Identification of a DNA nonhomologous end-joining complex in bacteria

CRISPR/Cas9-mediated genome editing of Shewanella oneidensis MR-1 using a broad host-range pBBR1-based plasmid

Here, we developed an all-in-one, broad host-range CRISPR/Cas9 vector system widely applicable to genome editing of proteobacteria. Plasmid pBBR1-Cas9 was constructed by cloning the cas9 gene from Streptococcus pyogenes into the broad host-range plasmid pBBR1MCS-2. We evaluated its applicability for frameshift mutagenesis of Shewanella oneidensis MR-1. Significant cell death was observed when MR-1 cells were transformed with a pBBR1-Cas9 derivative that expressed a single-guide RNA targeting the crp gene. However, cell death was partially prevented when a donor DNA fragment containing a modified crp sequence with a frameshift mutation was introduced using the same vector. All transformants (9 colonies) contained the expected frameshift mutation in their chromosomal crp genes. These results indicate that this vector system efficiently introduced CRISPR/Cas9-mediated double-strand DNA breaks and subsequent homology-directed repair. This work provides a simple and powerful genome-editing tool for proteobacteria that can harbor pBBR1-based plasmids.

Stress-inducible NHEJ in bacteria: function in DNA repair and acquisition of heterologous DNA

DNA double-strand breaks (DSB) in bacteria can be repaired by non-homologous end-joining (NHEJ), a two-component system relying on Ku and LigD. While performing a genetic characterization of NHEJ in Sinorhizobium meliloti, a representative of bacterial species encoding several Ku and LigD orthologues, we found that at least two distinct functional NHEJ repair pathways co-exist: one is dependent on Ku2 and LigD2, while the other depends on Ku3, Ku4 and LigD4. Whereas Ku2 likely acts as canonical bacterial Ku homodimers, genetic evidences suggest that Ku3-Ku4 form eukaryotic-like heterodimers. Strikingly, we found that the efficiency of both NHEJ systems increases under stress conditions, including heat and nutrient starvation. We found that this stimulation results from the transcriptional up-regulation of the ku and/or ligD genes, and that some of these genes are controlled by the general stress response regulator RpoE2. Finally, we provided evidence that NHEJ not only repairs DSBs, but can also capture heterologous DNA fragments into genomic breaks. Our data therefore suggest that NHEJ could participate to horizontal gene transfer from distantly related species, bypassing the need of homology to integrate exogenous DNA. This supports the hypothesis that NHEJ contributes to evolution and adaptation of bacteria under adverse environmental conditions.

A New Single Gene Differential Biomarker for Mycobacterium tuberculosis Complex and Non-tuberculosis Mycobacteria

Background

Tuberculosis (TB) and non-tuberculous mycobacteriosis are serious threats to health worldwide. A simple non-sequencing method is needed for rapid diagnosis, especially in less experienced hospitals, but there is no specific biomarker commonly used for all mycobacteria. The ku gene of the prokaryotic error-prone non-homologous end joining system (NHEJ) has the potential to be a highly specific detection biomarker for mycobacteria.

Methods

A total of 7294 mycobacterial genomes and 14 complete genomes of other families belonging to Corynebacteriales with Mycobacteriaceae were downloaded and analyzed for the existence and variation of the ku gene. Mycobacterium tuberculosis complex (MTBC) and non-tuberculosis mycobacteria (NTM)- specific primers were designed and the actual amplification and identification efficiencies were tested with 150 strains of 40 Mycobacterium species and 10 kinds of common respiratory pathogenic bacteria.

Results

The ku gene of the NHEJ system was ubiquitous in all genome sequenced Mycobacterium species and absent in other families of Corynebacteriales. On the one hand, as a single gene non-sequencing biomarker, its specific primers could effectively distinguish mycobacteria from other bacteria, MTBC from NTM, which would make the clinical detection of mycobacteria easy and have great clinical practical value. On the other hand, the sequence of ku gene can effectively distinguish NTM to species level with high resolution.

Conclusion

The Ku protein existed before the differentiation of Mycobacterium species, which was an important protein involved in maintaining of the genome’s integrity and related to the special growth stage of mycobacteria. It was rare in prokaryotes. These features made it a highly special differential biomarker for Mycobacterium.

Causes and Effects of Loss of Classical Nonhomologous End Joining Pathway in Parasitic Eukaryotes

We report frequent losses of components of the classical nonhomologous end joining pathway (C-NHEJ), one of the main eukaryotic tools for end joining repair of DNA double-strand breaks, in several lineages of parasitic protists. Moreover, we have identified a single lineage among trypanosomatid flagellates that has lost Ku70 and Ku80, the core C-NHEJ components, and accumulated numerous insertions in many protein-coding genes. We propose a correlation between these two phenomena and discuss the possible impact of the C-NHEJ loss on genome evolution and transition to the parasitic lifestyle.IMPORTANCE Parasites tend to evolve small and compact genomes, generally endowed with a high mutation rate, compared with those of their free-living relatives. However, the mechanisms by which they achieve these features, independently in unrelated lineages, remain largely unknown. We argue that the loss of the classical nonhomologous end joining pathway components may be one of the crucial steps responsible for characteristic features of parasite genomes.

Repair-independent functions of DNA-PKcs protect irradiated cells from mitotic slippage and accelerated senescence

The binding of DNA-dependent protein kinase catalytic subunit (DNA-PKcs, also known as PRKDC) to Ku proteins at DNA double-strand breaks (DSBs) has long been considered essential for non-homologous end joining (NHEJ) repair, providing a rationale for use of DNA-PKcs inhibitors as cancer therapeutics. Given lagging clinical translation, we reexamined mechanisms and observed instead that DSB repair can proceed independently of DNA-PKcs. While repair of radiation-induced DSBs was blocked in cells expressing shRNAs targeting Ku proteins or other NHEJ core factors, DSBs were repaired on schedule despite targeting DNA-PKcs. Although we failed to observe a DSB repair defect, the γH2AX foci that formed at sites of DNA damage persisted indefinitely after irradiation, leading to cytokinesis failure and accumulation of binucleated cells. Following this mitotic slippage, cells with decreased DNA-PKcs underwent accelerated cellular senescence. We identified downregulation of ataxia-telangiectasia mutated kinase (ATM) as the critical role of DNA-PKcs in recovery from DNA damage, insofar as targeting ATM restored γH2AX foci resolution and cytokinesis. Considering the lack of direct impact on DSB repair and emerging links between senescence and resistance to cancer therapy, these results suggest reassessing DNA-PKcs as a target for cancer treatment.

