Smc5/6 coordinates formation and resolution of joint molecules with chromosome morphology to ensure meiotic divisions

During meiosis, Structural Maintenance of Chromosome (SMC) complexes underpin two fundamental features of meiosis: homologous recombination and chromosome segregation. While meiotic functions of the cohesin and condensin complexes have been delineated, the role of the third SMC complex, Smc5/6, remains enigmatic. Here we identify specific, essential meiotic functions for the Smc5/6 complex in homologous recombination and the regulation of cohesin. We show that Smc5/6 is enriched at centromeres and cohesin-association sites where it regulates sister-chromatid cohesion and the timely removal of cohesin from chromosomal arms, respectively. Smc5/6 also localizes to recombination hotspots, where it promotes normal formation and resolution of a subset of joint-molecule intermediates. In this regard, Smc5/6 functions independently of the major crossover pathway defined by the MutLγ complex. Furthermore, we show that Smc5/6 is required for stable chromosomal localization of the XPF-family endonuclease, Mus81-Mms4Eme1. Our data suggest that the Smc5/6 complex is required for specific recombination and chromosomal processes throughout meiosis and that in its absence, attempts at cell division with unresolved joint molecules and residual cohesin lead to severe recombination-induced meiotic catastroph

Pervasive and Essential Roles of the Top3-Rmi1 Decatenase Orchestrate Recombination and Facilitate Chromosome Segregation in Meiosis

The Bloom's helicase ortholog, Sgs1, plays central roles to coordinate the formation and resolution of joint molecule intermediates (JMs) during meiotic recombination in budding yeast. Sgs1 can associate with type-I topoisomerase Top3 and its accessory factor Rmi1 to form a conserved complex best known for its unique ability to decatenate double-Holliday junctions. Contrary to expectations, we show that the strand-passage activity of Top3-Rmi1 is required for all known functions of Sgs1 in meiotic recombination, including channeling JMs into physiological crossover and noncrossover pathways, and suppression of non-allelic recombination. We infer that Sgs1 always functions in the context of the Sgs1-Top3-Rmi1 complex to regulate meiotic recombination. In addition, we reveal a distinct late role for Top3-Rmi1 in resolving recombination-dependent chromosome entanglements to allow segregation at anaphase. Surprisingly, Sgs1 does not share this essential role of Top3-Rmi1. These data reveal an essential and pervasive role for the Top3-Rmi1 decatenase during meiosis

Estimation of maize evapotraspiration under drought stress - A case study of Huaibei Plain, China.

Given the importance and complexity of crop evapotranspiration estimation under drought stress, an experiment tailored for maize under drought stress was completed using six sets of large-scale weighing lysimeters at the Xinmaqiao Comprehensive Experimental Irrigation and Drainage Station, Anhui Province, China. Our aim was to analyze maize evapotranspiration under different drought conditions. Based on estimates of maize evapotranspiration under no drought stress using the dual crop coefficient approach, we optimized and calibrated basic crop coefficients Kcbini, Kcbmid, Kcbend, and the maximum crop coefficient Kcmax using a genetic algorithm. Measurements of solar radiation at the experimental station were used to derive the empirical parameters a and b from the Angstrom formula through the genetic algorithm, and then evapotranspiration was calculated for the reference crop (ET0). We then estimated the maize evapotranspiration under drought using the dual crop coefficient approach. The results indicated that a slight water deficit during the earlier stage of vegetative growth may stimulate the maize homeostatic mechanism and increase tolerance to drought stress in later growth periods. Maize evapotranspiration significantly decreased if drought stress continued into the elongation stage, and the same degree of drought stress had a greater influence on the middle and later stages of vegetative and reproductive growth. The calibrated results for Kcbini, Kcbmid, Kcbend, and Kcmax were 0.155, 1.218, 0.420 and 1.497 respectively. We calculated the root-mean-square error (RMSE), mean absolute error (MAE), and mean relative error (MRE) of maize evapotranspiration under no drought stress over the full growing season using a dual crop coefficient approach, and the results were 1.33 mm/day, 0.99 mm/day, and 1.30%, respectively, or 18.40%, 17.50%, and 91.11% lower than results using the recommended coefficients. The RMSE, MAE, and MRE results for maize under drought stress during two full growth periods were 1.18 mm/day, 0.98 mm/day, and 13.92%, respectively. These results were higher than maize without drought stress, but better than the estimated results based on FAO-56 recommended values. Therefore, maize evapotranspiration estimation under drought stress using the dual crop coefficient approach and genetic algorithm was reasonable and reliable. This study provides a theoretical basis for developing suitable regional irrigation programs and decreasing losses due to agricultural drought

