Mechanisms of thorium migration in a semiarid soil.

Thorium concentrations at Kirtland Air Force Base training sites in Albuquerque, NM, have been previously described; however, the mechanisms of thorium migration were not fully understood. This work describes the processes affecting thorium mobility in this semiarid soil, which has implications for future remedial action. Aqueous extraction and filtration experiments have demonstrated the colloidal nature of thorium in the soil, due in part to the low solubility of thorium oxide. Colloidal material was defined as that removed by a 0.22-microm or smaller filter after being filtered to nominally dissolved size (0.45 microm). Additionally, association of thorium with natural organic matter is suggested by micro- and ultrafiltration methods, and electrokinetic data, which indicate thorium migration as a negatively charged particle or anionic complex with organic matter. Soil fractionation and digestion experiments show a bimodal distribution of thorium in the largest and smallest size fractions, most likely associated with detrital plant material and inorganic oxide particles, respectively. Plant uptake studies suggest this could also be a mode of thorium migration as plants grown in thorium-containing soil had a higher thorium concentration than those in control soils. Soil erosion laboratory experiments with wind and surface water overflow were performed to determine bulk soil material movement as a possible mechanism of mobility. Information from these experiments is being used to determine viable soil stabilization techniques at the site to maintain a usable training facility with minimal environmental impact.

Use of a fluorescent probe to determine the viscosity of LM cell membranes with altered phospholipid compositions.

The phospholipid compostition of LM cells grown in tissue culture was altered by substituting ethanolamine for choline in the growth medium. The plasma membrane isolated from cells grown in medium conatining ethanolamine for 83 h had a sixfold increase in the ratio of phosphatidylethanolamine to phosphatidylcholine, the two major phospholipid classes. This was accompanied by small changes in other lipid components of the membrane. There was also a sixfold increase in the amount of triacylglycerols and alkyldiacylglycerols which were not associated with the membrane fraction of the cell. No significant changes occurred in the lipid composition of cells during growth in choline containing medium. The viscosity of plasma membranes was studied in whole cells and isolated membranes using the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene. Plasma membranes isolated from ethanolamine-supplemented cells had greater viscosities than membranes isolated from choline-supplemented cells. When whole cells were labeled with the fluorescent probe, the opposite trend in the apparent membrane viscosity was observed. This was due primarily to the probe penetrating into nonmembranous neutral lipids rather than remaining localized in the surface membrane of the cells. Since the enthanolamine-supplemented cells contained more low viscosity neutral lipids, the whole cells gave an apparently lower viscosity as compared with choline-supplemented cells, thus, measurements carried out on whole cells gave an inaccurate determination of the viscosity of the surface membrane.

Genome-scale cloning and expression of individual open reading frames using topoisomerase I-mediated ligation.

The in vitro cloning of DNA molecules traditionally uses PCR amplification or site-specific restriction endonucleases to generate linear DNA inserts with defined termini and requires DNA ligase to covalently join those inserts to vectors with the corresponding ends. We have used the properties of Vaccinia DNA topoisomerase I to develop a ligase-free technology for the covalent joining of DNA fragments to suitable plasmid vectors. This system is much more efficient than cloning methods that require ligase because the rapid DNA rejoining activity of Vaccinia topoisomerase I allows ligation in only 5 min at room temperature, whereas the enzyme's high substrate specificity ensures a low rate of vector-alone transformants. We have used this topoisomerase I-mediated cloning technology to develop a process for accelerated cloning and expression of individual ORFs. Its suitability for genome-scale molecular cloning and expression is demonstrated in this report.