Proteomic identification of histone post-translational modifications and proteins enriched at a DNA double-strand break

RNAPII Degradation Factor Def1 Is Required for Development, Stress Response, and Full Virulence of Magnaporthe oryzae

The RNA polymerase II degradation factor Degradation Factor 1 (Def1) is important for DNA damage repair and plays various roles in eukaryotes; however, the biological role in plant pathogenic fungi is still unknown. In this study, we investigated the role of Def1 during the development and infection of the rice blast fungus Magnaporthe oryzae. The deletion mutant of Def1 displayed slower mycelial growth, less conidial production, and abnormal conidial morphology. The appressoria of Δdef1 was impaired in the penetration into host cells, mainly due to blocking in the utilization of conidial storages, such as glycogen and lipid droplets. The invasive growth of the Δdef1 mutant was also retarded and accompanied with the accumulation of reactive oxygen species (ROS) inside the host cells. Furthermore, compared with the wild type, Δdef1 was more sensitive to multiple stresses, such as oxidative stress, high osmotic pressure, and alkaline/acidic pH. Interestingly, we found that Def1 was modified by O-GlcNAcylation at Ser232, which was required for the stability of Def1 and its function in pathogenicity. Taken together, the O-GlcNAc modified Def1 is required for hyphae growth, conidiation, pathogenicity, and stress response in M. oryzae. This study reveals a novel regulatory mechanism of O-GlcNAc-mediated Def1 in plant pathogenic fungi.

DEF1: Much more than an RNA polymerase degradation factor

Degradation Factor 1 was discovered 20 years ago as a yeast protein copurifying with Rad26, a helicase involved in transcription-coupled DNA repair. It was subsequently shown to control the ubiquitylation and destruction of the large subunit of DNA damage-arrested RNA Polymerase II. Since that time, much has been learned about Def1's role in polymerase destruction and new functions of the protein have been revealed. We now understand that Def1 is involved in more than just RNA polymerase II regulation. Most of its known functions are associated with maintaining chromosome and genomic integrity, but other exciting activities outside this realm have been suggested. Here we review this fascinating protein, describe its regulation and present a hypothesis that Def1 is a central coordinator of ubiquitin signaling pathways in cells.

Host Chromatin Regulators Required for Aggregatibacter actinomycetemcomitans Cytolethal Distending Toxin Activity in Saccharomyces cerevisiae Model

Cytolethal distending toxin (CDT) is a bacterial genotoxin that causes host cell cycle arrest and death. We previously employed a Saccharomyces cerevisiae model with inducible expression of the CDT catalytic subunit from Aggregatibacter actinomycetemcomitans, AaCdtB, and showed that a wide variety of host factors play a role in facilitating the activity of CdtB. Our observation that a yeast H2B mutant defective in chromatin condensation was partially resistant to CdtB implies that chromatin structure may affect CDT function. In this study, we identified host chromatin regulatory genes required for CdtB cytotoxicity. We found that the deletion of HTZ1 or certain subunits of SWR, INO80, and SIR complexes increased cellular resistance to CdtB. We hypothesized that CdtB may interact with Htz1 or the chromatin, but immunoprecipitation experiments failed to detect physical interaction between CdtB and Htz1 or the chromatin. However, we observed reduced nuclear localization of CdtB in several mutants, suggesting that impaired nuclear translocation may, at least partly, explain the mechanisms of CdtB resistance. In addition, mutations in chromatin regulatory genes induce changes in the global gene expression profile, and these may indirectly affect CdtB toxicity. Our results suggest that decreased expression of endoplasmic reticulum (ER)-Golgi transport-related genes that may be involved in CdtB transport and/or increased expression of DNA repair genes may contribute to CdtB resistance. These results suggest that the functions of chromatin regulators may contribute to the activity of CDT in host cells.

The functional complexity of the RNA-binding protein Yra1: mRNA biogenesis, genome stability and DSB repair

The mRNA export adaptor Yra1 is essential in S. cerevisiae, and conserved from yeast to human (ALY/REF). It is well characterized for its function during transcription elongation, 3' processing and mRNA export. Recently, different studies linked Yra1 to genome stability showing that Yra1 overexpression causes DNA Double Strand Breaks through DNA:RNA hybrids stabilization, and that Yra1 depletion affects DSB repair. However, the mechanisms through which Yra1 contributes to genome stability maintenance are not fully understood. Interestingly, our results showed that the Yra1 C-box domain is required for Yra1 recruitment to an HO-induced irreparable DSB following extensive resection, and that it is essential to repair an HO-induced reparable DSB. Furthermore, we defined that the C-box domain of Yra1 plays a crucial role in DSB repair through homologous recombination but not through non-homologous end joining. Future studies aim at deciphering the mechanism by which Yra1 contributes to DSB repair by searching for Yra1 partners important for this process. This review focuses on the functional complexity of the Yra1 protein, not only summarizing its role in mRNA biogenesis but also emphasizing its auto-regulation and implication in genome integrity either through DNA:RNA hybrids stabilization or DNA double strand break repair in S. cerevisiae.

