Lipid- and glucose-lowering effects of Rhamnan sulphate from Monostroma nitidum with altered gut microbiota in mice.

Rhamnan sulphate (RS) is a sulphated polysaccharide found in green algae such as Monostroma nitidum that exhibits various biological functions, including anticoagulant, antitumour, antiviral, and anti-obesity properties. In our previous clinical trial, we demonstrated that RS intake improves constipation. However, no specific bacteria showed a significant (p < .05) change. Notably, these results were obtained after a short RS inoculation period of only 2 weeks. In the present study, to evaluate the long-term effects of RS on the gut microbiota, we orally administered RS to BALB/c mice for 11 weeks, analyzed their blood biochemical data, and performed 16s rRNA-sequencing. Oral administration of RS increased body weight with increased food intake, whereas plasma total cholesterol and fasting plasma glucose levels decreased. RS-fed mice showed lower fasting insulin levels (p < .1) and decreased homeostatic model assessment for insulin resistance (HOMA-IR, p < .0001), suggesting that RS improved insulin resistance. In the feces of mice, the amounts of acetic and propionic acids increased. In the gut microbiota, predictive metagenomic profiling using the phylogenetic investigation of communities by reconstruction of unobserved states (PICRUSt2) revealed functional alterations in Kyoto Encyclopaedia of Genes and Genomes (KEGG) pathways in RS-fed mice. Corresponding to the blood glucose-lowering effect, the glycolysis and tricarboxylic acid (TCA) cycle pathways were activated. In addition, the Firmicutes/Bacteroides (F/B) ratio, which may be associated with various health outcomes, was also reduced. These results suggest that the blood glucose-lowering effect, improvement in insulin resistance, and lipid-lowering effect of RS may be due to changes in the intestinal microbiota.

Oral Administration of Rhamnan Sulfate from Monostroma nitidum Suppresses Atherosclerosis in ApoE-Deficient Mice Fed a High-Fat Diet.

Oral administration of rhamnan sulfate (RS), derived from the seaweed Monostroma nitidum, markedly suppresses inflammatory damage in the vascular endothelium and organs of lipopolysaccharide-treated mice. This study aimed to analyze whether orally administered RS inhibits the development of atherosclerosis, a chronic inflammation of the arteries. ApoE-deficient female mice were fed a normal or high-fat diet (HFD) with or without RS for 12 weeks. Immunohistochemical and mRNA analyses of atherosclerosis-related genes were performed. The effect of RS on the migration of RAW264.7 cells was also examined in vitro. RS administration suppressed the increase in blood total cholesterol and triglyceride levels. In the aorta of HFD-fed mice, RS reduced vascular smooth muscle cell proliferation, macrophage accumulation, and elevation of VCAM-1 and inhibited the reduction of Robo4. Increased mRNA levels of Vcam1, Mmp9, and Srebp1 in atherosclerotic areas of HFD-fed mice were also suppressed with RS. Moreover, RS directly inhibited the migration of RAW264.7 cells in vitro. Thus, in HFD-fed ApoE-deficient mice, oral administration of RS ameliorated abnormal lipid metabolism and reduced vascular endothelial inflammation and hyperpermeability, macrophage infiltration and accumulation, and smooth muscle cell proliferation in the arteries leading to atherosclerosis. These results suggest that RS is an effective functional food for the prevention of atherosclerosis.

Beneficial effects of seaweed-derived components on metabolic syndrome via gut microbiota modulation.

Metabolic syndrome comprises a group of conditions that collectively increase the risk of abdominal obesity, diabetes, atherosclerosis, cardiovascular diseases, and cancer. Gut microbiota is involved in the pathogenesis of metabolic syndrome, and microbial diversity and function are strongly affected by diet. In recent years, epidemiological evidence has shown that the dietary intake of seaweed can prevent metabolic syndrome via gut microbiota modulation. In this review, we summarize the current in vivo studies that have reported the prevention and treatment of metabolic syndrome via seaweed-derived components by regulating the gut microbiota and the production of short-chain fatty acids. Among the surveyed related articles, animal studies revealed that these bioactive components mainly modulate the gut microbiota by reversing the Firmicutes/Bacteroidetes ratio, increasing the relative abundance of beneficial bacteria, such as Bacteroides, Akkermansia, Lactobacillus, or decreasing the abundance of harmful bacteria, such as Lachnospiraceae, Desulfovibrio, Lachnoclostridium. The regulated microbiota is thought to affect host health by improving gut barrier functions, reducing LPS-induced inflammation or oxidative stress, and increasing bile acid production. Furthermore, these compounds increase the production of short-chain fatty acids and influence glucose and lipid metabolism. Thus, the interaction between the gut microbiota and seaweed-derived bioactive components plays a critical regulatory role in human health, and these compounds have the potential to be used for drug development. However, further animal studies and human clinical trials are required to confirm the functional roles and mechanisms of these components in balancing the gut microbiota and managing host health.

