RAD18 O-GlcNAcylation promotes translesion DNA synthesis and homologous recombination repair.

RAD18, an important ubiquitin E3 ligase, plays a dual role in translesion DNA synthesis (TLS) and homologous recombination (HR) repair. However, whether and how the regulatory mechanism of O-linked N-acetylglucosamine (O-GlcNAc) modification governing RAD18 and its function during these processes remains unknown. Here, we report that human RAD18, can undergo O-GlcNAcylation at Ser130/Ser164/Thr468, which is important for optimal RAD18 accumulation at DNA damage sites. Mechanistically, abrogation of RAD18 O-GlcNAcylation limits CDC7-dependent RAD18 Ser434 phosphorylation, which in turn significantly reduces damage-induced PCNA monoubiquitination, impairs Polη focus formation and enhances UV sensitivity. Moreover, the ubiquitin and RAD51C binding ability of RAD18 at DNA double-strand breaks (DSBs) is O-GlcNAcylation-dependent. O-GlcNAcylated RAD18 promotes the binding of RAD51 to damaged DNA during HR and decreases CPT hypersensitivity. Our findings demonstrate a novel role of RAD18 O-GlcNAcylation in TLS and HR regulation, establishing a new rationale to improve chemotherapeutic treatment.

Symmetric control of sister chromatid cohesion establishment.

Besides entrapping sister chromatids, cohesin drives other high-order chromosomal structural dynamics like looping, compartmentalization and condensation. ESCO2 acetylates a subset of cohesin so that cohesion must be established and only be established between nascent sister chromatids. How this process is precisely achieved remains unknown. Here, we report that GSK3 family kinases provide higher hierarchical control through an ESCO2 regulator, CRL4MMS22L. GSK3s phosphorylate Thr105 in MMS22L, resulting in homo-dimerization of CRL4MMS22L and ESCO2 during S phase as evidenced by single-molecule spectroscopy and several biochemical approaches. A single phospho-mimicking mutation on MMS22L (T105D) is sufficient to mediate their dimerization and rescue the cohesion defects caused by GSK3 or MMS22L depletion, whereas non-phosphorylable T105A exerts dominant-negative effects even in wildtype cells. Through cell fractionation and time-course measurements, we show that GSK3s facilitate the timely chromatin association of MMS22L and ESCO2 and subsequently SMC3 acetylation. The necessity of ESCO2 dimerization implicates symmetric control of cohesion establishment in eukaryotes.

Phosphorylation of the LCB1 subunit of Arabidopsis serine palmitoyltransferase stimulates its activity and modulates sphingolipid biosynthesis.

Sphingolipids are the structural components of membrane lipid bilayers and act as signaling molecules in many cellular processes. Serine palmitoyltransferase (SPT) is the first committed and rate-limiting enzyme in the de novo sphingolipids biosynthetic pathway. The core SPT enzyme is a heterodimer consisting of LONG-CHAIN BASE1 (LCB1) and LCB2 subunits. SPT activity is inhibited by orosomucoid proteins and stimulated by small subunits of SPT (ssSPTs). However, whether LCB1 is modified and how such modification might regulate SPT activity have to date been unclear. Here, we show that activation of MITOGEN-ACTIVATED PROTEIN KINASE 3 (MPK3) and MPK6 by upstream MKK9 and treatment with Flg22 (a pathogen-associated molecular pattern) increases SPT activity and induces the accumulation of sphingosine long-chain base t18:0 in Arabidopsis thaliana, with activated MPK3 and MPK6 phosphorylating AtLCB1. Phosphorylation of AtLCB1 strengthened its binding with AtLCB2b, promoted its binding with ssSPTs, and stimulated the formation of higher order oligomeric and active SPT complexes. Our findings therefore suggest a novel regulatory mechanism for SPT activity.

AtMCM10 promotes DNA replication-coupled nucleosome assembly in Arabidopsis.