Super Black Material from Low-Density Carbon Aerogels with Subwavelength Structures

Many scientists have devoted themselves to the study of the interaction between subwavelength structures and electromagnetic waves. These structures are commonly composed of regular arrays of subwavelength protuberances, which can be artificially designed. However, extending from 2D periodic patterns to 3D disordered subwavelength structures has not been studied yet. In this study, we studied the total diffuse reflectivity of carbon aerogels with various 3D networks of randomly oriented particle-like nanostructures by using normally incident visible light (430–675 nm). We observed that the different 3D network nanostructures of carbon aerogels, especially for the structures with the minimum size, reduced the reflectivity effectively. It was found that the key mechanism for the subwavelength-structure-induced ultralow reflectivity property is due to the decrease of the amplitude of electron vibration forced by the electromagnetic wave, which provides a simple method for designing perfect black materials

Regulated Proteolysis of MutSγ Controls Meiotic Crossing Over

Crossover recombination is essential for accurate chromosome segregation during meiosis. The MutSγ complex, Msh4-Msh5, facilitates crossing over by binding and stabilizing nascent recombination intermediates. We show that these activities are governed by regulated proteolysis. MutSγ is initially inactive for crossing over due to an N-terminal degron on Msh4 that renders it unstable by directly targeting proteasomal degradation. Activation of MutSγ requires the Dbf4-dependent kinase Cdc7 (DDK), which directly phosphorylates and thereby neutralizes the Msh4 degron. Genetic requirements for Msh4 phosphorylation indicate that DDK targets MutSγ only after it has bound to nascent joint molecules (JMs) in the context of synapsing chromosomes. Overexpression studies confirm that the steady-state level of Msh4, not phosphorylation per se, is the critical determinant for crossing over. At the DNA level, Msh4 phosphorylation enables the formation and crossover-biased resolution of double-Holliday Junction intermediates. Our study establishes regulated protein degradation as a fundamental mechanism underlying meiotic crossing over

Sgs1 and MutLγ are functional in <i>smc5/6</i>.

<p>(A) Representative images of 2D analysis from <i>sgs1</i> mutant (<i>P_CLB2-3HA-SGS1</i>) in combination with <i>nse4</i>. (B) Quantification of total joint molecules, crossovers and non-crossovers, and total joint molecule levels at meiotic endpoints (13 hours). Quantification from three independent diploids; error bars represent the standard deviation. (C) Representative images of crossover formation in <i>mlh3</i>Δ mutants, in combination with <i>nse4</i>. (D) Quantification of crossovers, noncrossovers, and total joint molecules levels from three independent diploids (13 hours). (E,F) Analysis of Zip3 foci. Representative images and Tukey-Kramer box-and-whisker plot of 30 nuclei from each strain (boxes represent the 25^th–75^th percentile; the median value is denoted by the horizontal bar, and the whiskers are 1.5× the 25–75^th percentile or max or min. values- whichever are the lowest). Fold increase in Zip3-GFP foci relative to wild type was calculated based on the arithmetic mean (horizontal bar, magenta). Note that the Zip3-GFP causes some polycomplex formation of Zip1 predominantly in the mutants but also in the wild type. The distributions of all four mutant strains were significantly different from wild type (p<0.01, Kruskall-Wallace). Strains: WT (Y1435), <i>sgs1</i> (Y3591), <i>smc5</i> (Y3514), <i>nse4</i> (Y3511), <i>sgs1 nse4</i> (Y3636).</p

Retained arm cohesin at anaphase I contributes to the chromosome resolution defect in the <i>smc5</i> and <i>nse4</i> mutants.