The phage T4 DNA ligase mediates bacterial chromosome DSBs repair as single component non-homologous end joining

DNA double-strand breaks (DSBs) are one of the most lethal forms of DNA damage that is not efficiently repaired in prokaryotes. Certain microorganisms can handle chromosomal DSBs using the error-prone non-homologous end joining (NHEJ) system and ultimately cause genome mutagenesis. Here, we demonstrated that Enterobacteria phage T4 DNA ligase alone is capable of mediating in vivo chromosome DSBs repair in Escherichia coli. The ligation efficiency of DSBs with T4 DNA ligase is one order of magnitude higher than the NHEJ system from Mycobacterium tuberculosis. This process introduces chromosome DNA excision with different sizes, which can be manipulated by regulating the activity of host-exonuclease RecBCD. The DNA deletion length reduced either by inactivating recB or expressing the RecBCD inhibitor Gam protein from λ phage. Furthermore, we also found single nucleotide substitutions at the DNA junction, suggesting that T4 DNA ligase, as a single component non-homologous end joining system, has great potential in genome mutagenesis, genome reduction and genome editing.

Bacterial NHEJ: a never ending story

Double-strand breaks (DSBs) are the most detrimental DNA damage encountered by bacterial cells. DBSs can be repaired by homologous recombination thanks to the availability of an intact DNA template or by Non-Homologous End Joining (NHEJ) when no intact template is available. Bacterial NHEJ is performed by sets of proteins of growing complexity from Bacillus subtilis and Mycobacterium tuberculosis to Streptomyces and Sinorhizobium meliloti. Here, we discuss the contribution of these models to the understanding of the bacterial NHEJ repair mechanism as well as the involvement of NHEJ partners in other DNA repair pathways. The importance of NHEJ and of its complexity is discussed in the perspective of regulation through the biological cycle of the bacteria and in response to environmental stimuli. Finally, we consider the role of NHEJ in genome evolution, notably in horizontal gene transfer.

Development and application of a CRISPR/Cas9 system for Bacillus licheniformis genome editing

A highly efficient genome editing system for Bacillus licheniformis was developed based on single-plasmid CRISPR/Cas9. For highly efficient genome editing the shuttle vector pWH1520 was selected to construct the knockout plasmids. A construct harboring a pS promoter driving cas9 endonuclease expression, a strong pLY-2 promoter driving the transcription of a single guide RNA was demonstrated as being the most effective. To verify the feasibility of the method the uprT gene coding uracil phosphoribosyltransferase was selected as the reporter gene. The efficiency of introducing nucleotide point mutations and single gene deletion reached an editing efficiency of up to 99.2% and 97.3%, respectively. After a upp-deficient strain was engineered, the system and strain were applied to introduce genomic deletions of another two genes, amyL and chiA (encoding amylase and chitinase, respectively) with about 90% deletion efficiency. As two native extracellular proteins with relatively high secretion in the host, amylase and chitinase can hamper the secretion and expression of alkaline protease. It was demonstrated that the mutant with deletions of the two genes effectively improved the alkaline protease yield by 24.8%. The results illustrated that the establishment of a CRISPR/Cas9 system for Bacillus licheniformis is of significance, and confirmed the system's high efficiency. The system provides support for effective molecular modification and metabolic regulation of Bacillus licheniformis, and offers promise for applications in genetic modification of other industrially relevant Bacillus species.

Phage Mu Gam protein promotes NHEJ in concert with Escherichia coli ligase

The Gam protein of transposable phage Mu is an ortholog of eukaryotic and bacterial Ku proteins, which carry out nonhomologous DNA end joining (NHEJ) with the help of dedicated ATP-dependent ligases. Many bacteria carry Gam homologs associated with either complete or defective Mu-like prophages, but the role of Gam in the life cycle of Mu or in bacteria is unknown. Here, we show that MuGam is part of a two-component bacterial NHEJ DNA repair system. Ensemble and single-molecule experiments reveal that MuGam binds to DNA ends, slows the progress of RecBCD exonuclease, promotes binding of NAD⁺-dependent Escherichia coli ligase A, and stimulates ligation. In vivo, Gam equally promotes both precise and imprecise joining of restriction enzyme-digested linear plasmid DNA, as well as of a double-strand break (DSB) at an engineered I-SceI site in the chromosome. Cell survival after the induced DSB is specific to the stationary phase. In long-term growth competition experiments, particularly upon treatment with a clastogen, the presence of gam in a Mu lysogen confers a distinct fitness advantage. We also show that the role of Gam in the life of phage Mu is related not to transposition but to protection of genomic Mu copies from RecBCD when viral DNA packaging begins. Taken together, our data show that MuGam provides bacteria with an NHEJ system and suggest that the resulting fitness advantage is a reason that bacteria continue to retain the gam gene in the absence of an intact prophage.

DNA-based memory devices for recording cellular events

Measuring biological data across time and space is critical for understanding complex biological processes and for various biosurveillance applications. However, such data are often inaccessible or difficult to directly obtain. Less invasive, more robust and higher-throughput biological recording tools are needed to profile cells and their environments. DNA-based cellular recording is an emerging and powerful framework for tracking intracellular and extracellular biological events over time across living cells and populations. Here, we review and assess DNA recorders that utilize CRISPR nucleases, integrases and base-editing strategies, as well as recombinase and polymerase-based methods. Quantitative characterization, modelling and evaluation of these DNA-recording modalities can guide their design and implementation for specific application areas.

CRISPR-Cas immunity, DNA repair and genome stability

Co-opting of CRISPR-Cas 'Interference' reactions for editing the genomes of eukaryotic and prokaryotic cells has highlighted crucial support roles for DNA repair systems that strive to maintain genome stability. As front-runners in genome editing that targets DNA, the class 2 CRISPR-Cas enzymes Cas9 and Cas12a rely on repair of DNA double-strand breaks (DDSBs) by host DNA repair enzymes, using mechanisms that vary in how well they are understood. Data are emerging about the identities of DNA repair enzymes that support genome editing in human cells. At the same time, it is becoming apparent that CRISPR-Cas systems functioning in their native environment, bacteria or archaea, also need DNA repair enzymes. In this short review, we survey how DNA repair and CRISPR-Cas systems are intertwined. We consider how understanding DNA repair and CRISPR-Cas interference reactions in nature might help improve the efficacy of genome editing procedures that utilise homologous or analogous systems in human and other cells.

Single-Homology-Arm Linear DNA Recombination by the Nonhomologous End Joining Pathway as a Novel and Simple Gene Inactivation Method: a Proof-of-Concept Study in Dietzia sp. Strain DQ12-45-1b

Nonhomologous end joining (NHEJ) is critical for genome stability because of its roles in double-strand break repair. Ku and ligase D (LigD) are the crucial proteins in this process, and strains expressing Ku and LigD can cyclize linear DNA in vivo Here, we established a proof-of-concept single-homology-arm linear DNA recombination for gene inactivation or genome editing by which cyclization of linear DNA in vivo by NHEJ could be used to generate nonreplicable circular DNA and could allow allelic exchanges between the circular DNA and the chromosome. We achieved this approach in Dietzia sp. strain DQ12-45-1b, which expresses Ku and LigD homologs and presents NHEJ activity. By transforming the strain with a linear DNA single homolog to the sequence in the chromosome, we mutated the genome. This method did not require the screening of suitable plasmids and was easy and time-effective. Bioinformatic analysis showed that more than 20% of prokaryotic organisms contain Ku and LigD, suggesting the wide distribution of NHEJ activities. Moreover, an Escherichia coli strain also showed NHEJ activity when the Ku and LigD of Dietzia sp. DQ12-45-1b were introduced and expressed in it. Therefore, this method may be a widely applicable genome editing tool for diverse prokaryotic organisms, especially for nonmodel microorganisms.IMPORTANCE Many nonmodel Gram-positive bacteria lack efficient genetic manipulation systems, but they express genes encoding Ku and LigD. The NHEJ pathway in Dietzia sp. DQ12-45-1b was evaluated and was used to successfully knock out 11 genes in the genome. Since bioinformatic studies revealed that the putative genes encoding Ku and LigD ubiquitously exist in phylogenetically diverse bacteria and archaea, the single-homology-arm linear DNA recombination by the NHEJ pathway could be a potentially applicable genetic manipulation method for diverse nonmodel prokaryotic organisms.

