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 ...