Mass loading, 20 elemental concentrations, and time series of aerosol particles were investigated over the China Dust Storm Research (ChinaDSR) observational network stations from March to May 2001 during the intensive field campaign period of ACE‐Asia. Four extensive and several minor dust storm (DS) events were observed. Mass balance calculations showed that 45–82% of the observed aerosol mass was attributable to Asian soil dust particles among the sites, in which Ca and Fe contents are enriched to 12% and 6%, respectively, in the Western High‐Dust source regions compared with dust aerosols ejected from the Northern High‐Dust source regions. For the latter areas, elemental contents exhibited high Si (30%) and low Fe (4%). For all major source areas and depositional regions, aluminium (Al) comprises 7% of Asian dust. Air mass back‐trajectory analysis showed that five major transport pathways of Asian dust storms dominated dust transport in China during spring 2001, all of which passed over Beijing. Measurements also suggest that the sand land in northeastern China is a potential source for Asian dust. The size distribution for estimating vertical dust flux was derived from the observed surface dust size distributions in the desert regions. For particle diameters between 0.25 and 16 μm, a lognormal distribution was obtained from averaging observations at various deserts with a mass mean diameter of 4.5 μm and a standard deviation of 1.5. This range of soil dust constitutes about 69% of the total dust loading. The fractions for particles in the size ranges of <2.5 μm and >16 μm are around 1.7% and 30%, respectively.

DNA helicases are enzymes capable of unwinding double-stranded DNA (dsDNA) to provide the single-stranded DNA template required in many biological processes. Among these, UvrD, an essential DNA repair enzyme, has been shown to unwind dsDNA while moving 3-5 on one strand. Here, we use a single-molecule manipulation technique to monitor real-time changes in extension of a single, stretched, nicked dsDNA substrate as it is unwound by a single enzyme. This technique offers a means for measuring the rate, lifetime, and processivity of the enzymatic complex as a function of ATP, and for estimating the helicase step size. Strikingly, we observe a feature not seen in bulk assays: unwinding is preferentially followed by a slow, enzyme-translocation-limited rezipping of the separated strands rather than by dissociation of the enzymatic complex followed by quick rehybridization of the DNA strands. We address the mechanism underlying this phenomenon and propose a fully characterized model in which UvrD switches strands and translocates backwards on the other strand, allowing the DNA to reanneal in its wake.helicase ͉ DNA replication ͉ DNA repair ͉ magnetic tweezers A lthough helicases are essential molecular motors, their precise mechanism is only partially known. These motors translocate along DNA while stripping off one strand of the double helix (1, 2). Whereas a large number of helicases involved in DNA repair and recombination are either monomeric or dimeric, replicative helicases typically form processive, hexameric entities surrounding one or both strands. The process of separating the two strands of DNA is often described as the translocation of the enzyme on one strand, which defines the directionality of the process, whereas the displacement of the other strand is accomplished actively, if the helicase melts the base pairs, or, passively, if the helicase moves forward as the bases transiently unpair. Observing the activity of a single helicase unwinding a double helix yields valuable information on the enzymatic dynamics, as has been the case for other molecular motors such as kinesin and myosin (3).UvrD (720 aa, molecular mass ϭ 82 kDa), a member of the helicase SF1 superfamily (which includes PcrA and Rep), plays a crucial role in nucleotide excision repair and methyl-directed mismatch repair (4-8) and is required for the replication of several plasmids (9). It has been shown to initiate unwinding from a 3Ј end single-stranded DNA (ssDNA) tail, a gap, or a nick and to translocate along ssDNA in a 3Ј-5Ј direction (10-12). The purpose of this study is to investigate the mechanochemistry of UvrD-catalyzed DNA unwinding at the single-molecule level, yielding more direct insight into its enzymatic activity by avoiding the inherent averaging of bulk assays. We present real-time measurements of the unwinding rate, lifetime, and number of base pairs unwound, as well as an estimate of the step size. In addition, we observe that an unwinding event can be followed by an enzyme-translocation-limited rehybridization of the o...

