The early macrophage response to pathogens requires dynamic regulation of the nuclear paraspeckle.

To ensure a robust immune response to pathogens without risking immunopathology, the kinetics and amplitude of inflammatory gene expression in macrophages need to be exquisitely well controlled. There is a growing appreciation for stress-responsive membraneless organelles (MLOs) regulating various steps of eukaryotic gene expression in response to extrinsic cues. Here, we implicate the nuclear paraspeckle, a highly ordered biomolecular condensate that nucleates on the Neat1 lncRNA, in tuning innate immune gene expression in murine macrophages. In response to a variety of innate agonists, macrophage paraspeckles rapidly aggregate (0.5 h poststimulation) and disaggregate (2 h poststimulation). Paraspeckle maintenance and aggregation require active transcription and MAPK signaling, whereas paraspeckle disaggregation requires degradation of Neat1 via the nuclear RNA exosome. In response to lipopolysaccharide treatment, Neat1 KO macrophages fail to properly express a large cohort of proinflammatory cytokines, chemokines, and antimicrobial mediators. Consequently, Neat1 KO macrophages cannot control replication of Salmonella enterica serovar Typhimurium or vesicular stomatitis virus. These findings highlight a prominent role for MLOs in orchestrating the macrophage response to pathogens and support a model whereby dynamic assembly and disassembly of paraspeckles reorganizes the nuclear landscape to enable inflammatory gene expression following innate stimuli.

Mad1's ability to interact with Mad2 is essential to regulate and monitor meiotic synapsis in C. elegans.

Meiotic homolog synapsis is essential to ensure accurate segregation of chromosomes during meiosis. In C. elegans, proper regulation of synapsis and a checkpoint that monitors synapsis relies on the spindle checkpoint components, Mad1 and Mad2, and Pairing Centers (PCs), cis-acting loci that interact with the nuclear envelope to mobilize chromosomes within the nucleus. Here, we test what specific functions of Mad1 and Mad2 are required to regulate and monitor synapsis. We find that a mutation that prevents Mad1's localization to the nuclear periphery abolishes the synapsis checkpoint but has no effect on Mad2's localization to the nuclear periphery or synapsis. By contrast, a mutation that prevents Mad1's interaction with Mad2 abolishes the synapsis checkpoint, delays synapsis and fails to localize Mad2 to the nuclear periphery. These data indicate that Mad1's primary role in regulating synapsis is through control of Mad2 and that Mad2 can bind other factors at the nuclear periphery. We also tested whether Mad2's ability to adopt a specific conformation associated with its activity during spindle checkpoint function is required for its role in meiosis. A mutation that prevents Mad2 from adopting its active conformer fails to localize to the nuclear periphery, abolishes the synapsis checkpoint and exhibits substantial defects in meiotic synapsis. Thus, Mad2, and its regulation by Mad1, is an important regulator of meiotic synapsis in C. elegans.

PCH-2 collaborates with CMT-1 to proofread meiotic homolog interactions.

The conserved ATPase, PCH-2/TRIP13, is required during both the spindle checkpoint and meiotic prophase. However, its specific role in regulating meiotic homolog pairing, synapsis and recombination has been enigmatic. Here, we report that this enzyme is required to proofread meiotic homolog interactions. We generated a mutant version of PCH-2 in C. elegans that binds ATP but cannot hydrolyze it: pch-2E253Q. In vitro, this mutant can bind a known substrate but is unable to remodel it. This mutation results in some non-homologous synapsis and impaired crossover assurance. Surprisingly, worms with a null mutation in PCH-2's adapter protein, CMT-1, the ortholog of p31comet, localize PCH-2 to meiotic chromosomes, exhibit non-homologous synapsis and lose crossover assurance. The similarity in phenotypes between cmt-1 and pch-2E253Q mutants suggest that PCH-2 can bind its meiotic substrates in the absence of CMT-1, in contrast to its role during the spindle checkpoint, but requires its adapter to hydrolyze ATP and remodel them.

The absence of the RecN protein suppresses the cellular defects of Deinococcus radiodurans irradiated cells devoid of the PprA protein by limiting recombinational repair of DNA lesions.