Minichromosome Maintenance protein 10 (MCM10) is essential for DNA replication initiation and DNA elongation in yeasts and animals. Although the functions of MCM10 in DNA replication and repair have been well documented, the detailed mechanisms for MCM10 in these processes are not well known. Here, we identified AtMCM10 gene through a forward genetic screening for releasing a silenced marker gene. Although plant MCM10 possesses a similar crystal structure as animal MCM10, AtMCM10 is not essential for plant growth or development in Arabidopsis. AtMCM10 can directly bind to histone H3-H4 and promotes nucleosome assembly in vitro. The nucleosome density is decreased in Atmcm10, and most of the nucleosome density decreased regions in Atmcm10 are also regulated by newly synthesized histone chaperone Chromatin Assembly Factor-1 (CAF-1). Loss of both AtMCM10 and CAF-1 is embryo lethal, indicating that AtMCM10 and CAF-1 are indispensable for replication-coupled nucleosome assembly. AtMCM10 interacts with both new and parental histones. Atmcm10 mutants have lower H3.1 abundance and reduced H3K27me1/3 levels with releasing some silenced transposons. We propose that AtMCM10 deposits new and parental histones during nucleosome assembly, maintaining proper epigenetic modifications and genome stability during DNA replication.

HDA-2-Containing Complex Is Required for Activation of Catalase-3 Expression in Neurospora crassa.

It is essential for aerobic organisms to maintain the homeostasis of intracellular reactive oxygen species (ROS) for survival and adaptation to the environment. In line with other eukaryotes, the catalase of Neurospora crassa is an important enzyme for clearing ROS, and its expression is tightly regulated by the growth phase and various oxidative stresses. Our study reveals that, in N. crassa, histone deacetylase 2 (HDA-2) and its catalytic activity positively regulate the expression of the catalase-3 (cat-3) gene. HDA-2, SIF-2, and SNT-1 may form a subcomplex with such a regulation role. As expected, deletion of HDA-2 or SIF-2 subunit increased acetylation levels of histone H4, indicating that loss of HDA-2 complex fails to deacetylate H4 at the cat-3 locus. Furthermore, loss of HDA-2 or its catalytic activity led to dramatic decreases of TFIIB and RNA polymerase II (RNAP II) recruitment at the cat-3 locus and also resulted in high deposition of H2A.Z at the promoter and transcription start site (TSS) regions of the cat-3 gene. Collectively, this study strongly demonstrates that the HDA-2-containing complex activates the transcription of the cat-3 gene by facilitating preinitiation complex (PIC) assembly and antagonizing the inhibition of H2A.Z at the cat-3 locus through H4 acetylation. IMPORTANCE Clearance of reactive oxygen species (ROS) is critical to the survival of aerobic organisms. In the model filamentous fungus Neurospora crassa, catalase-3 (cat-3) expression is activated in response to H2O2-induced ROS stress. We found that histone deacetylase 2 (HDA-2) positively regulates cat-3 transcription in N. crassa; this is widely divergent from the classical repressive role of most histone deacetylases. Like HDA-2, the SIF-2 or SNT-1 subunit of HDA-2-containing complex plays a positive role in cat-3 transcription. Furthermore, we also found that HDA-2-containing complex provides an appropriate chromatin environment to facilitate PIC assembly and to antagonize the inhibition role of H2A.Z at the cat-3 locus through H4 acetylation. Taken together, our results establish a mechanism for how the HDA-2-containing complex regulates transcription of the cat-3 gene in N. crassa.

Synthetic lethality between TP53 and ENDOD1.

The atypical nuclease ENDOD1 functions with cGAS-STING in innate immunity. Here we identify a previously uncharacterized ENDOD1 function in DNA repair. ENDOD1 is enriched in the nucleus following H2O2 treatment and ENDOD1-/- cells show increased PARP chromatin-association. Loss of ENDOD1 function is synthetic lethal with homologous recombination defects, with affected cells accumulating DNA double strand breaks. Remarkably, we also uncover an additional synthetic lethality between ENDOD1 and p53. ENDOD1 depletion in TP53 mutated tumour cells, or p53 depletion in ENDOD1-/- cells, results in rapid single stranded DNA accumulation and cell death. Because TP53 is mutated in ~50% of tumours, ENDOD1 has potential as a wide-spectrum target for synthetic lethal treatments. To support this we demonstrate that systemic knockdown of mouse EndoD1 is well tolerated and whole-animal siRNA against human ENDOD1 restrains TP53 mutated tumour progression in xenograft models. These data identify ENDOD1 as a potential cancer-specific target for SL drug discovery.