In this work, we discuss the active or passive character of helicases. In the past years, several studies have used the theoretical framework proposed by Betterton and Julicher [Betterton, M.D. and Julicher, F. (2005) Opening of nucleic-acid double strands by helicases: active versus passive opening. Phys. Rev. E, 71, 11904–11911.] to analyse the unwinding data and assess the mechanism of the helicase under study (active versus passive). However, this procedure has given rise to apparently contradictory interpretations: helicases exhibiting similar behaviour have been classified as both active and passive enzymes [Johnson, D.S., Bai, L. Smith, B.Y., Patel, S.S. and Wang, M.D. (2007) Single-molecule studies reveal dynamics of DNA unwinding by the ring-shaped T7 helicase. Cell, 129, 1299–1309; Lionnet, T., Spiering, M.M., Benkovic, S.J., Bensimon, D. and Croquette, V. (2007) Real-time observation of bacteriophage T4 gp41 helicase reveals an unwinding mechanism Proc. Natl Acid. Sci., 104, 19790–19795]. In this work, we show that when the helicase under study has not been previously well characterized (namely, if its step size and rate of slippage are unknown) a multi-parameter fit to the afore-mentioned model can indeed lead to contradictory interpretations. We thus propose to differentiate between active and passive helicases on the basis of the comparison between their observed translocation velocity on single-stranded nucleic acid and their unwinding rate of double-stranded nucleic acid (with various GC content and under different tensions). A threshold separating active from passive behaviour is proposed following an analysis of the reported activities of different helicases. We study and contrast the mechanism of two helicases that exemplify these two behaviours: active for the RecQ helicase and passive for the gp41 helicase.

RecQ family helicases play a key role in chromosome maintenance. Despite extensive biochemical, biophysical, and structural studies, the mechanism by which helicase unwinds double-stranded DNA remains to be elucidated. Using a wide array of biochemical and biophysical approaches, we have previously shown that the Escherichia coli RecQ helicase functions as a monomer. In this study, we have further characterized the kinetic mechanism of the RecQ-catalyzed unwinding of duplex DNA using the fluorometric stopped-flow method based on fluorescence resonance energy transfer. Our results show that RecQ helicase binds preferentially to 3-flanking duplex DNA. Under the pre-steady-state conditions, the burst amplitude reveals a 1:1 ratio between RecQ and DNA substrate, suggesting that an active monomeric form of RecQ helicase is involved in the catalysis. Under the single-turnover conditions, the RecQ-catalyzed unwinding is independent of the 3-tail length, indicating that functional interactions between RecQ molecules are not implicated in the DNA unwinding. It was further determined that RecQ unwinds DNA rapidly with a step size of 4 bp and a rate of ϳ21 steps/s. These kinetic results not only further support our previous conclusion that E. coli RecQ functions as a monomer but also suggest that some of the Superfamily 2 helicases may function through an "inchworm" mechanism.Helicases are molecular motor proteins that use the energy of nucleotide triphosphate hydrolysis to translocate along and separate the complementary strands of a nucleic acid duplex. These enzymes play essential roles in most aspects of the DNA metabolic pathway, such as replication, repair, recombination, and transcription (1-5). A large number of helicases have been identified; however, the mechanisms by which helicases unwind double-stranded DNA (dsDNA) 3 remain obscure.One of the major concerns in studying the unwinding mechanism of a DNA helicase is its oligomeric state. This is because an oligomeric structure could provide multiple potential binding sites for the DNA substrate and nucleotide cofactors, which are absolutely required for the helicase to translocate along the DNA track during the processive DNA unwinding. Indeed, the biochemical and structural studies have shown that some helicases assemble into stable cooperative hexameric rings (3). These helicases include the Escherichia coli DnaB (6, 7) and Rho (8, 9), bacteriophage T4 gp41 (10, 11), and T7 gp4 (12)(13)(14). DNA passes through such a central channel and is unwound with a high processivity. For the non-ring helicases, the enzymes could be assembled into oligomers, and the DNA binding sites may be located on the separate subunits. On the basis of quantitative analyses of DNA binding properties, a "rolling model" was proposed to explain factorial DNA unwinding (15). This model suggests that each monomer of the Rep dimer binds alternatively to ssDNA and dsDNA. This process is regulated by repeated binding and hydrolysis of ATP and release of ADP. In this way, Rep dimer rolls along ...