The Deinococcus radiodurans bacterium is one of the most radioresistant organisms known. It can repair hundreds of radiation-induced DNA double-strand breaks without loss of viability and reconstitute an intact genome through RecA-dependent and RecA-independent DNA repair pathways. Among the Deinococcus specific proteins required for radioresistance, the PprA protein was shown to play a major role for accurate chromosome segregation and cell division after completion of DNA repair. Here, we analyzed the cellular role of the deinococcal RecN protein belonging to the SMC family and, surprisingly, observed that the absence of the RecN protein suppressed the sensitivity of cells devoid of the PprA protein to γ- and UV-irradiation and to treatment with MMC or DNA gyrase inhibitors. This suppression was not observed when ΔpprA cells were devoid of SMC or SbcC, two other proteins belonging to the SMC family. The absence of RecN also alleviated the DNA segregation defects displayed by ΔpprA cells recovering from γ-irradiation. When exposed to 5 kGy γ-irradiation, ΔpprA, ΔrecN and ΔpprA ΔrecN cells repaired their DNA with a delay of about one hour, as compared to the wild type cells. After irradiation, the absence of RecN reduced recombination between chromosomal and plasmid DNA, indicating that the deinococcal RecN protein is important for recombinational repair of DNA lesions. The transformation efficiency of genomic DNA was also reduced in the absence of the RecN protein. Here, we propose a model in which RecN, via its cohesin activity, might favor recombinational repair of DNA double strand breaks. This might increase, in irradiated cells, DNA constraints with PprA protein being required to resolve them via its ability to recruit DNA gyrase and to stimulate its decatenation activity.

PprA Protein Is Involved in Chromosome Segregation via Its Physical and Functional Interaction with DNA Gyrase in Irradiated Deinococcus radiodurans Bacteria.

PprA, a radiation-induced Deinococcus-specific protein, was previously shown to be required for cell survival and accurate chromosome segregation after exposure to ionizing radiation. Here, we used an in vivo approach to determine, by shotgun proteomics, putative PprA partners coimmunoprecipitating with PprA when cells were exposed to gamma rays. Among them, we found the two subunits of DNA gyrase and, thus, chose to focus our work on characterizing the activities of the deinococcal DNA gyrase in the presence or absence of PprA. Loss of PprA rendered cells hypersensitive to novobiocin, an inhibitor of the B subunit of DNA gyrase. We showed that treatment of bacteria with novobiocin resulted in induction of the radiation desiccation response (RDR) regulon and in defects in chromosome segregation that were aggravated by the absence of PprA. In vitro, the deinococcal DNA gyrase, like other bacterial DNA gyrases, possesses DNA negative supercoiling and decatenation activities. These two activities are inhibited in vitro by novobiocin and nalidixic acid, whereas PprA specifically stimulates the decatenation activity of DNA gyrase. Together, these results suggest that PprA plays a major role in chromosome decatenation via its interaction with the deinococcal DNA gyrase when D. radiodurans cells are recovering from exposure to ionizing radiation. IMPORTANCE D. radiodurans is one of the most radiation-resistant organisms known. This bacterium is able to cope with high levels of DNA lesions generated by exposure to extreme doses of ionizing radiation and to reconstruct a functional genome from hundreds of radiation-induced chromosomal fragments. Here, we identified partners of PprA, a radiation-induced Deinococcus-specific protein, previously shown to be required for radioresistance. Our study leads to three main findings: (i) PprA interacts with DNA gyrase after irradiation, (ii) treatment of cells with novobiocin results in defects in chromosome segregation that are aggravated by the absence of PprA, and (iii) PprA stimulates the decatenation activity of DNA gyrase. Our results extend the knowledge of how D. radiodurans cells survive exposure to extreme doses of gamma irradiation and point out the link between DNA repair, chromosome segregation, and DNA gyrase activities in the radioresistant D. radiodurans bacterium.

The abundant and essential HU proteins in Deinococcus deserti and Deinococcus radiodurans are translated from leaderless mRNA.