Cohesin in DNA damage response and double-strand break repair.

Cohesin, a four-subunit ring comprising SMC1, SMC3, RAD21 and SA1/2, tethers sister chromatids by DNA replication-coupled cohesion (RC-cohesion) to guarantee correct chromosome segregation during cell proliferation. Postreplicative cohesion, also called damage-induced cohesion (DI-cohesion), is an emerging critical player in DNA damage response (DDR). In this review, we sum up recent progress on how cohesin regulates the DNA damage checkpoint activation and repair pathway choice, emphasizing postreplicative cohesin loading and DI-cohesion establishment in yeasts and mammals. DI-cohesion and RC-cohesion show distinct features in many aspects. DI-cohesion near or far from the break sites might undergo different regulations and execute different tasks in DDR and DSB repair. Furthermore, some open questions in this field and the significance of this new scenario to our understanding of genome stability maintenance and cohesinopathies are discussed.

Metabolic remodeling maintains a reducing environment for rapid activation of the yeast DNA replication checkpoint.

Nucleotide metabolism fuels normal DNA replication and is also primarily targeted by the DNA replication checkpoint when replication stalls. To reveal a comprehensive interconnection between genome maintenance and metabolism, we analyzed the metabolomic changes upon replication stress in the budding yeast S. cerevisiae. We found that upon treatment of cells with hydroxyurea, glucose is rapidly diverted to the oxidative pentose phosphate pathway (PPP). This effect is mediated by the AMP-dependent kinase, SNF1, which phosphorylates the transcription factor Mig1, thereby relieving repression of the gene encoding the rate-limiting enzyme of the PPP. Surprisingly, NADPH produced by the PPP is required for efficient recruitment of replication protein A (RPA) to single-stranded DNA, providing the signal for the activation of the Mec1/ATR-Rad53/CHK1 checkpoint signaling kinase cascade. Thus, SNF1, best known as a central energy controller, determines a fast mode of replication checkpoint activation through a redox mechanism. These findings establish that SNF1 provides a hub with direct links to cellular metabolism, redox, and surveillance of DNA replication in eukaryotes.

Improved Production of Xylanase in Pichia pastoris and Its Application in Xylose Production From Xylan.

Xylanases with high specific activity has been focused with great interest as a useful enzyme in biomass utilization. The production of recombinant GH11 xylanase (MYCTH_56237) from Myceliophthora thermophila has been improved through N-terminal signal peptide engineering in P. pastoris. The production of newly recombinant xylanase (termed Mtxyn11C) was improved from 442.53 to 490.7 U/mL, through a replacement of α-factor signal peptide with the native xylanase signal peptide segment (MVSVKAVLLLGAAGTTLA) in P. pastoris. Scaling up of Mtxyn11C production in a 7.5 L fermentor was improved to the maximal production rate of 2503 U/mL. In this study, the degradation efficiency of Mtxyn11C was further examined. Analysis of the hydrolytic mode of action towards the birchwood xylan (BWX) revealed that Mtxyn11C was clearly more effective than commercial xylanase and degrades xylan into xylooligosaccharides (xylobiose, xylotriose, xylotetraose). More importantly, Mtxyn11C in combination with a single multifunctional xylanolytic enzyme, improved the hydrolysis of BWX into single xylose by 40%. Altogether, this study provided strategies for improved production of xylanase together with rapid conversion of xylose from BWX, which provides sustainable, cost-effective and environmental friendly approaches to produce xylose/XOSs for biomass energy or biofuels production.

Novel ß-mannanase/GLP-1 fusion peptide high effectively ameliorates obesity in a mouse model by modifying balance of gut microbiota.