<p>(A) Diagram of bivalent resolution by cohesin (Rec8) cleavage along arms regions Abbreviations: MT-microtubules, CEN-centromeres, scissors depict TEV protease. (B) TEV-9Myc expression after induction during a meiotic time course and the TEV cleavage site introduced into Rec8. Note that Rec8-TEV287-PK retains its two separase (Esp1) cleavage sites. (C) Rec8-TEV287-PK cleavage by TEV protease in <i>ndt80</i>Δ <i>ubr1</i>Δ cells. TEV protease was induced 6 hours into meiosis when >80% are arrested in pachynema. FL-full length Rec8-TEV287-PK. Left panel shows no TEV induction; the middle panels shows TEV induction; and the right panel shows TEV induction and cyclohexamide treatment (CHX) 1.15 hours after induction. Pgk1 was used as a loading control. Strain: Y3380. (D) Experimental set up of TEV protease induction after meiotic prophase by simultaneous induction of TEV protease and prophase exit (<i>NDT80-IN</i>). (E) Analysis of protein levels of Rec8-TEV₂₈₇-PK in arrested and released (<i>NDT80-IN</i>) cells. (F) Nuclear separation at anaphase I. Bar graph shows proportion of tetrads with fully separated, ‘stretched’ or compacted nuclear appearance. The *denotes statistically significant differences (p<0.01, G-test) in the distribution of classes. (G) DNA encapsulation into spores. Bar graph shows proportion of tetrads with fully encapsulated DNA. The *denotes statistically significant differences (p<0.05, G-test) in the distribution of classes. Strains: WT (Y3264- no TEV and Y3299), <i>smc5</i> (Y3261- no TEV and Y3237), and <i>nse4</i> (Y3258- no TEV and Y3240).</p

Model for Smc5/6 function during meiosis.

<p>(A) In wild type cells, Smc5/6 is present and ensures the formation of IH-dHJs either directly or perhaps by removing mcJMs and IS-dHJs, returning them to an interhomolog fate. This could be done in co-operation with helicases and resolvases, potentially Mus81-Mms4. (B) In the absence of Smc5/6, second end regulation is aberrant and cells enter late prophase with increased mcJMs and IS-dHJs. These are not cleaved by Mus81-Mms4, which is hyperphosphorylated by Cdc5, because it requires Smc5/6. Since the joint molecules do not appear to trigger a prophase I checkpoint, <i>smc5/6</i> mutants enter the nuclear divisions with joint molecules as well as precociously separated sister kinetochores that prevent chromosome segregation, leading to meiotic catastrophe.</p

Assessment of meiotic recombination at the <i>HIS4LEU2</i> hotspot.

<p>(A–C) The <i>HIS4LEU2</i> hotspot. mcJM: multichromatid joint molecules (abbreviations: M-Mom, D-Dad), IS-dHJ intersister double Holliday Junctions, IH-dHJ interhomolog double Holliday Junctions, SEI- single-end invasions, DSBs- double strand breaks. Digesting with <i>Xho</i>I gives diagnostic band sizes from parental molecules, Mom and Dad, as well as recombinant fragment lengths (R1 and R2). These are predominantly crossovers. The different molecules can be separated on 1D (A) and shape-dependent separation on 2D gels (C). Further digestion with <i>NgoMIV</i> differentiates noncrossovers from parental molecules (B). The * indicates a non-specific signal.</p