Role of DNA Repair and Protective Components in Bacillus subtilis Spore Resistance to Inactivation by 400-nm-Wavelength Blue Light

The high intrinsic decontamination resistance of Firmicutes spores is important medically (disease) and commercially (food spoilage). Effective methods of spore eradication would be of considerable interest in the health care and medical product industries, particularly if the decontamination method effectively killed spores while remaining benign to both humans and sensitive equipment. Intense blue light at a ∼400 nm wavelength is one such treatment that has drawn significant interest. This work has determined the resistance of spores to blue light in an extensive panel of Bacillus subtilis strains, including wild-type strains and mutants that (i) lack protective components such as the spore coat and its pigment(s) or the DNA protective α/β-type small, acid-soluble spore proteins (SASP); (ii) have an elevated spore core water content; or (iii) lack enzymes involved in DNA repair, including those for homologous recombination and nonhomologous end joining (HR and NHEJ), apurinic/apyrimidinic endonucleases, nucleotide and base excision repair (NER and BER), translesion synthesis (TLS) by Y-family DNA polymerases, and spore photoproduct (SP) removal by SP lyase (SPL). The most important factors in spore blue light resistance were determined to be spore coats/pigmentation, α/β-type SASP, NER, BER, TLS, and SP repair. A major conclusion from this work is that blue light kills spores by DNA damage, and the results in this work indicate at least some of the specific DNA damage. It appears that high-intensity blue light could be a significant addition to the agents used to kill bacterial spores in applied settings.IMPORTANCE Effective methods of spore inactivation would be of considerable interest in the health care and medical products industries, particularly if the decontamination method effectively killed spores while remaining benign to both humans and sensitive equipment. Intense blue light radiation is one such treatment that has drawn significant interest. In this work, all known spore-protective features, as well as universal and spore-specific DNA repair mechanisms, were tested in a systematic fashion for their contribution to the resistance of spores to blue light radiation.

A CRISPR-Cas9-Based Toolkit for Fast and Precise In Vivo Genetic Engineering of Bacillus subtilis Phages

Phages are currently under discussion as a solution for the antibiotic crisis, as they may cure diseases caused by multi-drug-resistant pathogens. However, knowledge of phage biology and genetics is limited, which impedes risk assessment of therapeutic applications. In order to enable advances in phage genetic research, the aim of this work was to create a toolkit for simple and fast genetic engineering of phages recruiting Bacillus subtilis as host system. The model organism B. subtilis represents a non-pathogenic surrogate of its harmful relatives, such as Bacillus anthracis or Bacillus cereus. This toolkit comprises the application CutSPR, a bioinformatic tool for rapid primer design, and facilitates the cloning of specific CRISPR-Cas9-based mutagenesis plasmids. The employment of the prophage-free and super-competent B. subtilis TS01 strain enables an easy and fast introduction of specific constructs for in vivo phage mutagenesis. Clean gene deletions and a functional clean gene insertion into the genome of the model phage vB_BsuP-Goe1 served as proof of concept and demonstrate reliability and high efficiency. The here presented toolkit allows comprehensive investigation of the diverse phage genetic pool, a better understanding of phage biology, and safe phage applications.

Genome plasticity is governed by double strand break DNA repair in Streptomyces

The linear chromosome of the bacterium Streptomyces exhibits a remarkable genetic organization with grossly a central conserved region flanked by variable chromosomal arms. The terminal diversity co-locates with an intense DNA plasticity including the occurrence of large deletions associated to circularization and chromosomal arm exchange. These observations prompted us to assess the role of double strand break (DSB) repair in chromosome plasticity following. For that purpose, DSBs were induced along the chromosome using the meganuclease I-SceI. DSB repair in the central region of the chromosome was mutagenic at the healing site but kept intact the whole genome structure. In contrast, DSB repair in the chromosomal arms was mostly associated to the loss of the targeted chromosomal arm and extensive deletions beyond the cleavage sites. While homologous recombination occurring between copies of DNA sequences accounted for the most part of the chromosome rescue events, Non Homologous End Joining was involved in mutagenic repair as well as in huge genome rearrangements (i.e. circularization). Further, NHEJ repair was concomitant with the integration of genetic material at the healing site. We postulate that DSB repair drives genome plasticity and evolution in Streptomyces and that NHEJ may foster horizontal transfer in the environment.

Genome editing of Ralstonia eutropha using an electroporation-based CRISPR-Cas9 technique

BACKGROUND

Ralstonia eutropha is an important bacterium for the study of polyhydroxyalkanoates (PHAs) synthesis and CO₂ fixation, which makes it a potential strain for industrial PHA production and attractive host for CO₂ conversion. Although the bacterium is not recalcitrant to genetic manipulation, current methods for genome editing based on group II introns or single crossover integration of a suicide plasmid are inefficient and time-consuming, which limits the genetic engineering of this organism. Thus, developing an efficient and convenient method for R. eutropha genome editing is imperative.

RESULTS

An efficient genome editing method for R. eutropha was developed using an electroporation-based CRISPR-Cas9 technique. In our study, the electroporation efficiency of R. eutropha was found to be limited by its restriction-modification (RM) systems. By searching the putative RM systems in R. eutropha H16 using REBASE database and comparing with that in E. coli MG1655, five putative restriction endonuclease genes which are related to the RM systems in R. eutropha were predicated and disrupted. It was found that deletion of H16_A0006 and H16_A0008-9 increased the electroporation efficiency 1658 and 4 times, respectively. Fructose was found to reduce the leaky expression of the arabinose-inducible pBAD promoter, which was used to optimize the expression of cas9, enabling genome editing via homologous recombination based on CRISPR-Cas9 in R. eutropha. A total of five genes were edited with efficiencies ranging from 78.3 to 100%. The CRISPR-Cpf1 system and the non-homologous end joining mechanism were also investigated, but failed to yield edited strains.

CONCLUSIONS

We present the first genome editing method for R. eutropha using an electroporation-based CRISPR-Cas9 approach, which significantly increased the efficiency and decreased time to manipulate this facultative chemolithoautotrophic microbe. The novel technique will facilitate more advanced researches and applications of R. eutropha for PHA production and CO₂ conversion.

