Tatsuhiro Mori

Destabilization of nucleosomes by an unusual DNA conformation adopted by poly(dA)·poly(dT) tracts in vivo

Poly(dA)·poly(dT) tracts are common and often found upstream of genes in eukaryotes. It has been suggested that poly(dA)·poly(dT) promotes transcription in vivo by affecting nucleosome formation. On the other hand, in vitro studies show that poly(dA)·poly(dT) can be easily incorporated into nucleosomes. Therefore, the roles of these tracts in nucleosome organization in vivo remain to be established. We have developed an assay system that can evaluate nucleosome formation in yeast cells, and demonstrated that relatively longer tracts such as A15TATA16 and A34 disrupt an array of positioned nucleosomes, whereas a shorter A5TATA4 tract is incorporated in positioned nucleosomes of yeast minichromosomes. Thus, nucleosomes are destabilized by poly(dA)·poly(dT) in vivo in a length‐dependent manner. Furthermore, in vivo UV footprinting revealed that the longer tracts adopt an unusual DNA structure in yeast cells that corresponds to the B′ conformation described in vitro. Our results support a mechanism in which a unique poly(dA)· poly(dT) conformation presets chromatin structure to which transcription factors are accessible.

Detection of light-absorbing iron oxide particles using a modified single-particle soot photometer

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Improved technique for measuring the size distribution of black carbon particles in liquid water

ABSTRACT We developed an improved technique for measuring the size distribution of black carbon (BC) particles suspended in liquid water to facilitate quantitative studies of the wet deposition of BC. The measurement system, which consists of a nebulizer and a single-particle soot photometer, incorporates two improvements into the system that we developed earlier. First, we extended the upper limit of the detectable BC size from 0.9 μm to about 4.0 μm by modifying the photo-detector for measuring the laser-induced incandescence signal. Second, we introduced a pneumatic nebulizer (Marin-5) with a high extraction efficiency (∼50.0%) that was independent of particle diameter up to 2.0 μm. For BC mass concentrations less than 70 μg L−1, we experimentally showed that the diameters of BC particles did not appreciably change during the Marin-5 extraction process, consistent with theoretical calculations. Finally, we demonstrated by laboratory experiments that the size distributions of ambient BC particles changed little during their growth into cloud droplets under supersaturation of water vapor. Using our improved system, we measured the size distributions of BC particles simultaneously in air and rainwater in Tokyo during summer 2014. We observed that the size distributions of BC particles in rainwater shifted to larger sizes compared with those observed in ambient air, indicating that larger BC particles in air were removed more efficiently by precipitation. Copyright

A key process controlling the wet removal of aerosols: new observational evidence.

The lifetime and spatial distributions of accumulation-mode aerosols in a size range of approximately 0.05–1 μm, and thus their global and regional climate impacts, are primarily constrained by their removal via cloud and precipitation (wet removal). However, the microphysical process that predominantly controls the removal efficiency remains unidentified because of observational difficulties. Here, we demonstrate that the activation of aerosols to cloud droplets (nucleation scavenging) predominantly controls the wet removal efficiency of accumulation-mode aerosols, using water-insoluble black carbon as an observable particle tracer during the removal process. From simultaneous ground-based observations of black carbon in air (prior to removal) and in rainwater (after removal) in Tokyo, Japan, we found that the wet removal efficiency depends strongly on particle size, and the size dependence can be explained quantitatively by the observed size-dependent cloud-nucleating ability. Furthermore, our observational method provides an estimate of the effective supersaturation of water vapour in precipitating cloud clusters, a key parameter controlling nucleation scavenging. These novel data firmly indicate the importance of quantitative numerical simulations of the nucleation scavenging process to improve the model’s ability to predict the atmospheric aerosol burden and the resultant climate forcings, and enable a new validation of such simulations.