A germline-specific role for unconventional components of the γ-tubulin complex in Caenorhabditis elegans.

The γ-tubulin complex (γTuC) is a widely conserved microtubule nucleator, but some of its components, namely GCP4, GCP5 and GCP6 (also known as TUBGCP4, TUBGCP5 and TUBGCP6, respectively), have not been detected in Caenorhabditis elegans. Here, we identified two γTuC-associated proteins in C. elegans, GTAP-1 and GTAP-2, for which apparent orthologs were detected only in the genus Caenorhabditis. GTAP-1 and GTAP-2 were found to localize at centrosomes and the plasma membrane of the germline, and their centrosomal localization was interdependent. In early C. elegans embryos, whereas the conserved γTuC component MZT-1 (also known as MOZART1 and MZT1) was essential for the localization of centrosomal γ-tubulin, depletion of GTAP-1 and/or GTAP-2 caused up to 50% reduction of centrosomal γ-tubulin and precocious disassembly of spindle poles during mitotic telophase. In the adult germline, GTAP-1 and GTAP-2 contributed to efficient recruitment of the γTuC to the plasma membrane. Depletion of GTAP-1, but not of GTAP-2, severely disrupted both the microtubule array and the honeycomb-like structure of the adult germline. We propose that GTAP-1 and GTAP-2 are unconventional components of the γTuC that contribute to the organization of both centrosomal and non-centrosomal microtubules by targeting the γTuC to specific subcellular sites in a tissue-specific manner.

Anti-Inflammatory Activity of Orally Administered Monostroma nitidum Rhamnan Sulfate against Lipopolysaccharide-Induced Damage to Mouse Organs and Vascular Endothelium.

We previously reported that rhamnan sulfate (RS) purified from Monostroma nitidum significantly suppressed lipopolysaccharide (LPS)-induced inflammation in cultured human vascular endothelial cells. Here, we analyzed the effect of orally administered RS on LPS-induced damage to mouse organs and vascular endothelium. RS (1 mg) was orally administered daily to BALB/c mice, 50 µg of LPS was intraperitoneally administered on day 8, and Evans blue was injected into the tail vein 6 h later. After 30 min, LPS-treated mice showed pulmonary Evans blue leakage and elevated plasma levels of liver damage markers, whereas this reaction was suppressed in LPS + RS-treated mice. Immunohistochemical and Western blot analysis of mouse organs 24 h after LPS treatment showed significant neutrophil infiltration into the lung, liver, and jejunum tissues of LPS-treated mice and high expression levels of inflammation-related factors in these tissues. Expression levels of these factors were significantly suppressed in LPS + RS-treated mice. Analysis of lung glycocalyx showed a significant reduction in glycocalyx in LPS-treated mice but not in LPS + RS-treated mice. Levels of syndecan-4, one of the glycocalyx components, decreased in LPS-treated mice and increased in LPS + RS-treated mice. The current results suggest that orally administered RS protects organs and vascular endothelium from LPS-induced inflammation and maintains blood circulation.

Rhamnan sulphate from green algae Monostroma nitidum improves constipation with gut microbiome alteration in double-blind placebo-controlled trial.

Rhamnan sulphate (RS), a sulphated polysaccharide from Monostroma nitidum, possesses several biological properties that help in treating diseases such as viral infection, thrombosis, and obesity. In the present study, we first administered RS (0.25 mg/g food volume) orally to high-fat diet-treated mice for 4 weeks. RS increased the faecal volume and calorie excretion with decreased plasma lipids, which was in accordance with the results of our previous zebrafish study. Notably, as the excretion amount by RS increased in the mice, we hypothesised that RS could decrease the chance of constipation in mice and also in human subjects because RS is considered as a dietary fibre. We administrated RS (100 mg/day) to subjects with low defaecation frequencies (3-5 times/week) for 2 weeks in double-blind placebo-controlled manner. As a result, RS administration significantly increased the frequency of dejection without any side effects, although no effect was observed on the body weight and blood lipids. Moreover, we performed 16s rRNA-seq analysis of the gut microbiota in these subjects. Metagenomics profiling using PICRUSt revealed functional alternation of the KEGG pathways, which could be involved in the therapeutic effect of RS for constipation.

