Second harmonic generation response by gold nanoparticles at the polarized water/2-octanone interface: from dispersed to aggregated particles

Gold nanoparticles with a diameter of approximately 20 nm have been observed at the polarized water/2-octanone interface by the nonlinear optical technique of second harmonic generation. Electric field induced adsorption of the gold particles at this liquid/liquid interface is clearly observed and confirms that these are negatively charged. The process is quasi-reversible at high potential sweep rates, but aggregation at the interface is observed at slower sweep rates through the loss of the nonlinear optical signal. The time evolution of the second harmonic signal is also reported during potential step experiments. After a rapid increase due to adsorption, a continuous decrease in the nonlinear optical signal intensity is observed due to aggregation of the particles into large islands at the interface. Diffusion of these large islands at the interface was observed for a longer timescale through large signal fluctuations

Ultrasound Effectiveness of Steroid Injection for hand Psoriatic Dactylitis: Results from a Longitudinal Observational Study

Introduction: To assess clinical and ultrasound effectiveness of steroid injection (local treatment, LT) into the digital flexor tendon sheath for the treatment of psoriatic dactylitis compared to systemic treatment (ST) alone. Methods: In this observational, multicentre, prospective study, 88 cases of symptomatic hand dactylitis were evaluated clinically and sonographically by high-frequency ultrasound (US) probe in both greyscale (GS) and power Doppler (PD). The presence of flexor tenosynovitis (FT), soft tissue oedema (STO), peritendon extensor inflammation and synovitis was assessed (including DACtylitis glObal Sonographic—DACTOS—score) before treatment, at 1-month (T1) and 3-months (T3) follow-up. LT was proposed to all patients. Patients refusing LT were treated with oral NSAIDs. Patients continued the same baseline csDMARDs and/or corticosteroid therapy during the whole follow-up period. US response was defined for DACTOS score < 3 and US remission for DACTOS score = 0. Results: At T3 evaluation the ST group showed a significantly higher persistence (grade > 1) of FT and STO (p < 0.001 for all) and MCP synovitis (p = 0.001). US remission was achieved only in the LT group (at T3 31% vs. 0, p < 0.001). The percentage of patients with DACTOS < 3 was significantly greater in the LT group compared with ST group, at both T1 (49% vs. 5%, p < 0.001) and T3 evaluation (76% vs. 7%, p < 0.001). In multiple conditional logistic regression analysis, the only factor associated with US remission was LT (T3 odds ratio = 41.21, p < 0.001). Conclusions: US confirmed the effectiveness of steroid injection for dactylitis by demonstrating that it involves the resolution of extra-articular inflammation, in particular FT and STO

Single-molecule studies of the stringency factors and rates governing the polymerization of RecA on double-stranded DNA

RecA is a key protein in homologous recombination. During recombination, one single-stranded DNA (ssDNA) bound to site I in RecA exchanges Watson–Crick pairing with a sequence-matched ssDNA that was part of a double-stranded DNA molecule (dsDNA) bound to site II in RecA. After strand exchange, heteroduplex dsDNA is bound to site I. In vivo, direct polymerization of RecA on dsDNA through site I does not occur, though it does in vitro. The mechanisms underlying the difference have been unclear. We use single-molecule experiments to decouple the two steps involved in polymerization: nucleation and elongation. We find that elongation is governed by a fundamental clock that is insensitive to force and RecA concentration from 0.2 and 6 µM, though rates depend on ionic conditions. Thus, we can probe nucleation site stability by creating nucleation sites at high force and then measuring elongation as a function of applied force. We find that in the presence of ATP hydrolysis a minimum force is required for polymerization. The minimum force decreases with increasing RecA or ATP concentrations. We propose that force reduces the off-rate for nucleation site binding and that nucleation site stability is the stringency factor that prevents in vivo polymerization

Dynamics and Regulation of RecA Polymerization and De-Polymerization on Double-Stranded DNA

10.1371/journal.pone.0066712PLoS ONE86

Rad51 Polymerization Reveals a New Chromatin Remodeling Mechanism

Rad51 protein is a well known protagonist of homologous recombination in eukaryotic cells. Rad51 polymerization on single-stranded DNA and its role in presynaptic filament formation have been extensively documented. Rad51 polymerizes also on double-stranded DNA but the significance of this filament formation remains unclear. We explored the behavior of Saccharomyces cerevisiae Rad51 on dsDNA and the influence of nucleosomes on Rad51 polymerization mechanism to investigate its putative role in chromatin accessibility to recombination machinery. We combined biochemical approaches, transmission electron microscopy (TEM) and atomic force microscopy (AFM) for analysis of the effects of the Rad51 filament on chromatinized templates. Quantitative analyses clearly demonstrated the occurrence of chromatin remodeling during nucleoprotein filament formation. During Rad51 polymerization, recombinase proteins moved all the nucleosomal arrays in front of the progressing filament. This polymerization process had a powerful remodeling effect, as Rad51 destabilized the nucleosomes along considerable stretches of DNA. Similar behavior was observed with RecA. Thus, recombinase polymerization is a powerful mechanism of chromatin remodeling. These remarkable features open up new possibilities for understanding DNA recombination and reveal new types of ATP-dependent chromatin dynamics

Structural basis for inhibition of homologous recombination by the RecX protein

The RecA/RAD51 nucleoprotein filament is central to the reaction of homologous recombination (HR). Filament activity must be tightly regulated in vivo as unrestrained HR can cause genomic instability. Our mechanistic understanding of HR is restricted by lack of structural information about the regulatory proteins that control filament activity. Here, we describe a structural and functional analysis of the HR inhibitor protein RecX and its mode of interaction with the RecA filament. RecX is a modular protein assembled of repeated three-helix motifs. The relative arrangement of the repeats generates an elongated and curved shape that is well suited for binding within the helical groove of the RecA filament. Structure-based mutagenesis confirms that conserved basic residues on the concave side of RecX are important for repression of RecA activity. Analysis of RecA filament dynamics in the presence of RecX shows that RecX actively promotes filament disassembly. Collectively, our data support a model in which RecX binding to the helical groove of the filament causes local dissociation of RecA protomers, leading to filament destabilisation and HR inhibition