DNA methylation functions in gene silencing and the maintenance of genome integrity. In plants, non-CG DNA methylation is linked through a self-reinforcing loop with histone 3 lysine 9 dimethylation (H3K9me2). The plant-specific SUPPRESSOR OF VARIEGATION 3-9 HOMOLOG (SUVH) family H3K9 methyltransferases (MTases) bind to DNA methylation marks and catalyze H3K9 methylation. Here, we analyzed the structure and function of SUVH6 to understand how this class of enzyme maintains methylation patterns in the genome. We reveal that SUVH6 has a distinct 5-methyl-dC (5mC) base-flipping mechanism involving a thumb loop element. Autoinhibition of H3 substrate entry is regulated by a SET domain loop, and a conformational transition in the post-SET domain upon cofactor binding may control catalysis. In vitro DNA binding and in vivo ChIP-seq data reveal that the different SUVH family H3K9 MTases have distinct DNA binding preferences, targeting H3K9 methylation to sites with different methylated DNA sequences, explaining the context biased non-CG DNA methylation in plants.

[1] The heterogeneous oxidation of SO 2 by O 3 on NaCl particles has been studied using diffuse reflectance infrared Fourier transform spectroscopy. The formation of sulfite and sulfate on the surface was identified, and the roles of O 3 and water in the oxidation processes were determined. The results showed that in the presence of O 3 , SO 2 could be oxidized to sulfate on the surface of NaCl particles. The reaction is first order in O 3 and zero order in SO 2 . The initial reactive uptake coefficient for SO 2 [(0.6-9.8) Â 10 14 molecule cm À3 ] oxidation by O 3 [(1.2-12) Â 10 14 molecule cm À3 ] was determined to be (4.8-0.7) Â 10 À8 using the Brunauer-Emmett-Teller area as the reactive area and (9.8-1.4) Â 10 À5 using the geometric area at 40% relative humidity. A three-stage mechanism that involves the adsorption of O 3 results in an alkalescent surface, the adsorption of SO 2 followed by O 3 oxidation is proposed, and the adsorption of O 3 on the NaCl surface is the rate-determining step. The proposed mechanism can well explain the experiment results. Furthermore, the surface oxidation on mixtures of NaCl with other components such as CaCO 3 , Al 2 O 3 , TiO 2 , MgCl 2 6H 2 O, MgO, elemental carbon, and soot were studied. The reactivity of mixtures can be predicted from the reactivity of the single component with each component weighted by its abundance in the mixture. The catalytic and basic additives could enhance the production of sulfate on the NaCl surface.

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Accurate numerical modeling of fracture propagation and deflection in porous media is important in the development of georesources. To this end, we propose a novel modeling framework to simulate nonplanar three‐dimensional fracture growth within poroelastic media, using an iteratively coupled approach based on time‐/scale‐dependent fracture stiffness. In this approach, the propagating fractures are explicitly tracked and fitted at each growth step using triangular elements that are independent of the matrix discretized by hexahedral grids. The finite volume/finite element method is employed to solve the hydromechanical system, based on the embedded discrete fracture model. The calculated pressure in fractures and the stress state of the host grid of the embedded fractures constitute the boundary conditions for the boundary element method. The boundary element method module, in turn, renders the evolving fracture stiffness and aperture for the finite volume/finite element method module. Finally, the total stresses and the fracture tip displacements are computed at the end of each time step to estimate the velocity and direction of newly created fractures ahead of the fracture tip. The proposed model is first validated against analytical solutions. Then, in three different examples, results are shown from the fracture's footprint under layered stress conditions, simultaneous propagation of two nonplanar three‐dimensional fractures, and the mechanical interaction of en échelon arrays. This work presents an efficient framework to simulate propagation of nonplanar fractures and establishes the foundation to build an integrated simulator for fracture propagation, proppant transport, and production forecasting in unconventional formations.

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Summary Massive hydraulic fracturing requires an enormous consumption of water and introduces many potential environmental issues. In addition, water-based fluid tends to be trapped in formations, reducing oil/gas-phase relative permeability, and causes clay-mineral swelling, which lowers absolute permeability. Carbon dioxide (CO2) is seen as a promising alternative working fluid that poses no formation-damage risk, and it can stimulate more-complex and extensive fracture networks. However, very little, if any, extant research has quantitatively analyzed the effectiveness of CO2 fracturing, except for some qualitative fracturing experiments that are based on acoustic emissions. In this study, we systematically examine water and CO2 fracturing, and compare their performance on the basis of a rigorously coupled geomechanics and a fluid-heat-flow model. Parameters investigated include fluid viscosity, compressibility, in-situ stress, and rock permeability, illustrating how they affect breakdown pressure (BP) and leakoff, as well as fracturing effectiveness. It is found that (1) CO2 has the potential to lower BP, benefiting the propagation of fractures; (2) water fracturing tends to create wider and longer tensile fractures compared with CO2 fracturing, thereby facilitating proppant transport and placement; (3) CO2 fracturing could dramatically enhance the complexity of artificial fracture networks even under high-stress-anisotropy conditions; (4) thickened CO2 tends to generate simpler fracture networks than does supercritical CO2 (SC-CO2), but still more-complex fracture networks than fresh water; and (5) the alternative fracturing scheme (i.e., SC-CO2 fracturing followed by thickened-CO2 fracturing) can readily create complex fracture networks and carry proppant to keep hydraulic fractures open. This study reveals that, for intact reservoirs, water-based fracturing can achieve better fracturing performance than CO2 fracturing; however, for naturally fractured reservoirs, CO2 fracturing can constitute an effective way to stimulate tight/shale oil/gas reservoirs, thereby improving oil/gas production.

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Summary A fully coupled thermal/hydromechanical (THM) model for hydraulic-fracturing treatments is developed in this study. In this model, the mixed finite-volume/finite-element method is used to solve the coupled system, in which the multipoint flux approximation L-method is used to calculate interelement fluid and heat flux. The Gu et al. (2011) crossing criterion is extended to a 3D scenario to delineate the crossing behaviors as hydraulic fractures meet inclined natural fractures. Moreover, the modified Barton et al. (1985) model proposed by Asadollahi et al. (2010) is used to estimate the fracture aperture and model the shear-dilation effect. After being (partially) verified by means of comparison with results from the literature, the developed model is used to investigate complex-fracture-network propagation in naturally fractured reservoirs. Numerical experiments show that the key factors controlling the complexity of the induced-fracture networks include stress anisotropy, injection rate, natural-fracture distribution (fracture-dip angle, strike angle, spacing, density, and length), fracture-filling properties (the degree of cementation and permeability), fracture-surface properties (cohesion and friction angle), and tensile strength of intact rock. It is found that the smaller the stress anisotropy and/or the lower the injection rate, the more complex the fracture network; a high rock tensile strength could increase the possibility of the occurrence of shear fractures; and under conditions of large permeability of fracture filling combined with small cohesive strength and friction coefficient, shear slip could become the dominant mechanism for generating complex-fracture networks. The model developed and the results presented can be used to understand the propagation of complex-fracture networks and aid in the design and optimization of hydraulic-fracturing treatments.