Anti-Influenza A Virus Activity of Rhamnan Sulfate from Green Algae Monostroma nitidum in Mice with Normal and Compromised Immunity.

Influenza viruses cause a significant public health burden each year despite the availability of anti-influenza drugs and vaccines. Therefore, new anti-influenza virus agents are needed. Rhamnan sulfate (RS) is a sulfated polysaccharide derived from the green alga Monostroma nitidum. Here, we aimed to demonstrate the antiviral activity of RS, especially against influenza A virus (IFV) infection, in vitro and in vivo. RS showed inhibitory effects on viral proliferation of enveloped viruses in vitro. Evaluation of the anti-IFV activity of RS in vitro showed that it inhibited both virus adsorption and entry steps. The oral administration of RS in IFV-infected immunocompetent and immunocompromised mice suppressed viral proliferation in both mouse types. The oral administration of RS also had stimulatory effects on neutralizing antibody production. Fluorescent analysis showed that RS colocalized with M cells in Peyer's patches, suggesting that RS bound to the M cells and may be incorporated into the Peyer's patches, which are essential to intestinal immunity. In summary, RS inhibits influenza virus infection and promotes antibody production, suggesting that RS is a potential candidate for the treatment of influenza virus infections.

Biological Activities of Rhamnan Sulfate Extract from the Green Algae Monostroma nitidum (Hitoegusa).

Monostroma nitidum is a green single-cell layered algae that grows on the southwest coast of Japan. It is often used for salad ingredients, boiled tsukudani, soups, etc., due to its health benefits. M. nitidum is composed of many cell aggregates, and the various substances that fill the intercellular space are dietary fibers, vitamins, and minerals. Rhamnan sulfate (RS), a sulfated polysaccharide, is main the component of the fiber extracted from M. nitidum. Recently, some biological properties of RS have been demonstrated by in vitro and in vivo studies that probably protect human subjects from viruses and ameliorate vascular dysfunction caused by metabolic disorders, especially lifestyle-related diseases. In this review, we focus on the antithrombotic effects of RS and introduce its antiviral and other biological activities.

Rhamnan sulfate extracted from Monostroma nitidum attenuates blood coagulation and inflammation of vascular endothelial cells.

Rhamnan sulfate (RS) is a polysaccharide with a rhamnose backbone isolated from Monostroma nitidum. Like heparin, it exerts anticoagulant activity in the presence of antithrombin. Endothelial cells facilitate the crosstalk between blood coagulation and vascular inflammation. In this study, we compared the effect of RS with that of heparin on blood coagulation and vascular endothelial cells in the presence or absence of inflammatory factors, using human umbilical vein endothelial cells. We found that RS significantly enhances inhibition of thrombin and factor Xa in the presence of antithrombin as well as heparin, and that RS inhibits tissue factor expression and von Willebrand factor release from the endothelial cells treated with or without lipopolysaccharide, tumor necrosis factor-α, or thrombin. Heparin did not show any effects on endothelial cell inflammation. Our findings suggest that RS, like heparin, is an antithrombin-dependent anticoagulant and, unlike heparin, is a potent anti-inflammatory agent acting on vascular endothelial cells.

Novel Anti-Obesity Properties of Palmaria mollis in Zebrafish and Mouse Models.

(1) Background: The red seaweed Palmaria mollis (PM), which has a bacon-like taste, is increasingly being included in Western diets. In this study, we evaluate anti-obesity effects of PM using diet-induced obese (DIO) zebrafish and mice models. (2) Methods: We fed PM-containing feed to DIO-zebrafish and mice, and evaluated the anti-obesity effects We also analyzed gene expression changes in their liver and visceral adipose tissues (VAT). (3) Results: PM ameliorated several anti-obesity traits in both animals, including dyslipidaemia, hepatic steatosis, and visceral adiposity. In liver tissues of DIO-zebrafish and mice, PM upregulated gene expressions involved in peroxisome proliferator-activated receptor alpha (PPARA) pathways, and downregulated peroxisome proliferator-activated receptor gamma (PPARG) pathways, suggesting that the lipid-lowering effect of PM might be caused by activation of beta-oxidation and inhibition of lipogenesis. In VAT, PM downregulated genes involved in early and late adipocyte differentiation in zebrafish, but not in mice. (4) Conclusions: We have demonstrated that PM can prevent hepatic steatosis and visceral adiposity for the first time. Dietary supplementation of PM as a functional food may be suitable for obesity prevention and reduction in the prevalence of obesity-related diseases.

