Thermal neutron flux evaluation by a single crystal CVD diamond detector in LHD deuterium experiment

The single crystal CVD diamond detector (SDD) was installed in the torus hall of the Large Helical Device (LHD) to measure neutrons with high time resolution and neutron energy resolution. The LiF foil with 95.62 % of 6Li isotope enrichment pasted on the detector was used as the thermal neutron convertor as the energetic ions of 2.0 MeV alpha and 2.7 MeV triton particles generated in LiF foil and deposited the energy into SDD. SDD were exposed to the neutron field in the torus hall of the LHD during the 2nd campaign of the deuterium experiment. The total pulse height in SDD was linearly propotional to the neutron yield in a plasma operation in LHD over 4 orders of magnitude. The energetic alpha and triton were separately measured by SDD with LiF with the thickness of 1.9 μm, although SDD with LiF with the thickness of 350 μm showed a broadened peak due to the large energy loss of energetic particles generated in the bulk of LiF. The modeling with MCNP and PHITS codes well interpreted the pulse height spectra for SDD with LiF with different thicknesses. The results above demonstrated the sufficient time resolution and energy discrimination of SDD used in this work

Direct and indirect control of the initiation of meiotic recombination by DNA damage checkpoint mechanisms in budding yeast

Meiotic recombination plays an essential role in the proper segregation of chromosomes at meiosis I in many sexually reproducing organisms. Meiotic recombination is initiated by the scheduled formation of genome-wide DNA double-strand breaks (DSBs). The timing of DSB formation is strictly controlled because unscheduled DSB formation is detrimental to genome integrity. Here, we investigated the role of DNA damage checkpoint mechanisms in the control of meiotic DSB formation using budding yeast. By using recombination defective mutants in which meiotic DSBs are not repaired, the effect of DNA damage checkpoint mutations on DSB formation was evaluated. The Tel1 (ATM) pathway mainly responds to unresected DSB ends, thus the sae2 mutant background in which DSB ends remain intact was employed. On the other hand, the Mec1 (ATR) pathway is primarily used when DSB ends are resected, thus the rad51 dmc1 double mutant background was employed in which highly resected DSBs accumulate. In order to separate the effect caused by unscheduled cell cycle progression, which is often associated with DNA damage checkpoint defects, we also employed the ndt80 mutation which permanently arrests the meiotic cell cycle at prophase I. In the absence of Tel1, DSB formation was reduced in larger chromosomes (IV, VII, II and XI) whereas no significant reduction was found in smaller chromosomes (III and VI). On the other hand, the absence of Rad17 (a critical component of the ATR pathway) lead to an increase in DSB formation (chromosomes VII and II were tested). We propose that, within prophase I, the Tel1 pathway facilitates DSB formation, especially in bigger chromosomes, while the Mec1 pathway negatively regulates DSB formation. We also identified prophase I exit, which is under the control of the DNA damage checkpoint machinery, to be a critical event associated with down-regulating meiotic DSB formation

Shaping meiotic chromosomes with SUMO: a feedback loop controls the assembly of the synaptonemal complex in budding yeast

The synaptonemal complex (SC) is a meiosis-specific chromosomal structure in which homologous chromosomes are intimately linked through arrays of specialized proteins called transverse filaments (TF). Widely conserved in eukaryote meiosis, the SC forms during prophase I and is essential for accurate segregation of homologous chromosomes at meiosis I. However, the basic mechanism overlooking formation and regulation of the SC has been poorly understood. By using the budding yeast Saccharomyces cerevisiae, we recently showed that SC formation is controlled through the attachment of multiple molecules of small ubiquitin-like modifier (SUMO) to a regulator of TF assembly. Intriguingly, this SUMOylation is activated by TF, implicating the involvement of a positive feedback loop in the control of SC assembly. We discuss the implication of this finding and possible involvement of a similar mechanism in regulating other processes

Quantitative analysis of meiotic DSB formation.

<p>DSB numbers were calculated using Southern blot data and the formula described in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0065875#s4" target="_blank">Materials and Methods</a>. (A) The effect of the <i>tel1</i> mutation on DSB formation. (B) The <i>tel1</i> mutation reduces DSB formation in the absence of Ndt80 in chromosome VII and II. (C) <i>rad17-mn</i> effect on DSB formation in the presence of various mutations. A whole chromosome was used for DSB number calculation in the <i>sae2</i> mutant strains while one third of a chromosome was employed in the <i>rad51 dmc1</i> mutant strains (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0065875#s4" target="_blank">Materials and Methods</a>). Error bars represent standard error. *, statistically significant (p<0.05, unpaired t-test). The actual data used to calculate DSB numbers are shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0065875#pone.0065875.s003" target="_blank">Table S2</a>.</p

DSB formation is reduced by the <i>tel1</i> mutation and the effect is chromosome specific.

<p>Diploid <i>sae2</i>, <i>sae2 pch2</i> and <i>sae2 tel1</i> mutants in the <i>NDT80</i> positive background or the <i>ndt80</i> mutant background were introduced into meiosis and DSB formation was detected at indicated time points in chromosomes VII (A) and II (B). Lane profiles of 10 and 12 hours in each mutant background were normalized and averaged to obtain the profiles shown on the right. Cells from the same time course were used to examine both chromosomes VII and II. The Southern blot data used for <i>sae2</i> and <i>sae2 pch2</i> are the same as previously shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0065875#pone.0065875-Farmer1" target="_blank">[14]</a>.</p

Comparison of lane profiles of broken meiotic chromosomes.

<p>Lane profiles of Southern blot signals shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0065875#pone-0065875-g003" target="_blank">Figure 3</a> were compared between various mutants as indicated. Lane profiles of 10 and 12 hours in each mutant background were normalized and averaged to obtain the profiles shown.</p

Positive and negative effect of the <i>rad17</i>-mutation on DSB formation.

<p>Diploid <i>rad51 dmc1</i> strains carrying various mutations as indicated, in the <i>NDT80</i> positive background or the <i>ndt80</i> mutant background, were introduced into meiosis and DSB formation was detected at indicated time points in chromosomes VII (A) and II (B).</p