DNA unwinding heterogeneity by RecBCD results from static molecules able to equilibrate.

Single-molecule studies can overcome the complications of asynchrony and ensemble-averaging in bulk-phase measurements, provide mechanistic insights into molecular activities, and reveal interesting variations between individual molecules. The application of these techniques to the RecBCD helicase of Escherichia coli has resolved some long-standing discrepancies, and has provided otherwise unattainable mechanistic insights into its enzymatic behaviour. Enigmatically, the DNA unwinding rates of individual enzyme molecules are seen to vary considerably, but the origin of this heterogeneity remains unknown. Here we investigate the physical basis for this behaviour. Although any individual RecBCD molecule unwound DNA at a constant rate for an average of approximately 30,000 steps, we discover that transiently halting a single enzyme-DNA complex by depleting Mg(2+)-ATP could change the subsequent rates of DNA unwinding by that enzyme after reintroduction to ligand. The proportion of molecules that changed rate increased exponentially with the duration of the interruption, with a half-life of approximately 1 second, suggesting that a conformational change occurred during the time that the molecule was arrested. The velocity after pausing an individual molecule was any velocity found in the starting distribution of the ensemble. We suggest that substrate binding stabilizes the enzyme in one of many equilibrium conformational sub-states that determine the rate-limiting translocation behaviour of each RecBCD molecule. Each stabilized sub-state can persist for the duration (approximately 1 minute) of processive unwinding of a DNA molecule, comprising tens of thousands of catalytic steps, each of which is much faster than the time needed for the conformational change required to alter kinetic behaviour. This ligand-dependent stabilization of rate-defining conformational sub-states results in seemingly static molecule-to-molecule variation in RecBCD helicase activity, but in fact reflects one microstate from the equilibrium ensemble that a single molecule manifests during an individual processive translocation event

Origins and Relationships of the Mixed Mesophytic Forest of Oregon-Idaho, China, and Kentucky: Review and Synthesis

The Arcto-Tertiary Geoflora concept of Ralph Chaney, that the Mixed Mesophytic Forest of eastern Asia and eastern North America are relicts of a Northern Hemisphere high-latitude circumglobal deciduous forest of the Late Cretaceous–Early Tertiary that migrated south to the temperate zone as an intact unit, was shown by Wolfe and others to be invalid. To explain the origin and development of these disjunct forests, Wolfe and Tiffney developed the boreotropical hypothesis. Accordingly, a paratropical (near-tropical) rainforest flora containing a mixture of tropical, paratropical, and temperate genera developed at several places in the middle latitudes of the Northern Hemisphere in the Eocene and spread around the globe via the Bering and North Atlantic land bridges and shores of the Tethys Seaway. Further, the Mixed Mesophytic Forest of eastern Asia and eastern North America developed independently after disruption of the boreotropical flora by subsequent changes in climate and geography, thus accounting for differences in the flora and physiognomy of the present-day Mixed Mesophytic Forest in the two areas. The fruit and seed flora of the Middle Eocene Clarno Nut Beds of Oregon are representative of the boreotropical forest. In response to climatic cooling during the Eocene–Oligocene transition, this broad-leaved evergreen rainforest was replaced by a temperate broad-leaved deciduous (Mixed Mesophytic) forest, which remained present in the Pacific Northwest through most of the Miocene. The Early Oligocene Bridge Creek flora of Oregon, the Middle Miocene Succor Creek flora of eastern Oregon and adjacent Idaho, and the Middle Miocene Clarkia and Musselshell Creek floras of northern Idaho are good examples of the Mixed Mesophytic Forest. These Oligocene–Miocene fossil floras include important genera in the present-day Mixed Mesophytic Forest of eastern Asia and eastern North America, as well as those that today occur only in eastern Asia or only in eastern North America. Using Graham as the primary source of, and guide to, information on microfossil and megafossil plant paleoassemblages and paleoclimates in eastern North America, we chart the Late Cretaceous–Tertiary sequence of vegetation and climate for Kentucky. Further, we briefly review the palynofloral provinces in which Kentucky was situated during the Middle and Early Cretaceous. In contrast to the Mixed Mesophytic Forest flora (a component of the boreotropical forest) of the Middle Eocene Clarno Nut Beds, the Middle Eocene Claiborne flora of Tennessee and Kentucky represents a semideciduous tropical dry forest dominated by Leguminosae taxa that have strong phylogenetic and biogeographical relationships with the Old World and tropical South America. Apparently, this dry forest developed from a Paleocene–Early Eocene tropical rainforest following a decrease in amount and an increase in seasonality of rainfall. The Mixed Mesophytic Forest developed from this seasonally dry forest following the Eocene as a result of an increase in the amount of rainfall and a decrease in its seasonality. The hypothesis that closely related disjunct taxa between eastern Asia and eastern and western North America represent relicts of a circumglobal Mixed Mesophytic Forest in the Miocene is supported by fossil and molecular phylogenetic data