The MRX Complex Ensures NHEJ Fidelity through Multiple Pathways Including Xrs2-FHA-Dependent Tel1 Activation.

Because DNA double-strand breaks (DSBs) are one of the most cytotoxic DNA lesions and often cause genomic instability, precise repair of DSBs is vital for the maintenance of genomic stability. Xrs2/Nbs1 is a multi-functional regulatory subunit of the Mre11-Rad50-Xrs2/Nbs1 (MRX/N) complex, and its function is critical for the primary step of DSB repair, whether by homologous recombination (HR) or non-homologous end joining. In human NBS1, mutations result truncation of the N-terminus region, which contains a forkhead-associated (FHA) domain, cause Nijmegen breakage syndrome. Here we show that the Xrs2 FHA domain of budding yeast is required both to suppress the imprecise repair of DSBs and to promote the robust activation of Tel1 in the DNA damage response pathway. The role of the Xrs2 FHA domain in Tel1 activation was independent of the Tel1-binding activity of the Xrs2 C terminus, which mediates Tel1 recruitment to DSB ends. Both the Xrs2 FHA domain and Tel1 were required for the timely removal of the Ku complex from DSB ends, which correlates with a reduced frequency of imprecise end-joining. Thus, the Xrs2 FHA domain and Tel1 kinase work in a coordinated manner to maintain DSB repair fidelity.

Double-strand break repair-adox: Restoration of suppressed double-strand break repair during mitosis induces genomic instability.

Double-strand breaks (DSBs) are one of the severest types of DNA damage. Unrepaired DSBs easily induce cell death and chromosome aberrations. To maintain genomic stability, cells have checkpoint and DSB repair systems to respond to DNA damage throughout most of the cell cycle. The failure of this process often results in apoptosis or genomic instability, such as aneuploidy, deletion, or translocation. Therefore, DSB repair is essential for maintenance of genomic stability. During mitosis, however, cells seem to suppress the DNA damage response and proceed to the next G1 phase, even if there are unrepaired DSBs. The biological significance of this suppression is not known. In this review, we summarize recent studies of mitotic DSB repair and discuss the mechanisms of suppression of DSB repair during mitosis. DSB repair, which maintains genomic integrity in other phases of the cell cycle, is rather toxic to cells during mitosis, often resulting in chromosome missegregation and aberration. Cells have multiple safeguards to prevent genomic instability during mitosis: inhibition of 53BP1 or BRCA1 localization to DSB sites, which is important to promote non-homologous end joining or homologous recombination, respectively, and also modulation of the non-homologous end joining core complex to inhibit DSB repair. We discuss how DSBs during mitosis are toxic and the multiple safeguard systems that suppress genomic instability.

Canonical non-homologous end joining in mitosis induces genome instability and is suppressed by M-phase-specific phosphorylation of XRCC4.

DNA double-strand breaks (DSBs) can be repaired by one of two major pathways-non-homologous end-joining (NHEJ) and homologous recombination (HR)-depending on whether cells are in G1 or S/G2 phase, respectively. However, the mechanisms of DSB repair during M phase remain largely unclear. In this study, we demonstrate that transient treatment of M-phase cells with the chemotherapeutic topoisomerase inhibitor etoposide induced DSBs that were often associated with anaphase bridge formation and genome instability such as dicentric chromosomes. Although most of the DSBs were carried over into the next G1 phase, some were repaired during M phase. Both NHEJ and HR, in particular NHEJ, promoted anaphase-bridge formation, suggesting that these repair pathways can induce genome instability during M phase. On the other hand, C-terminal-binding protein interacting protein (CtIP) suppressed anaphase bridge formation, implying that CtIP function prevents genome instability during mitosis. We also observed M-phase-specific phosphorylation of XRCC4, a regulatory subunit of the ligase IV complex specialized for NHEJ. This phosphorylation required cyclin-dependent kinase (CDK) activity as well as polo-like kinase 1 (Plk1). A phosphorylation-defective XRCC4 mutant showed more efficient M-phase DSB repair accompanied with an increase in anaphase bridge formation. These results suggest that phosphorylation of XRCC4 suppresses DSB repair by modulating ligase IV function to prevent genome instability during M phase. Taken together, our results indicate that XRCC4 is required not only for the promotion of NHEJ during interphase but also for its M-phase-specific suppression of DSB repair.