High-coverage proteomics reveals methionine auxotrophy in Deinococcus radiodurans

Deinococcus radiodurans is a robust bacterium best known for its capacity to resist to radiation. In this study, the SDS-PAGE coupled with high-precision LC-MS/MS was used to study the D. radiodurans proteome. A total of 1951 proteins were identified which covers 63.18% protein-coding genes. Comparison of the identified proteins to the key enzymes in amino acid biosyntheses from KEGG database showed the methionine biosynthesis module is incomplete while other amino acid biosynthesis modules are complete, which indicated methionine auxotrophy in D. radiodurans. The subsequent amino acid-auxotrophic screening has verified methionine instead of other amino acids is essential for the growth of D. radiodurans. With molecular evolutionary genetic analysis, we found the divergence in methionine biosynthesis during the evolution of the common ancestor of bacteria. We also found D. radiodurans lost the power of synthesizing methionine because of the missing metA and metX in two types of methionine biosyntheses. For the first time, this study used high-coverage proteome analysis to identify D. radiodurans amino acid auxotrophy, which provides the important reference for the development of quantitative proteomics analysis using stable isotope labeling in metabolomics of D. radiodurans and in-depth analysis of the molecular mechanism of radiation resistance.

Inhibition of NHEJ repair by type II-A CRISPR-Cas systems in bacteria

Type II CRISPR-Cas systems introduce double-strand breaks into DNA of invading genetic material and use DNA fragments to acquire novel spacers during adaptation. These breaks can be the substrate of several DNA repair pathways, paving the way for interactions. We report that non-homologous end-joining (NHEJ) and type II-A CRISPR-Cas systems only co-occur once among 5563 fully sequenced prokaryotic genomes. We investigated experimentally the possible molecular interactions using the NHEJ pathway from Bacillus subtilis and the type II-A CRISPR-Cas systems from Streptococcus thermophilus and Streptococcus pyogenes. Our results suggest that the NHEJ system has no effect on CRISPR immunity. On the other hand, we provide evidence for the inhibition of NHEJ repair by the Csn2 protein. Our findings give insights on the complex interactions between CRISPR-Cas systems and repair mechanisms in bacteria, contributing to explain the scattered distribution of CRISPR-Cas systems in bacterial genome.

Copy Number Variation Disorders

PURPOSE OF REVIEW

Copy number variation (CNV) disorders arise from the dosage imbalance of one or more gene(s), resulting from deletions, duplications or other genomic rearrangements that lead to the loss or gain of genetic material. Several disorders, characterized by multiple birth defects and neurodevelopmental abnormalities, have been associated with relatively large (>1 Mb) and often recurrent CNVs. CNVs have also been implicated in the etiology of neuropsychiatric disorders including autism and schizophrenia as well as other common complex diseases. Thus, CNVs have a significant impact on human health and disease.

RECENT FINDINGS

The use of increasingly higher resolution, genomewide analysis has greatly enhanced the detection of genetic variation, including CNVs. Furthermore, the availability of comprehensive genetic variation data from large cohorts of healthy controls has the potential to greatly improve the identification of disease associated genetic variants in patient samples.

SUMMARY

This review discusses the current knowledge about CNV disorders, including the mechanisms underlying their formation and phenotypic outcomes, and the advantages and limitations of current methods of detection and disease association.

Guardians of the mycobacterial genome: A review on DNA repair systems in Mycobacterium tuberculosis

The genomic integrity of Mycobacterium tuberculosis is continuously threatened by the harsh survival conditions inside host macrophages, due to immune and antibiotic stresses. Faithful genome maintenance and repair must be accomplished under stress for the bacillus to survive in the host, necessitating a robust DNA repair system. The importance of DNA repair systems in pathogenesis is well established. Previous examination of the M. tuberculosis genome revealed homologues of almost all the major DNA repair systems, i.e. nucleotide excision repair (NER), base excision repair (BER), homologous recombination (HR) and non-homologous end joining (NHEJ). However, recent developments in the field have pointed to the presence of novel proteins and pathways in mycobacteria. Homologues of archeal mismatch repair proteins were recently reported in mycobacteria, a pathway previously thought to be absent. RecBCD, the major nuclease-helicase enzymes involved in HR in E. coli, were implicated in the single-strand annealing (SSA) pathway. Novel roles of archeo-eukaryotic primase (AEP) polymerases, previously thought to be exclusive to NHEJ, have been reported in BER. Many new proteins with a probable role in DNA repair have also been discovered. It is now realized that the DNA repair systems in M. tuberculosis are highly evolved and have redundant backup mechanisms to mend the damage. This review is an attempt to summarize our current understanding of the DNA repair systems in M. tuberculosis.

DNA Ligase C and Prim-PolC participate in base excision repair in mycobacteria

Prokaryotic Ligase D is a conserved DNA repair apparatus processing DNA double-strand breaks in stationary phase. An orthologous Ligase C (LigC) complex also co-exists in many bacterial species but its function is unknown. Here we show that the LigC complex interacts with core BER enzymes in vivo and demonstrate that together these factors constitute an excision repair apparatus capable of repairing damaged bases and abasic sites. The polymerase component, which contains a conserved C-terminal structural loop, preferentially binds to and fills-in short gapped DNA intermediates with RNA and LigC ligates the resulting nicks to complete repair. Components of the LigC complex, like LigD, are expressed upon entry into stationary phase and cells lacking either of these pathways exhibit increased sensitivity to oxidising genotoxins. Together, these findings establish that the LigC complex is directly involved in an excision repair pathway(s) that repairs DNA damage with ribonucleotides during stationary phase.

Ligase D is a conserved DNA repair protein complex that repairs double-strand breaks in stationary phase prokaryotes. Here the authors show that orthologous Ligase C has a role in base excision repair during stationary phase.

Evidence for double-strand break mediated mitochondrial DNA replication in Saccharomyces cerevisiae

The mechanism of mitochondrial DNA (mtDNA) replication in Saccharomyces cerevisiae is controversial. Evidence exists for double-strand break (DSB) mediated recombination-dependent replication at mitochondrial replication origin ori5 in hypersuppressive ρ⁻ cells. However, it is not clear if this replication mode operates in ρ⁺ cells. To understand this, we targeted bacterial Ku (bKu), a DSB binding protein, to the mitochondria of ρ⁺ cells with the hypothesis that bKu would bind persistently to mtDNA DSBs, thereby preventing mtDNA replication or repair. Here, we show that mitochondrial-targeted bKu binds to ori5 and that inducible expression of bKu triggers petite formation preferentially in daughter cells. bKu expression also induces mtDNA depletion that eventually results in the formation of ρ⁰ cells. This data supports the idea that yeast mtDNA replication is initiated by a DSB and bKu inhibits mtDNA replication by binding to a DSB at ori5, preventing mtDNA segregation to daughter cells. Interestingly, we find that mitochondrial-targeted bKu does not decrease mtDNA content in human MCF7 cells. This finding is in agreement with the fact that human mtDNA replication, typically, is not initiated by a DSB. Therefore, this study provides evidence that DSB-mediated replication is the predominant form of mtDNA replication in ρ⁺ yeast cells.

