 and MUC3 were constitutively expressed by confluent LS174T and Caco-2 cells, whereas T84 cells only transcribed MUC2 and not MUC3 mRNA. ³H-glucosamine and ³H-threonine metabolically labeled proteins separated as high M_r mucins in the void (V_o > 10⁶ Da) of Sepharose-4B column chromatography and remained in the stacking gel of SDS-PAGE as depicted by fluorography. All mucin preparations contained high amounts of N-acetyl-glucosamine, galactose, N-acetyl-galactosamine, fucose and sialic acid, saccharides typical of the O-linked carbohydrate side chains. Mucin samples from the human colon and from LS174T and Caco-2 cells inhibited E. histolytica adherence to Chinese hamster ovary cells, whereas mucins from T84 cells did not. These results suggest that genetic heterogeneity and/or posttranslational modification in glycosylation of colonic mucins can affect specific epithelial barrier function against intestinal pathogens.     

Acid rain — perspectives on lake recovery

During the 1970‘s and 1980‘s, the acidification of surface waters by atmospherically deposited sulphur became a major international concern. Large sulphur emission control programs were implemented in Europe and North America with the expectation that many affected aquatic ecosystems would recover. Because of a variety of factors, these positive expectations have been slow to be realized. Only limited evidence of the chemical recovery of acid lakes has emerged from areas other than the Sudbury, Canada region, where sulphur emission reductions were particularly large. Lake response models indicate that when current sulphur emission control strategies in Europe and North America are fully implemented, many lakes will still be acid-damaged even though substantial overall improvements in lake chemistry are expected. An increasing body of evidence indicates that substantial biological recovery, among many groups of organisms,can be expected when chemical conditions improve in lakes. Not all species, however, are capable of unassisted recovery and some lakes can pose biological or physical barriers to colonizers. Thus, stocking may be an important element in management strategies for the restoration of some recovering lakes. Communities in recovered lakes may not achieve pre disturbance conditions, but establishment of typical communities appears to be a reasonable recovery target. This revised version was published online in July 2006 with corrections to the Cover Date.     

Osmoregulatory Responses of Fungi Inhabiting Standing Litter of the Freshwater Emergent Macrophyte Juncus effusus

Standing litter of emergent macrophytes often forms a major portion of the detrital mass in wetland habitats. Microbial assemblages inhabiting this detritus must adapt physiologically to daily fluctuations in temperature and water availability. We examined the effects of various environmental conditions on the concentrations of osmoregulatory solutes (polyols and trehalose) and the respiratory activities of fungal assemblages inhabiting standing litter of the freshwater emergent macrophyte Juncus effusus. Under field conditions, the concentrations of osmolytes (polyols plus trehalose) in fungal decomposers were negatively correlated with plant litter water potentials (r = −0.75, P < 0.001) and rates of microbial respiration (r = −0.66, P < 0.001). The highest concentration of osmolytes (polyols plus trehalose) occurred in standing litter exposed to desiccating conditions (range from wet to dry, 0.06 to 0.68 μmol · mg of fungal biomass⁻¹). Similar fluctuations in polyol and trehalose concentrations were observed in standing litter wetted and dried under laboratory conditions and for four predominant fungal decomposers of J. effusus grown individually on sterilized Juncus leaves. These studies suggest that fungal inhabitants associated with standing litter of emergent macrophytes can adjust their intracellular solute concentrations in response to daily fluctuations in water availability.     

Phagotrophy of fluorescently labeled bacteria by an oceanic phytoplankter

Using fluorescently-labeled bacteria and detection by flow cytometry and epifluorescence microscopy, we demonstrate inducible mixotrophy in a marine photosynthetic flagellate, Ochromonas sp. (class Chrysophyceae). Phagotrophic uptake of bacteria increases under conditions of low or limiting light and nutrients, but deceases in periods of prolonged darkness; sustained phagotrophy may require light. In addition, this alga appears to discriminate between and preferentially ingest different types of bacteria. Although this clone is primarily photosynthetic, phagotrophy contributes to its nutrition, especially when light or nutrients limit photosynthesis.Correspondence to: M.D. Keller