Cyclin-dependent kinase-dependent phosphorylation of Lif1 and Sae2 controls imprecise nonhomologous end joining accompanied by double-strand break resection.

DNA double-strand breaks (DSBs) are repaired by two distinct pathways, homologous recombination (HR) and nonhomologous end joining (NHEJ). NHEJ includes two pathways, that is, precise and imprecise end joining. We found that Lif1, a component of the DNA ligase IV complex in Saccharomyces cerevisiae, was phosphorylated by cyclin-dependent kinase (CDK) at Ser261 during the S to G2 phase but not during G1 phase. This phosphorylation was required for efficient NHEJ in G2/M cells, rather than in G1 cells. It also promotes the stable binding of Lif1 protein to DSBs, specifically in G2/M-arrested cells, which shows the resection of DSB ends. Thus, Lif1 phosphorylation plays a critical role in a certain type of imprecise NHEJ accompanied by DSB end resection and micro-homology. Lif1 phosphorylation at Ser261 is probably involved in micro-homology-dependent end joining associated with producing single-stranded DSB ends that are formed by Sae2 as early intermediates in the HR pathway. CDK-dependent modification of the NHEJ pathway might make DSB ends compatible for NHEJ and thus prevent competition between HR and NHEJ in hierarchy on the choice of DSB repair pathways.

A kinase-independent role for Aurora A in the assembly of mitotic spindle microtubules in Caenorhabditis elegans embryos.

The assembly of a functional mitotic spindle is crucial for achieving successful mitosis. Aurora A kinase is one of the key regulators of mitotic events, including mitotic entry, centrosome maturation and spindle bipolarity. Caenorhabditis elegans Aurora A (AIR-1) is responsible for the assembly of γ-tubulin-independent microtubules in early embryos; however, the mechanism by which AIR-1 contributes to microtubule assembly during mitosis has been unclear. Here we show by live-cell imaging and RNA-mediated interference (RNAi)-based modulation of gene activity that AIR-1 has a crucial role in the assembly of chromatin-stimulated microtubules that is independent of the γ-tubulin complex. Surprisingly, the kinase activity of AIR-1 is dispensable for this process. Although the kinase-inactive form of AIR-1 was detected along the microtubules as well as on centrosomes, the kinase-active form of AIR-1 was restricted to centrosomes. Thus, we propose that AIR-1 has a kinase-dependent role at centrosomes and a kinase-independent role for stabilizing spindle microtubules and that coordination of these two roles is crucial for the assembly of mitotic spindles.

Caenorhabditis elegans ortholog of the p24/p22 subunit, DNC-3, is essential for the formation of the dynactin complex by bridging DNC-1/p150(Glued) and DNC-2/dynamitin.

Dynactin is a multisubunit protein complex required for the activity of cytoplasmic dynein. In Caenorhabditis elegans, although 10 of the 11 dynactin subunits were identified based on the sequence similarities to their orthologs, the p24/p22 subunit has not been detected in the genome. Here, we demonstrate that DNC-3 (W10G11.20) is the functional counterpart of the p24/p22 subunit in C. elegans. RNAi phenotypes and subcellular localization of DNC-3 in early C. elegans embryos were nearly identical to those of the known dynactin components. All other dynactin subunits were co-immunoprecipitated with DNC-3, indicating that DNC-3 is a core component of dynactin. Furthermore, the overall secondary structure of DNC-3 resembles to those of the mammalian and yeast p24/p22. We found that DNC-3 is required for the localization of the DNC-1/p150(Glued) and DNC-2/dynamitin, the two components of the projection arm of dynactin, to the nuclear envelope of meiotic nuclei in the adult gonad. Moreover, DNC-3 physically interacted with DNC-1 and DNC-2 and significantly enhanced the binding ability between DNC-1 and DNC-2 in vitro. These results suggest that DNC-3 is essential for the formation of the projection arm subcomplex of dynactin.

Sae2p phosphorylation is crucial for cooperation with Mre11p for resection of DNA double-strand break ends during meiotic recombination in Saccharomyces cerevisiae.