Novel Antibiotic Resistance Determinants from Agricultural Soil Exposed to Antibiotics Widely Used in Human Medicine and Animal Farming

Antibiotic resistance has emerged globally as one of the biggest threats to human and animal health. Although the excessive use of antibiotics is recognized as accelerating the selection for resistance, there is a growing body of evidence suggesting that natural environments are "hot spots" for the development of both ancient and contemporary resistance mechanisms. Given that pharmaceuticals can be entrained onto agricultural land through anthropogenic activities, this could be a potential driver for the emergence and dissemination of resistance in soil bacteria. Using functional metagenomics, we interrogated the "resistome" of bacterial communities found in a collection of Canadian agricultural soil, some of which had been receiving antibiotics widely used in human medicine (macrolides) or food animal production (sulfamethazine, chlortetracycline, and tylosin) for up to 16 years. Of the 34 new antibiotic resistance genes (ARGs) recovered, the majority were predicted to encode (multi)drug efflux systems, while a few share little to no homology with established resistance determinants. We characterized several novel gene products, including putative enzymes that can confer high-level resistance against aminoglycosides, sulfonamides, and broad range of beta-lactams, with respect to their resistance mechanisms and clinical significance. By coupling high-resolution proteomics analysis with functional metagenomics, we discovered an unusual peptide, PPP^{AZI 4}, encoded within an alternative open reading frame not predicted by bioinformatics tools. Expression of the proline-rich PPP^{AZI 4} can promote resistance against different macrolides but not other ribosome-targeting antibiotics, implicating a new macrolide-specific resistance mechanism that could be fundamentally linked to the evolutionary design of this peptide.IMPORTANCE Antibiotic resistance is a clinical phenomenon with an evolutionary link to the microbial pangenome. Genes and protogenes encoding specialized and potential resistance mechanisms are abundant in natural environments, but understanding of their identity and genomic context remains limited. Our discovery of several previously unknown antibiotic resistance genes from uncultured soil microorganisms indicates that soil is a significant reservoir of resistance determinants, which, once acquired and "repurposed" by pathogenic bacteria, can have serious impacts on therapeutic outcomes. This study provides valuable insights into the diversity and identity of resistance within the soil microbiome. The finding of a novel peptide-mediated resistance mechanism involving an unpredicted gene product also highlights the usefulness of integrating proteomics analysis into metagenomics-driven gene discovery.

DNA double-strand break repair is involved in desiccation resistance of Sinorhizobium meliloti, but is not essential for its symbiotic interaction with Medicago truncatula

The soil bacterium Sinorhizobium meliloti, a nitrogen-fixing symbiont of legume plants, is exposed to numerous stress conditions in nature, some of which cause the formation of harmful DNA double-strand breaks (DSBs). In particular, the reactive oxygen species (ROS) and the reactive nitrogen species (RNS) produced during symbiosis, and the desiccation occurring in dry soils, are conditions which induce DSBs. Two major systems of DSB repair are known in S. meliloti: homologous recombination (HR) and non-homologous end-joining (NHEJ). However, their role in the resistance to ROS, RNS and desiccation has never been examined in this bacterial species, and the importance of DSB repair in the symbiotic interaction has not been properly evaluated. Here, we constructed S. meliloti strains deficient in HR (by deleting the recA gene) or in NHEJ (by deleting the four ku genes) or both. Interestingly, we observed that ku and/or recA genes are involved in S. meliloti resistance to ROS and RNS. Nevertheless, an S. meliloti strain deficient in both HR and NHEJ was not altered in its ability to establish and maintain an efficient nitrogen-fixing symbiosis with Medicago truncatula, showing that rhizobial DSB repair is not essential for this process. This result suggests either that DSB formation in S. meliloti is efficiently prevented during symbiosis or that DSBs are not detrimental for symbiosis efficiency. In contrast, we found for the first time that both recA and ku genes are involved in S. meliloti resistance to desiccation, suggesting that DSB repair could be important for rhizobium persistence in the soil.

Mycobacterium tuberculosis and Mycobacterium marinum non-homologous end-joining proteins can function together to join DNA ends in Escherichia coli

Mycobacterium tuberculosis and Mycobacterium smegmatis express a Ku protein and a DNA ligase D and are able to repair DNA double strand breaks (DSBs) by non-homologous end-joining (NHEJ). This pathway protects against DNA damage when bacteria are in stationary phase. Mycobacterium marinum is a member of this mycobacterium family and like M. tuberculosis is pathogenic. M. marinum lives in water, forms biofilms and infects fish and frogs. M. marinum is a biosafety level 2 (BSL2) organism as it can infect humans, although infections are limited to the skin. M. marinum is accepted as a model to study mycobacterial pathogenesis, as M. marinum and M. tuberculosis are genetically closely related and have similar mechanisms of survival and persistence inside macrophage. The aim of this study was to determine whether M. marinum could be used as a model to understand M. tuberculosis NHEJ repair. We identified and cloned the M. marinum genes encoding NHEJ proteins and generated E. coli strains that express the M. marinum Ku (Mm-Ku) and ligase D (Mm-Lig) individually or together (LHmKumLig strain) from expression vectors integrated at phage attachment sites in the genome. We demonstrated that Mm-Ku and Mm-Lig are both required to re-circularize Cla I-linearized plasmid DNA in E. coli. We compared repair of strain LHmKumLig with that of an E. coli strain (BWKuLig#2) expressing the M. tuberculosis Ku (Mt-Ku) and ligase D (Mt-Lig), and found that LHmKumLig performed 3.5 times more repair and repair was more accurate than BWKuLig#2. By expressing the Mm-Ku with the Mt-Lig, or the Mt-Ku with the Mm-Lig in E. coli, we have shown that the NHEJ proteins from M. marinum and M. tuberculosis can function together to join DNA DSBs. NHEJ repair is therefore conserved between the two species. Consequently, M. marinum is a good model to study NHEJ repair during mycobacterial pathogenesis.

Naphthalimides Selectively Inhibit the Activity of Bacterial, Replicative DNA Ligases and Display Bactericidal Effects against Tubercle Bacilli

The DNA ligases, enzymes that seal breaks in the backbones of DNA, are essential for all organisms, however bacterial ligases essential for DNA replication use β-nicotinamide adenine dinucleotide as their co-factor, whereas those that are essential in eukaryotes and viruses use adenosine-5'-triphosphate. This fact leads to the conclusion that NAD⁺-dependent DNA ligases in bacteria could be targeted by their co-factor specific inhibitors. The development of novel alternative medical strategies, including new drugs, are a top priority focus areas for tuberculosis research due to an increase in the number of multi-drug resistant as well as totally drug resistant tubercle bacilli strains. Here, through the use of a virtual high-throughput screen and manual inspection of the top 200 records, 23 compounds were selected for in vitro studies. The selected compounds were evaluated in respect to their Mycobacterium tuberculosis NAD⁺ DNA ligase inhibitory effect by a newly developed assay based on Genetic Analyzer 3500 Sequencer. The most effective agents (e.g., pinafide, mitonafide) inhibited the activity of M. tuberculosis NAD⁺-dependent DNA ligase A at concentrations of 50 µM. At the same time, the ATP-dependent (phage) DNA LigT₄ was unaffected by the agents at concentrations up to 2 mM. The selected compounds appeared to also be active against actively growing tubercle bacilli in concentrations as low as 15 µM.