We constructed a novel ß-mannanase/GLP-1 fusion peptide, termed MGLP_1, and evaluated its ability to ameliorate obesity in a high-fat/high-sugar diet (HFSD)-induced mouse model. Eight-wk MGLP_1 treatment notably reduced obesity, as reflected by significant changes of body weight, serum triglyceride level, fatty liver and adipose tissue distribution. Amelioration of HFSD-induced gut dysbiosis by MGLP_1 was evidenced by reduced abundance ratio of bacterial phyla Firmicutes to Bacteroidetes, enhanced abundance of beneficial probiotic genera (Bifidobacterium, Lachnospiraceae, Ileibacterium), and reduced abundance of harmful genera (Clostridium, Romboutsia). Mechanisms of weight loss were investigated by comparing effects of treatment with MGLP_1 vs. prebiotics manno-oligosaccharides (MOS). MGLP_1 ameliorated gut microbiota imbalance by enhancing carbohydrate catabolism, whereas MOS promoted glycan synthesis and metabolism. Our findings, taken together, indicate that MGLP_1 fusion peptide has strong potential for amelioration of obesity by modifying relationships between gut microbiota and lipid and glucose metabolism.

Mthfd2 Modulates Mitochondrial Function and DNA Repair to Maintain the Pluripotency of Mouse Stem Cells.

The pluripotency of stem cells determines their developmental potential. While the pluripotency states of pluripotent stem cells are variable and interconvertible, the mechanisms underlying the acquisition and maintenance of pluripotency remain largely elusive. Here, we identified that methylenetetrahydrofolate dehydrogenase (NAD+-dependent), methenyltetrahydrofolate cyclohydrolase (Mthfd2) plays an essential role in maintaining embryonic stem cell pluripotency and promoting complete reprogramming of induced pluripotent stem cells. Mechanistically, in mitochondria, Mthfd2 maintains the integrity of the mitochondrial respiratory chain and prevents mitochondrial dysfunction. In the nucleus, Mthfd2 stabilizes the phosphorylation of EXO1 to support DNA end resection and promote homologous recombination repair. Our results revealed that Mthfd2 is a dual-function factor in determining the pluripotency of pluripotent stem cells through both mitochondrial and nuclear pathways, ultimately ensuring safe application of pluripotent stem cells.

The acetyltransferase Eco1 elicits cohesin dimerization during S phase.

Cohesin is a DNA-associated protein complex that forms a tripartite ring controlling sister chromatid cohesion, chromosome segregation and organization, DNA replication, and gene expression. Sister chromatid cohesion is established by the protein acetyltransferase Eco1, which acetylates two conserved lysine residues on the cohesin subunit Smc3 and thereby ensures correct chromatid separation in yeast (Saccharomyces cerevisiae) and other eukaryotes. However, the consequence of Eco1-catalyzed cohesin acetylation is unknown, and the exact nature of the cohesive state of chromatids remains controversial. Here, we show that self-interactions of the cohesin subunits Scc1/Rad21 and Scc3 occur in a DNA replication-coupled manner in both yeast and human cells. Using cross-linking MS-based and in vivo disulfide cross-linking analyses of purified cohesin, we show that a subpopulation of cohesin may exist as dimers. Importantly, upon temperature-sensitive and auxin-induced degron-mediated Eco1 depletion, the cohesin-cohesin interactions became significantly compromised, whereas deleting either the deacetylase Hos1 or the Eco1 antagonist Wpl1/Rad61 increased cohesin dimer levels by ∼20%. These results indicate that cohesin dimerizes in the S phase and monomerizes in mitosis, processes that are controlled by Eco1, Wpl1, and Hos1 in the sister chromatid cohesion-dissolution cycle. These findings suggest that cohesin dimerization is controlled by the cohesion cycle and support the notion that a double-ring cohesin model operates in sister chromatid cohesion.

Post-Translational Modifications Aid Archaeal Survival.

Since the pioneering work of Carl Woese, Archaea have fascinated biologists of almost all areas given their unique evolutionary status, wide distribution, high diversity, and ability to grow in special environments. Archaea often thrive in extreme conditions such as high temperature, high/low pH, high salinity, and anoxic ecosystems. All of these are threats to the stability and proper functioning of biological molecules, especially proteins and nucleic acids. Post-translational modifications (PTMs), such as phosphorylation, methylation, acetylation, and glycosylation, are reportedly widespread in Archaea and represent a critical adaptive mechanism to extreme habitats. Here, we summarize our current understanding of the contributions of PTMs to aid in extremophile survival, with a particular focus on the maintenance of genome stability.