Meiotic recombination is initiated by the introduction of DNA double-strand breaks (DSBs) at recombination hotspots. DSB ends are resected to yield ssDNA, which is used in a homology search. Sae2p, which is involved in the resection of DSB ends, is phosphorylated by the Mec1p and Tel1p kinases during meiosis. To clarify the role of Sae2p phosphorylation in meiotic recombination, three mutants with alanine substitutions (at two putative Mec1/Tel1 phosphorylation sites near the N terminus, at three sites near the C terminus or at all five sites) were constructed. Analysis of DSB ends during meiotic recombination demonstrated that phosphorylation of the three C-terminal phosphorylation sites is necessary for DSB end resection and that phosphorylation of the two N-terminal phosphorylation sites is required for the efficient initiation of DSB end resection. Sae2p was localized on meiotic chromosomes in the rad50S and mre11-H125R mutants, which accumulate DSB ends. Alanine substitutions of all phosphorylation sites did not affect localization of Sae2p on meiotic chromosomes. Although colocalization of Sae2p with Mre11p and recombinant formation were observed in the N-terminally mutated and the C-terminally mutated strains, these processes were drastically impaired in the quintuple mutant. These results indicate that phosphorylation of Sae2p is required to initiate resection and to improve the efficiency of resection through cooperation with the Mre11-Rad50-Xrs2 complex.

Meiotic recombination-related DNA synthesis and its implications for cross-over and non-cross-over recombinant formation.

Meiotic recombination-related DNA synthesis (MRDS) was analyzed in Saccharomyces cerevisiae by specifically timed incorporation of thymidine analogs into chromosomes. Lengths and positions of incorporation tracts were determined relative to a known recombination hot spot along DNA, as was the timing and localization of incorporation relative to forming and formed synaptonemal complex in spread chromosomes. Distinct patterns could be specifically associated with the majority cross-over and non-cross-over recombination processes. The results obtained provide direct evidence for key aspects of current consensus recombination models, provide information regarding temporal and spatial relationships between non-cross-over formation and the synaptonemal complex, and raise the possibility that removal of RecA homolog Rad51 plays a key role in regulating onset of MRDS. Finally, classical observations on MRDS in Drosophila, mouse, and lily are readily mapped onto the findings presented here, providing further evidence for a broadly conserved meiotic recombination process.

Condensin loaded onto the replication fork barrier site in the rRNA gene repeats during S phase in a FOB1-dependent fashion to prevent contraction of a long repetitive array in Saccharomyces cerevisiae.

An average of 200 copies of the rRNA gene (rDNA) is clustered in a long tandem array in Saccharomyces cerevisiae. FOB1 is known to be required for expansion/contraction of the repeats by stimulating recombination, thereby contributing to the maintenance of the average copy number. In Deltafob1 cells, the repeats are still maintained without any fluctuation in the copy number, suggesting that another, unknown system acts to prevent repeat contraction. Here, we show that condensin acts together with FOB1 in a functionally complemented fashion to maintain the long tandem repeats. Six condensin mutants possessing severely contracted rDNA repeats were isolated in Deltafob1 cells but not in FOB1+ cells. We also found that the condensin complex associated with the nontranscribed spacer region of rDNA with a major peak coincided with the replication fork barrier (RFB) site in a FOB1-dependent fashion. Surprisingly, condensin association with the RFB site was established during S phase and was maintained until anaphase. These results indicate that FOB1 plays a novel role in preventing repeat contraction by regulating condensin association and suggest a link between replication termination and chromosome condensation and segregation.

Xrs2p regulates Mre11p translocation to the nucleus and plays a role in telomere elongation and meiotic recombination.

The Mre11-Rad50-Xrs2 (MRX) protein complex plays pivotal roles in meiotic recombination, repair of damaged DNA, telomere elongation, and cell cycle checkpoint control. Xrs2p is known to be essential for all the functions of the complex, but its role in the complex has not been clearly elucidated. A 32-amino acid region near the C terminus of Xrs2p was identified as an Mre11p-binding site. No more function of Xrs2p than translocation of Mre11p from the cytoplasm to the nucleus is necessary for response to DNA damage. However, domains in Xrs2p located both 49 amino acids upstream and 104 amino acids downstream of the Mre11p binding site are required for meiotic recombination and telomere elongation, respectively, in addition to the 32-amino acid region. These findings demonstrate that Xrs2p acts as a specificity factor that allows the MRX complex to function in meiotic recombination and in telomere elongation.