HU proteins have an important architectural role in nucleoid organization in bacteria. Compared with HU of many bacteria, HU proteins from Deinococcus species possess an N-terminal lysine-rich extension similar to the eukaryotic histone H1 C-terminal domain involved in DNA compaction. The single HU gene in Deinococcus radiodurans, encoding DrHU, is required for nucleoid compaction and cell viability. Deinococcus deserti contains three expressed HU genes, encoding DdHU1, DdHU2 and DdHU3. Here, we show that either DdHU1 or DdHU2 is essential in D. deserti. DdHU1 and DdHU2, but not DdHU3, can substitute for DrHU in D. radiodurans, indicating that DdHU3 may have a non-essential function different from DdHU1, DdHU2 and DrHU. Interestingly, the highly abundant DrHU and DdHU1 proteins, and also the less expressed DdHU2, are translated in Deinococcus from leaderless mRNAs, which lack a 5'-untranslated region and, hence, the Shine-Dalgarno sequence. Unexpectedly, cloning the DrHU or DdHU1 gene under control of a strong promoter in an expression plasmid, which results in leadered transcripts, strongly reduced the DrHU and DdHU1 protein level in D. radiodurans compared with that obtained from the natural leaderless gene. We also show that the start codon position for DrHU and DdHU1 should be reannotated, resulting in proteins that are 15 and 4 aa residues shorter than initially reported. The expression level and start codon correction were crucial for functional characterization of HU in Deinococcus.

DdrO is an essential protein that regulates the radiation desiccation response and the apoptotic-like cell death in the radioresistant Deinococcus radiodurans bacterium.

Deinococcus radiodurans is known for its extreme radioresistance. Comparative genomics identified a radiation-desiccation response (RDR) regulon comprising genes that are highly induced after DNA damage and containing a conserved motif (RDRM) upstream of their coding region. We demonstrated that the RDRM sequence is involved in cis-regulation of the RDR gene ddrB in vivo. Using a transposon mutagenesis approach, we showed that, in addition to ddrO encoding a predicted RDR repressor and irrE encoding a positive regulator recently shown to cleave DdrO in Deinococcus deserti, two genes encoding α-keto-glutarate dehydrogenase subunits are involved in ddrB regulation. In wild-type cells, the DdrO cell concentration decreased transiently in an IrrE-dependent manner at early times after irradiation. Using a conditional gene inactivation system, we showed that DdrO depletion enhanced expression of three RDR proteins, consistent with the hypothesis that DdrO acts as a repressor of the RDR regulon. DdrO-depleted cells loose viability and showed morphological changes evocative of an apoptotic-like response, including membrane blebbing, defects in cell division and DNA fragmentation. We propose that DNA repair and apoptotic-like death might be two responses mediated by the same regulators, IrrE and DdrO, but differently activated depending on the persistence of IrrE-dependent DdrO cleavage.

The PprA protein is required for accurate cell division of γ-irradiated Deinococcus radiodurans bacteria.

Deinococcus radiodurans, one of the most radioresistant organisms known to date is able to reconstruct an intact genome from hundreds of DNA fragments. Here, we investigate the in vivo role of PprA, a radiation-induced Deinococcus specific protein. We report that DNA double strand break repair in cells devoid of PprA and exposed to 3800Gy γ-irradiation takes place efficiently with a delay of only 1h as compared to the wild type, whereas massive DNA synthesis begins 90min after irradiation as in the wild type, a phenotype insufficient to explain the severe radiosensitivity of the ΔpprA mutant. We show that the slow kinetics of reassembly of DNA fragments in a ΔpprA ΔrecA double mutant was the same as that observed in a ΔrecA single mutant demonstrating that PprA does not play a major role in DNA repair through RecA-independent pathways. Using a tagged PprA protein and immunofluorescence microscopy, we show that PprA is recruited onto the nucleoid after γ-irradiation before DNA double strand break repair completion, and then is found as a thread across the septum in dividing cells. Moreover, whereas untreated cells devoid of PprA displayed a wild type morphology, they showed a characteristic cell division abnormality after irradiation not found in other radiosensitive mutants committed to die, as DNA is present equally in the two daughter cells but not separated at the division septum. We propose that PprA may play a crucial role in the control of DNA segregation and/or cell division after DNA double strand break repair.