MutS2 Promotes Homologous Recombination in Bacillus subtilis

Bacterial MutS proteins are subdivided into two families, MutS1 and MutS2. MutS1 family members recognize DNA replication errors during their participation in the well-characterized mismatch repair (MMR) pathway. In contrast to the well-described function of MutS1, the function of MutS2 in bacteria has remained less clear. In Helicobacter pylori and Thermus thermophilus, MutS2 has been shown to suppress homologous recombination. The role of MutS2 is unknown in the Gram-positive bacterium Bacillus subtilis In this work, we investigated the contribution of MutS2 to maintaining genome integrity in B. subtilis We found that deletion of mutS2 renders B. subtilis sensitive to the natural antibiotic mitomycin C (MMC), which requires homologous recombination for repair. We demonstrate that the C-terminal small MutS-related (Smr) domain is necessary but not sufficient for tolerance to MMC. Further, we developed a CRISPR/Cas9 genome editing system to test if the inducible prophage PBSX was the underlying cause of the observed MMC sensitivity. Genetic analysis revealed that MMC sensitivity was dependent on recombination and not on nucleotide excision repair or a symptom of prophage PBSX replication and cell lysis. We found that deletion of mutS2 resulted in decreased transformation efficiency using both plasmid and chromosomal DNA. Further, deletion of mutS2 in a strain lacking the Holliday junction endonuclease gene recU resulted in increased MMC sensitivity and decreased transformation efficiency, suggesting that MutS2 could function redundantly with RecU. Together, our results support a model where B. subtilis MutS2 helps to promote homologous recombination, demonstrating a new function for bacterial MutS2.

IMPORTANCE

Cells contain pathways that promote or inhibit recombination. MutS2 homologs are Smr-endonuclease domain-containing proteins that have been shown to function in antirecombination in some bacteria. We present evidence that B. subtilis MutS2 promotes recombination, providing a new function for MutS2. We found that cells lacking mutS2 are sensitive to DNA damage that requires homologous recombination for repair and have reduced transformation efficiency. Further analysis indicates that the C-terminal Smr domain requires the N-terminal portion of MutS2 for function in vivo Moreover, we show that a mutS2 deletion is additive with a recU deletion, suggesting that these proteins have a redundant function in homologous recombination. Together, our study shows that MutS2 proteins have adapted different functions that impact recombination.

Rad51 and RecA juxtapose dsDNA ends ready for DNA ligase-catalyzed end-joining under recombinase-suppressive conditions

RecA-family recombinase-catalyzed ATP-dependent homologous joint formation is critical for homologous recombination, in which RecA or Rad51 binds first to single-stranded (ss)DNA and then interacts with double-stranded (ds)DNA. However, when RecA or Rad51 interacts with dsDNA before binding to ssDNA, the homologous joint-forming activity of RecA or Rad51 is quickly suppressed. We found that under these and adenosine diphosphate (ADP)-generating suppressive conditions for the recombinase activity, RecA or Rad51 at similar optimal concentrations enhances the DNA ligase-catalyzed dsDNA end-joining (DNA ligation) about 30- to 40-fold. The DNA ligation enhancement by RecA or Rad51 transforms most of the substrate DNA into multimers within 2-5 min, and for this enhancement, ADP is the common and best cofactor. Adenosine triphosphate (ATP) is effective for RecA, but not for Rad51. Rad51/RecA-enhanced DNA ligation depends on dsDNA-binding, as shown by a mutant, and is independent of physical interactions with the DNA ligase. These observations demonstrate the common and unique activities of RecA and Rad51 to juxtapose dsDNA-ends in preparation for covalent joining by a DNA ligase. This new in vitro function of Rad51 provides a simple explanation for our genetic observation that Rad51 plays a role in the fidelity of the end-joining of a reporter plasmid DNA, by yeast canonical non-homologous end-joining (NHEJ) in vivo.

Multiple and Variable NHEJ-Like Genes Are Involved in Resistance to DNA Damage in Streptomyces ambofaciens

Non-homologous end-joining (NHEJ) is a double strand break (DSB) repair pathway which does not require any homologous template and can ligate two DNA ends together. The basic bacterial NHEJ machinery involves two partners: the Ku protein, a DNA end binding protein for DSB recognition and the multifunctional LigD protein composed a ligase, a nuclease and a polymerase domain, for end processing and ligation of the broken ends. In silico analyses performed in the 38 sequenced genomes of Streptomyces species revealed the existence of a large panel of NHEJ-like genes. Indeed, ku genes or ligD domain homologues are scattered throughout the genome in multiple copies and can be distinguished in two categories: the “core” NHEJ gene set constituted of conserved loci and the “variable” NHEJ gene set constituted of NHEJ-like genes present in only a part of the species. In Streptomyces ambofaciens ATCC23877, not only the deletion of “core” genes but also that of “variable” genes led to an increased sensitivity to DNA damage induced by electron beam irradiation. Multiple mutants of ku, ligase or polymerase encoding genes showed an aggravated phenotype compared to single mutants. Biochemical assays revealed the ability of Ku-like proteins to protect and to stimulate ligation of DNA ends. RT-qPCR and GFP fusion experiments suggested that ku-like genes show a growth phase dependent expression profile consistent with their involvement in DNA repair during spores formation and/or germination.

Multigene disruption in undomesticated Bacillus subtilis ATCC 6051a using the CRISPR/Cas9 system

Bacillus subtilis ATCC 6051a is an undomesticated strain used in the industrial production of enzymes. Because it is poorly transformable, genetic manipulation in this strain requires a highly efficient genome editing method. In this study, a Streptococcus pyogenes CRISPR/Cas9 system consisting of an all-in-one knockout plasmid containing a target-specific guide RNA, cas9, and a homologous repair template was established for highly efficient gene disruption in B. subtilis ATCC 6051a. With an efficiency of 33% to 53%, this system was used to disrupt the srfC, spoIIAC, nprE, aprE and amyE genes of B. subtilis ATCC 6051a, which hamper its use in industrial fermentation. Compared with B. subtilis ATCC 6051a, the final mutant, BS5 (ΔsrfC, ΔspoIIAC, ΔnprE, ΔaprE, ΔamyE), produces much less foam during fermentation, displays greater resistant to spore formation, and secretes 2.5-fold more β-cyclodextrin glycosyltransferase into the fermentation medium. Thus, the CRISPR/Cas9 system proved to be a powerful tool for targeted genome editing in an industrially relevant, poorly transformable strain.