Escherichia coli UvrD is a non-ring-shaped model helicase, displaying a 3 0 -5 0 polarity in DNA unwinding. Using a transverse magnetic tweezer and DNA hairpins, we measured the unwinding kinetics of UvrD at various DNAdestabilizing forces. The multiform patterns of unwinding bursts and the distributions of the off-times favour the mechanism that UvrD unwinds DNA as a dimer. The two subunits of the dimer coordinate to unwind DNA processively. They can jointly switch strands and translocate backwards on the other strand to allow slow (B40 bp/s) rewinding, or unbind simultaneously to allow quick rehybridization. Partial dissociation of the dimer results in pauses in the middle of the unwinding or increases the translocation rate from B40 to B150 nt/s in the middle of the rewinding. Moreover, the unwinding rate was surprisingly found to decrease from B45 to B10 bp/s when the force is increased from 2 to 12 pN. The results lead to a strained-inchworm mechanism in which a conformational change that bends and tenses the ssDNA is required to activate the dimer.

An improved model representation of mineral dust cycle is critical to reducing the uncertainty of dust-induced environmental and climatic impact. Here we present a mesoscale model study of the seasonal dust activity in the semiarid drylands of Central Asia, focusing on the effects of wind speed, soil moisture, surface roughness heterogeneity, and vegetation phenology on the threshold friction velocity (u*t) and dust emission during the dust season of 1 March to 31 October 2001. The dust model WRF-Chem-DuMo allows us to examine the uncertainties in seasonal dust emissions due to the selection of dust emission scheme and soil grain size distribution data. To account for the vegetation effects on the u*t, we use the Moderate Resolution Imaging Spectroradiometer monthly normalized difference vegetation index to derive the dynamic surface roughness parameters required by the physically based dust schemes of Marticorena and Bergametti (1995, hereinafter MB) and Shao et al. (1996, hereinafter Shao). We find the springtime u*t is strongly enhanced by the roughness effects of temperate steppe and desert ephemeral plants and, to less extent, the binding effects of increased soil moisture. The u*t decreases as the aboveground biomass dies back and soil moisture depletes during summer. The u*t dynamics determines the dust seasonality by causing more summer dust emission, despite a higher frequency of strong winds during spring. Due to the presence of more erodible materials in the saltation diameter range of 60–200 µm, the dry-sieved soil size distribution data lead to eight times more season-total dust emission than the soil texture data, but with minor differences in the temporal distribution. On the other hand, the Shao scheme produces almost the same amount of season-total dust emission as the MB scheme, but with a strong shift toward summer due to the strong sensitivity of the u*t to vegetation. By simply averaging the MB and Shao model experiments, we obtain a mean estimate (Exp_mean) of season-total dust emission of 255.6 Mt (megaton), of which 26.8%, 50.4%, and 22.8% are produced in spring (March-April-May), summer (June-July-August), and autumn (September-October), respectively. The Exp_mean estimate identifies the Ustyurt Plateau, dried seabed of Aral Sea (called Aralkum), Caspian Sea coast, and loess deserts as the strongest dust source areas in Central Asia. The spatial distribution and seasonality of the Exp_mean estimate are in general agreement with ground station dusty weather observations and satellite aerosol optical depth and absorbing aerosol index products. Compared to Cakmur et al. (2006), the Exp_mean estimate suggests Central Asia contributes 10–17% to the global dust emission in 2001.Key PointsThe WRF-Chem-DuMo model is used to study dust seasonality in Central Asia An accurate representation of u*t is critical for dust seasonality Multiexperiment mean dust emission estimate agrees with observations