Two dominant selectable markers for genetic manipulation in Neurospora crassa.

Neurospora crassa is an excellent model fungus for studies on molecular genetics, biochemistry, physiology, and molecular cell biology. Along with the rapid progress of Neurospora research, new tools facilitating more efficient and accurate genetic analysis are in high demand. Here, we tested whether the dominant selective makers widely used in yeasts are applicable in N. crassa. Among them, we found that the strains of N. crassa are sensitive to the aminoglycoside antibiotics, G418 and nourseothricin. 1000 µg/mL of G418 or 50 µg/mL of nourseothricin is sufficient to inhibit Neurospora growth completely. When the neomycin phosphotransferase gene (neo) used in mammalian cells is expressed, N. crassa shows potent resistance to G418. This establishes G418-resistant marker as a dominant selectable marker to use in N. crassa. Similarly, when the nourseothricin acetyltransferase gene (nat) from Streptomyces noursei is induced by qa-2 promoter in the presence of quinic acid (QA), N. crassa shows potent resistance to nourseothricin. When nat is constitutively expressed by full-length or truncated versions of the promoter from the N. crassa cfp gene (NCU02193), or by the trpC promoter of Aspergillus nidulans, the growth of N. crassa in the presence of nourseothricin is proportional to the expression levels of Nat. Finally, these two markers are used to knock-out wc-2 or al-1 gene from the N. crassa genome. The successful development of these two markers in this study expands the toolbox for N. crassa and very likely for other filamentous fungi as well.

The Emerging Roles of Fox Family Transcription Factors in Chromosome Replication, Organization, and Genome Stability.

The forkhead box (Fox) transcription factors (TFs) are widespread from yeast to humans. Their mutations and dysregulation have been linked to a broad spectrum of malignant neoplasias. They are known as critical players in DNA repair, metabolism, cell cycle control, differentiation, and aging. Recent studies, especially those from the simple model eukaryotes, revealed unexpected contributions of Fox TFs in chromosome replication and organization. More importantly, besides functioning as a canonical TF in cell signaling cascades and gene expression, Fox TFs can directly participate in DNA replication and determine the global replication timing program in a transcription-independent mechanism. Yeast Fox TFs preferentially recruit the limiting replication factors to a subset of early origins on chromosome arms. Attributed to their dimerization capability and distinct DNA binding modes, Fkh1 and Fkh2 also promote the origin clustering and assemblage of replication elements (replication factories). They can mediate long-range intrachromosomal and interchromosomal interactions and thus regulate the four-dimensional chromosome organization. The novel aspects of Fox TFs reviewed here expand their roles in maintaining genome integrity and coordinating the multiple essential chromosome events. These will inevitably be translated to our knowledge and new treatment strategies of Fox TF-associated human diseases including cancer.

Characterization of the dimeric CMG/pre-initiation complex and its transition into DNA replication forks.

The pre-initiation complex (pre-IC) has been proposed for two decades as an intermediate right before the maturation of the eukaryotic DNA replication fork. However, its existence and biochemical nature remain enigmatic. Here, through combining several enrichment strategies, we are able to isolate an endogenous dimeric CMG-containing complex (designated as d-CMG) distinct from traditional single CMG (s-CMG) and in vitro reconstituted dimeric CMG. D-CMG is assembled upon entry into the S phase and shortly matures into s-CMG/replisome, leading to the fact that only ~ 5% of the total CMG-containing complexes can be detected as d-CMG in vivo. Mass spectra reveal that RPA and DNA Pol α/primase co-purify with s-CMG, but not with d-CMG. Consistently, the former fraction is able to catalyze DNA unwinding and de novo synthesis, while the latter catalyzes neither. The two CMGs in d-CMG display flexibly orientated conformations under an electronic microscope. When DNA Pol α-primase is inactivated, d-CMG % rose up to 29%, indicating an incomplete pre-IC/fork transition. These findings reveal biochemical properties of the d-CMG/pre-IC and provide in vivo evidence to support the pre-IC/fork transition as a bona fide step in replication initiation.