The mRNA export adaptor Yra1 contributes to DNA double-strand break repair through its C-box domain

Yra1 is an mRNA export adaptor involved in mRNA biogenesis and export in S. cerevisiae. Yra1 overexpression was recently shown to promote accumulation of DNA:RNA hybrids favoring DNA double strand breaks (DSB), cell senescence and telomere shortening, via an unknown mechanism. Yra1 was also identified at an HO-induced DSB and Yra1 depletion causes defects in DSB repair. Previous work from our laboratory showed that Yra1 ubiquitination by Tom1 is important for mRNA export. Here, we found that Yra1 is also ubiquitinated by the SUMO-targeted ubiquitin ligases Slx5-Slx8 implicated in the interaction of irreparable DSB with nuclear pores. We further show that Yra1 binds an HO-induced irreparable DSB in a process dependent on resection. Importantly, a Yra1 mutant lacking the evolutionarily conserved C-box is not recruited to an HO-induced irreparable DSB and becomes lethal under DSB induction in a HO-cut reparable system. Together, the data provide evidence that Yra1 plays a crucial role in DSB repair via homologous recombination. While Yra1 sumoylation and/or ubiquitination are dispensable, the Yra1 C-box region is essential in this process.

A Snapshot on the Cis Chromatin Response to DNA Double-Strand Breaks

In eukaryotes, detection and repair of DNA double-strand breaks (DSBs) operate within chromatin, an incredibly complex structure that tightly packages and regulates DNA metabolism. Chromatin participates in the repair of these lesions at multiple steps, from detection to genomic sequence recovery and chromatin is itself extensively modified during the repair process. In recent years, new methodologies and dedicated techniques have expanded the experimental toolbox, opening up a new era granting the high-resolution analysis of chromatin modifications at annotated DSBs in a genome-wide manner. A complex picture is starting to emerge whereby chromatin is altered at various scales around DSBs, in a manner that relates to the repair pathway used, hence defining a 'repair histone code'. Here, we review the recent advances regarding our knowledge of the chromatin landscape induced in cis around DSBs, with an emphasis on histone post-translational modifications and histone variants.

Deletion of the DEF1 gene does not confer UV-immutability but frequently leads to self-diploidization in yeast Saccharomyces cerevisiae

In yeast Saccharomyces cerevisiae, the DEF1 gene is responsible for regulation of many cellular processes including ubiquitin-dependent degradation of DNA metabolism proteins. Recently it has been proposed that Def1 promotes degradation of the catalytic subunit of DNA polymerase δ at sites of DNA damage and regulates a switch to specialized polymerases and, as a consequence, DNA-damage induced mutagenesis. The idea was based substantially on the severe defects in induced mutagenesis observed in the def1 mutants. We describe that UV mutability of def1Δ strains is actually only moderately affected, while the virtual absence of UV mutagenesis in many def1Δ clones is caused by a novel phenotype of the def1 mutants, proneness to self-diploidization. Diploids are extremely frequent (90%) after transformation of wild-type haploids with def1::kanMX disruption cassette and are frequent (2.3%) in vegetative haploid def1 cultures. Such diploids look "UV immutable" when assayed for recessive forward mutations but have normal UV mutability when assayed for dominant reverse mutations. The propensity for frequent self-diploidization in def1Δ mutants should be taken into account in studies of the def1Δ effect on mutagenesis. The true haploids with def1Δ mutation are moderately UV sensitive but retain substantial UV mutagenesis for forward mutations: they are fully proficient at lower doses and only partially defective at higher doses of UV. We conclude that Def1 does not play a critical role in damage-induced mutagenesis.

Histone methylation in DNA repair and clinical practice: new findings during the past 5-years

DNA double-strand breaks (DSBs) are highly toxic lesions that can impair cellular homeostasis and genome stability to result in tumorigenesis for inappropriate repair. Although DSBs are repaired by homologous recombination (HR) or non-homologous end-joining (NHEJ), the related mechanisms are still incompletely unclear. Indeed, more and more evidences indicate that the methylation of histone lysine has an important role in choosing the pathways of DNA repair. For example, tri-methylated H3K36 is required for HR repair, while di-methylated H4K20 can recruit 53BP1 for NHEJ repair. Here, we reviewed the recent progress in the molecular mechanisms by which histone methylation functions in DNA double-strand breaks repair (DSBR). The insight into the mechanisms of histone methylation repairing DNA damage will supply important cues for clinical cancer treatment.