We present a comprehensive analysis of the interannual variability and trend of dust aerosol in Central Asia (37°-55°N, 50°-80°E) from 2000 to 2014, based on a set of dust emission simulations using the WRF-Chem-DuMo modeling system, observations of dust frequency derived from surface station synoptic weather records, and dust optical depth (DOD) derived from Moderate Resolution Imaging Spectroradiometer (MODIS) and Sea-viewing Wide Field-of-view Sensor (SeaWiFS) aerosol optical depth (AOD) products. Model simulations reveal that the soil grain size distribution has little impact on the interannual variability of dust fluxes but strongly affects their magnitude. The two physically based dust schemes based on Marticorena and Bergametti (1995) (MB) and Shao et al. (1996) (Shao) produce large differences in the dust flux magnitude and spatiotemporal distributions, largely due to different sensitivities of the threshold friction velocity to vegetation-induced surface roughness. By using a fixed threshold velocity, the dust scheme of Tegen and Fung (1995) (TF) relies on the dynamic dust source function to capture the dust variability associated with vegetation changes. Through a correlation analysis, the simulated dust fluxes show good consistency with the observed dust frequency, whereas only the Shao and TF dust fluxes are consistent with the MODIS Collection 5.1 and SeaWiFS DOD. The dust fluxes, dust frequency, and DOD (except MODIS Collection 6) are highly correlated with the frequency of strong surface winds but show different sensitivities to drought and soil erodibility factors (i.e., precipitation, soil moisture, and vegetation) which are influenced by El Niño-Southern Oscillation (ENSO). In general, La Niña years are associated with reduced precipitation, drier soils, less vegetation, and, consequently, more severe drought and enhanced dust activity in Central Asia. The averaged dust flux of the MB and Shao experiments shows a significant negative trend of À2.00 ± 0.59 × 10 À3 g m À2 yr À1 from 2000 to 2014, which is consistent with the trends in the TF dust flux (À1.74 ± 0.34 × 10 À3 g m À2 yr À1 ), dust frequency (À0.63 ± 0.21 × 10 À3 yr À1 ), and SeaWiFS AOD (À3.3 ± 1.3 × 10 À3 AOD yr À1 ), as well as the decreasing tendency in the MODIS AOD after the ENSO effect is removed. The negative dust trend is driven by a decline in the surface winds, which is likely due to changes in large-scale atmospheric circulation rather than the local effect of vegetation-induced surface roughness.

The RecQ helicase family is highly conserved from bacteria to men and plays a conserved role in the preservation of genome integrity. Its deficiency in human cells leads to a marked genomic instability that is associated with premature aging and cancer. DNA helicases catalyze the unwinding of duplex DNA (dsDNA) 1 to provide the single-stranded DNA (ssDNA) intermediate that is required for diverse DNA metabolic processes such as DNA replication, recombination, transcription, and DNA repair (1). DNA helicases function as molecular motors and use the chemical energy derived from nucleoside 5Ј-triphosphate binding or hydrolysis to mechanically disrupt the hydrogen bonds between the two strands in dsDNA and to translocate along DNA for processive unwinding (2).The RecQ helicase family, which is widespread in diverse organisms from Escherichia coli to humans, has become more interesting since the discovery that several human diseases, such as Bloom's, Werner's, and Rothmund-Thompson syndromes, are linked with these enzymes (3-5). Analyses of these and related diseases at the cellular and molecular levels led to the conclusion that RecQ helicase plays a conserved role in the preservation of genomic integrity (6 -8). An increasing amount of data has shown that the RecQ helicase family acts in concert with other proteins to perform functions in multiple processes in vivo. A number of proteins, including topoisomerases (9, 10), primase (11), polymerases (12, 13), proliferating cell nuclear antigen (10), replication protein A (14), P53 protein (15), and FLAG endonuclease 1 (16), have been identified to interact physically and functionally with Werner and Bloom syndrome proteins and RecQ helicases. The prototypical member of the RecQ helicase family, the E. coli RecQ protein, plays an essential role both in DNA recombination and in suppression of illegitimate recombination (17-19). E. coli RecQ helicase (610 amino acids, molecular mass, 68,920 Da) displays a 3Ј-5Ј polarity in DNA unwinding and can unwind diverse DNA substrates including DNA with blunt ends, with 5Ј or 3Ј overhangs, nicked or forked DNA, and threeor four-way junctions as well as G4 DNA (a guanine-rich parallel four-stranded DNA structure) (20, 21). Elucidation of the interactions between helicase and its DNA substrate is a very important step in understanding the mechanism by which helicases bind and unwind dsDNA. Currently little is known about the thermodynamic characteristics of the binding reaction between RecQ and DNA. Additionally, the effect of solution conditions, salt, and type of salt on the intrinsic affinities and the roles of nucleotide cofactors remain unclear. Thus, more knowledge concerning the properties and kinetic mechanism of RecQ helicase binding to DNA is needed to understand the molecular mechanism underlying helicase function. Toward this goal, we have investigated the DNA binding properties of E. coli RecQ helicase under equilibrium conditions using fluorescein-labeled DNA substrates. Free fluorescein-labeled oligonucleotide (ssDNA o...