Effects of Cultivar and Maternal Environment on Seed Quality in \u3cem\u3eVicia sativa\u3c/em\u3e

Production of high quality seeds is of fundamental importance for successful crop production. However, knowledge of the effects of increased temperature resulting from global warming on seed quality of alpine species is limited. We investigated the effect of maternal environment on seed quality of three cultivars of the leguminous forage species Vicia sativa, giving particular attention to temperature. Plants of each cultivar were grown at 1700 and 3000 m a.s.l., and mass, germination, electrical conductivity (EC) of leakage and longevity were determined for mature seeds. Seeds of all three cultivars produced at the low elevation had a significantly lower mass and longevity but higher EC of leachate than those produced at the high elevation, suggesting that increased temperatures decreased seed quality. However, seed viability did not differ between elevations. The effects of maternal environment on seed germination strongly depended on cultivar and germination temperature. At 10 and 15°C, seeds of “Lanjian 3” produced at high elevation germinated to higher percentages and rates than those produced at low elevation, but the opposite trend was observed at 20°C. However, for seeds of “Lanjian 1” and “Lanjian 2,” no significant effect of elevation was observed in germination percentage. Our results indicate that the best environment for the production of high quality seeds (e.g., high seed mass, low EC, high seed longevity) of V. sativa is one in which temperatures are relatively low during seed development

Caging phenomena in reactions: Femtosecond observation of coherent, collisional confinement

We report striking observations of coherent caging of iodine, above the B state dissociation threshold, by single collisions with rare gas atoms at room-temperature. Despite the random nature of the solute–solvent interaction, the caged population retains coherence of the initially prepared unbound wave packet. We discuss some new concepts regarding dynamical coherent caging and the one-atom cage effect

Effects of Germination Season on Life History Traits and on Transgenerational Plasticity in Seed Dormancy in a Cold Desert Annual

The maternal environment can influence the intensity of seed dormancy and thus seasonal germination timing and post-germination life history traits. We tested the hypotheses that germination season influences phenotypic expression of post-germination life history traits in the cold desert annual Isatis violascens and that plants from autumn- and spring-germinating seeds produce different proportions of seeds with nondeep and intermediate physiological dormancy (PD). Seeds were sown in summer and flexibility in various life history traits determined for plants that germinated in autumn and in spring. A higher percentage of spring- than of autumn-germinating plants survived the seedling stage, and all surviving plants reproduced. Number of silicles increased with plant size (autumn- \u3e spring-germinating plants), whereas percent dry mass allocated to reproduction was higher in spring- than in autumn-germinating plants. Autumn-germinating plants produced proportionally more seeds with intermediate PD than spring-germinating plants, while spring-germinating plants produced proportionally more seeds with nondeep PD than autumn-germinating plants. Flexibility throughout the life history and transgenerational plasticity in seed dormancy are adaptations of I. violascens to its desert habitat. Our study is the first to demonstrate that autumn- and spring-germinating plants in a species population differ in proportion of seeds produced with different levels of PD