C-terminal region of bacterial Ku controls DNA bridging, DNA threading and recruitment of DNA ligase D for double strand breaks repair

Non-homologous end joining is a ligation process repairing DNA double strand breaks in eukaryotes and many prokaryotes. The ring structured eukaryotic Ku binds DNA ends and recruits other factors which can access DNA ends through the threading of Ku inward the DNA, making this protein a key ingredient for the scaffolding of the NHEJ machinery. However, this threading ability seems unevenly conserved among bacterial Ku. As bacterial Ku differ mainly by their C-terminus, we evaluate the role of this region in the loading and the threading abilities of Bacillus subtilis Ku and the stimulation of the DNA ligase LigD. We identify two distinct sub-regions: a ubiquitous minimal C-terminal region and a frequent basic C-terminal extension. We show that truncation of one or both of these sub-regions in Bacillus subtilis Ku impairs the stimulation of the LigD end joining activity in vitro. We further demonstrate that the minimal C-terminus is required for the Ku-LigD interaction, whereas the basic extension controls the threading and DNA bridging abilities of Ku. We propose that the Ku basic C-terminal extension increases the concentration of Ku near DNA ends, favoring the recruitment of LigD at the break, thanks to the minimal C-terminal sub-region.

Identification of a conserved 5'-dRP lyase activity in bacterial DNA repair ligase D and its potential role in base excision repair

Bacillus subtilis is one of the bacterial members provided with a nonhomologous end joining (NHEJ) system constituted by the DNA-binding Ku homodimer that recruits the ATP-dependent DNA Ligase D (BsuLigD) to the double-stranded DNA breaks (DSBs) ends. BsuLigD has inherent polymerization and ligase activities that allow it to fill the short gaps that can arise after realignment of the broken ends and to seal the resulting nicks, contributing to genome stability during the stationary phase and germination of spores. Here we show that BsuLigD also has an intrinsic 5'-2-deoxyribose-5-phosphate (dRP) lyase activity located at the N-terminal ligase domain that in coordination with the polymerization and ligase activities allows efficient repairing of 2'-deoxyuridine-containing DNA in an in vitro reconstituted Base Excision Repair (BER) reaction. The requirement of a polymerization, a dRP removal and a final sealing step in BER, together with the joint participation of BsuLigD with the spore specific AP endonuclease in conferring spore resistance to ultrahigh vacuum desiccation suggest that BsuLigD could actively participate in this pathway. We demonstrate the presence of the dRP lyase activity also in the homolog protein from the distantly related bacterium Pseudomonas aeruginosa, allowing us to expand our results to other bacterial LigDs.

Double-Strand DNA Break Repair in Mycobacteria

Discontinuity of both strands of the chromosome is a lethal event in all living organisms because it compromises chromosome replication. As such, a diversity of DNA repair systems has evolved to repair double-strand DNA breaks (DSBs). In part, this diversity of DSB repair systems has evolved to repair breaks that arise in diverse physiologic circumstances or sequence contexts, including cellular states of nonreplication or breaks that arise between repeats. Mycobacteria elaborate a set of three genetically distinct DNA repair pathways: homologous recombination, nonhomologous end joining, and single-strand annealing. As such, mycobacterial DSB repair diverges substantially from the standard model of prokaryotic DSB repair and represents an attractive new model system. In addition, the presence in mycobacteria of a DSB repair system that can repair DSBs in nonreplicating cells (nonhomologous end joining) or when DSBs arise between repeats (single-strand annealing) has clear potential relevance to Mycobacterium tuberculosis pathogenesis, although the exact role of these systems in M. tuberculosis pathogenesis is still being elucidated. In this article we will review the genetics of mycobacterial DSB repair systems, focusing on recent insights.

Analysis of the distribution and evolution of the ATP-dependent DNA ligases of bacteria delineates a distinct phylogenetic group 'Lig E'

Prior to the discovery of a minimal ATP-dependent DNA ligase in Haemophilus influenzae, bacteria were thought to only possess a NAD-dependent ligase, which was involved in sealing of Okazaki fragments. We now know that a diverse range of bacterial species possess up to six of these accessory bacterial ATP-dependent DNA ligases (b-ADLs), which vary in size and enzymatic domain associations. Here we compare the domain structure of different types of b-ADLs and investigate their distribution among the bacterial domain to describe possible evolutionary trajectories that gave rise to the sequence and structural diversity of these enzymes. Previous biochemical and genetic analyses have delineated three main classes of these enzymes: Lig B, Lig C and Lig D, which appear to have descended from a common ancestor within the bacterial domain. In the present study, we delineate a fourth group of b-ADLs, Lig E, which possesses a number of unique features at the primary and tertiary structural levels. The biochemical characteristics, domain structure and inferred extracellular location sets this group apart from the other b-ADLs. The results presented here indicate that the Lig E type ligases were horizontally transferred into bacteria in a separate event from other b-ADLs possibly from a bacteriophage.

Primase-polymerases are a functionally diverse superfamily of replication and repair enzymes

Until relatively recently, DNA primases were viewed simply as a class of proteins that synthesize short RNA primers requisite for the initiation of DNA replication. However, recent studies have shown that this perception of the limited activities associated with these diverse enzymes can no longer be justified. Numerous examples can now be cited demonstrating how the term ‘DNA primase’ only describes a very narrow subset of these nucleotidyltransferases, with the vast majority fulfilling multifunctional roles from DNA replication to damage tolerance and repair. This article focuses on the archaeo-eukaryotic primase (AEP) superfamily, drawing on recently characterized examples from all domains of life to highlight the functionally diverse pathways in which these enzymes are employed. The broad origins, functionalities and enzymatic capabilities of AEPs emphasizes their previous functional misannotation and supports the necessity for a reclassification of these enzymes under a category called primase-polymerases within the wider functional grouping of polymerases. Importantly, the repositioning of AEPs in this way better recognizes their broader roles in DNA metabolism and encourages the discovery of additional functions for these enzymes, aside from those highlighted here.

NHEJ enzymes LigD and Ku participate in stationary-phase mutagenesis in Pseudomonas putida

Under growth-restricting conditions bacterial populations can rapidly evolve by a process known as stationary-phase mutagenesis. Bacterial nonhomologous end-joining (NHEJ) system which consists of the DNA-end-binding enzyme Ku and the multifunctional DNA ligase LigD has been shown to be important for survival of bacteria especially during quiescent states, such as late stationary-phase populations or sporulation. In this study we provide genetic evidence that NHEJ enzymes participate in stationary-phase mutagenesis in a population of carbon-starved Pseudomonas putida. Both the absence of LigD or Ku resulted in characteristic spectra of stationary-phase mutations that differed from each other and also from the wild-type spectrum. This indicates that LigD and Ku may participate also in mutagenic pathways that are independent from each other. Our results also imply that both phosphoesterase (PE) and polymerase (POL) domains of the LigD protein are involved in the occurrence of mutations in starving P. putida. The participation of both Ku and LigD in the occurrence of stationary-phase mutations was further supported by the results of the analysis of mutation spectra in stationary-phase sigma factor RpoS-minus background. The spectra of mutations identified in the RpoS-minus background were also distinct if LigD or Ku was absent. Interestingly, the effects of the presence of these enzymes on the frequency of occurrence of certain types of mutations were different or even opposite in the RpoS-proficient and deficient backgrounds. These results imply that RpoS affects performance of mutagenic pathways in starving P. putida that utilize LigD and/or Ku.