Mck1 defines a key S-phase checkpoint effector in response to various degrees of replication threats.

The S-phase checkpoint plays an essential role in regulation of the ribonucleotide reductase (RNR) activity to maintain the dNTP pools. How eukaryotic cells respond appropriately to different levels of replication threats remains elusive. Here, we have identified that a conserved GSK-3 kinase Mck1 cooperates with Dun1 in regulating this process. Deleting MCK1 sensitizes dun1Δ to hydroxyurea (HU) reminiscent of mec1Δ or rad53Δ. While Mck1 is downstream of Rad53, it does not participate in the post-translational regulation of RNR as Dun1 does. Mck1 phosphorylates and releases the Crt1 repressor from the promoters of DNA damage-inducible genes as RNR2-4 and HUG1. Hug1, an Rnr2 inhibitor normally silenced, is induced as a counterweight to excessive RNR. When cells suffer a more severe threat, Mck1 inhibits HUG1 transcription. Consistently, only a combined deletion of HUG1 and CRT1, confers a dramatic boost of dNTP levels and the survival of mck1Δdun1Δ or mec1Δ cells assaulted by a lethal dose of HU. These findings reveal the division-of-labor between Mck1 and Dun1 at the S-phase checkpoint pathway to fine-tune dNTP homeostasis.

Cul4-Ddb1 ubiquitin ligases facilitate DNA replication-coupled sister chromatid cohesion through regulation of cohesin acetyltransferase Esco2.

Cohesin acetyltransferases ESCO1 and ESCO2 play a vital role in establishing sister chromatid cohesion. How ESCO1 and ESCO2 are controlled in a DNA replication-coupled manner remains unclear in higher eukaryotes. Here we show a critical role of CUL4-RING ligases (CRL4s) in cohesion establishment via regulating ESCO2 in human cells. Depletion of CUL4A, CUL4B or DDB1 subunits substantially reduces the normal cohesion efficiency. We also show that MMS22L, a vertebrate ortholog of yeast Mms22, is one of DDB1 and CUL4-associated factors (DCAFs) involved in cohesion. Several lines of evidence show selective interaction of CRL4s with ESCO2 through LxG motif, which is lost in ESCO1. Depletion of either CRL4s or ESCO2 causes a defect in SMC3 acetylation, which can be rescued by HDAC8 inhibition. More importantly, both CRL4s and PCNA act as mediators for efficiently stabilizing ESCO2 on chromatin and catalyzing SMC3 acetylation. Taken together, we propose an evolutionarily conserved mechanism in which CRL4s and PCNA promote ESCO2-dependent establishment of sister chromatid cohesion.

Characterization of Two Endo-ß-1, 4-Xylanases from Myceliophthora thermophila and Their Saccharification Efficiencies, Synergistic with Commercial Cellulase.

The xylanases with high specific activity and resistance to harsh conditions are of high practical value for biomass utilization. In the present study, two new GH11 xylanase genes, MYCTH_56237 and MYCTH_49824, have been cloned from thermophilic fungus Myceliophthora thermophila and expressed in Pichia pastoris. The specific activities of purified xylanases reach approximately 1,533.7 and 1,412.5 U/mg, respectively. Based on multiple template-based homology modeling, the structures of their catalytic domains are predicted. Enzyme activity was more effective in 7.5 L fermentor, yielding 2,010.4 and 2,004.2 U/mL, respectively. Both enzymes exhibit optimal activity at 60°C with pH of 6.0 and 7.0, respectively. Their activities are not affected by EDTA and an array of metal ions. The kinetic constants have been determined for MYCTH_56237 (Km = 8.80 mg/mL, Vmax = 2,380 U/mg) and MYCTH_49824 (Km = 5.67 mg/mL, Vmax = 1,750 U/mg). More importantly, both xylanases significantly cooperate with the commercial cellulase Celluclast 1.5 L in terms of the saccharification efficiency. All these biochemical properties of the xylanases offer practical potential for future applications.