Genomic and proteomic evidences unravel the UV-resistome of the poly-extremophile Acinetobacter sp. Ver3

Ultraviolet radiation can damage biomolecules, with detrimental or even lethal effects for life. Even though lower wavelengths are filtered by the ozone layer, a significant amount of harmful UV-B and UV-A radiation reach Earth's surface, particularly in high altitude environments. high-altitude Andean lakes (HAALs) are a group of disperse shallow lakes and salterns, located at the Dry Central Andes region in South America at altitudes above 3,000 m. As it is considered one of the highest UV-exposed environments, HAAL microbes constitute model systems to study UV-resistance mechanisms in environmental bacteria at various complexity levels. Herein, we present the genome sequence of Acinetobacter sp. Ver3, a gammaproteobacterium isolated from Lake Verde (4,400 m), together with further experimental evidence supporting the phenomenological observations regarding this bacterium ability to cope with increased UV-induced DNA damage. Comparison with the genomes of other Acinetobacter strains highlighted a number of unique genes, such as a novel cryptochrome. Proteomic profiling of UV-exposed cells identified up-regulated proteins such as a specific cytoplasmic catalase, a putative regulator, and proteins associated to amino acid and protein synthesis. Down-regulated proteins were related to several energy-generating pathways such as glycolysis, beta-oxidation of fatty acids, and electronic respiratory chain. To the best of our knowledge, this is the first report on a genome from a polyextremophilic Acinetobacter strain. From the genomic and proteomic data, an "UV-resistome" was defined, encompassing the genes that would support the outstanding UV-resistance of this strain.

Mycobacterium tuberculosis RecA is indispensable for inhibition of the mitogen-activated protein kinase-dependent bactericidal activity of THP-1-derived macrophages in vitro

Our knowledge about the mechanisms utilized by Mycobacterium tuberculosis to survive inside macrophages is still incomplete. One of the mechanism that protects M. tuberculosis from the host's microbicidal products and allows bacteria to survive involves DNA repair systems such as the homologous recombination (HR) and nonhomologous end-joining (NHEJ) pathways. It is accepted that any pathway that contributes to genome maintenance should be considered as potentially important virulence factor. In these studies, we investigated reactive oxygen species, nitric oxide and tumor necrosis factor-α production by macrophages infected with wild-type M. tuberculosis, with an HR-defective mutant (∆recA), with an NHEJ-defective mutant [∆(ku,ligD)], with a mutant defective for both HR and NHEJ [∆(ku,ligD,recA)], or with appropriate complemented strains. We also assessed the involvement of extracellular signal-regulated kinases (ERKs) 1 and 2 in the response of macrophages to infection with the above-mentioned strains, and ERK1/2 phosphorylation in M. tuberculosis-infected macrophages. We found that mutants lacking RecA induced a greater bactericidal response by macrophages than did the wild-type strain or an NHEJ-defective mutant, and activated ERK1/2 was involved only in the response of macrophages to recA deletion mutants [∆(ku,ligD,recA) and ∆recA]. We also demonstrated that only the triple mutant induced ERK1/2 phosphorylation in phorbol-12-myristate-13-acetate-stimulated macrophages. Moreover, HR-defective mutants induced lower amounts of tumor necrosis factor-α secretion than did the wild-type or ∆(ku,ligD). Our results indicate that RecA contributes to M. tuberculosis virulence, and also suggest that diminished ERK1/2 activation in macrophages infected with M. tuberculosis possessing recA may be an important mechanism by which wild-type mycobacteria escape intracellular killing.

Archaeal replicative primases can perform translesion DNA synthesis

DNA replicases routinely stall at lesions encountered on the template strand, and translesion DNA synthesis (TLS) is used to rescue progression of stalled replisomes. This process requires specialized polymerases that perform translesion DNA synthesis. Although prokaryotes and eukaryotes possess canonical TLS polymerases (Y-family Pols) capable of traversing blocking DNA lesions, most archaea lack these enzymes. Here, we report that archaeal replicative primases (Pri S, primase small subunit) can also perform TLS. Archaeal Pri S can bypass common oxidative DNA lesions, such as 8-Oxo-2'-deoxyguanosines and UV light-induced DNA damage, faithfully bypassing cyclobutane pyrimidine dimers. Although it is well documented that archaeal replicases specifically arrest at deoxyuracils (dUs) due to recognition and binding to the lesions, a replication restart mechanism has not been identified. Here, we report that Pri S efficiently replicates past dUs, even in the presence of stalled replicase complexes, thus providing a mechanism for maintaining replication bypass of these DNA lesions. Together, these findings establish that some replicative primases, previously considered to be solely involved in priming replication, are also TLS proficient and therefore may play important roles in damage tolerance at replication forks.

A moonlighting function of Mycobacterium smegmatis Ku in zinc homeostasis?

Ku protein participates in DNA double-strand break repair via the nonhomologous end-joining pathway. The three-dimensional structure of eukaryotic Ku reveals a central core consisting of a β-barrel domain and pillar and bridge regions that combine to form a ring-like structure that encircles DNA. Homologs of Ku are encoded by a subset of bacterial species, and they are predicted to conserve this core domain. In addition, the bridge region of Ku from some bacteria is predicted from homology modeling and sequence analyses to contain a conventional HxxC and CxxC (where x is any residue) zinc-binding motif. These potential zinc-binding sites have either deteriorated or been entirely lost in Ku from other organisms. Using an in vitro metal binding assay, we show that Mycobacterium smegmatis Ku binds two zinc ions. Zinc binding modestly stabilizes the Ku protein (by ∼3°C) and prevents cysteine oxidation, but it has little effect on DNA binding. In vivo, zinc induces significant upregulation of the gene encoding Ku (∼sixfold) as well as a divergently oriented gene encoding a predicted zinc-dependent MarR family transcription factor. Notably, overexpression of Ku confers zinc tolerance on Escherichia coli. We speculate that zinc-binding sites in Ku proteins from M. smegmatis and other mycobacterial species have been evolutionarily retained to provide protection against zinc toxicity without compromising the function of Ku in DNA double-strand break repair.

Efficient processing of abasic sites by bacterial nonhomologous end-joining Ku proteins

Intracellular reactive oxygen species as well as the exposure to harsh environmental conditions can cause, in the single chromosome of Bacillus subtilis spores, the formation of apurinic/apyrimidinic (AP) sites and strand breaks whose repair during outgrowth is crucial to guarantee cell viability. Whereas double-stranded breaks are mended by the nonhomologous end joining (NHEJ) system composed of an ATP-dependent DNA Ligase D (LigD) and the DNA-end-binding protein Ku, repair of AP sites would rely on an AP endonuclease or an AP-lyase, a polymerase and a ligase. Here we show that B. subtilis Ku (BsuKu), along with its pivotal role in allowing joining of two broken ends by B. subtilis LigD (BsuLigD), is endowed with an AP/deoxyribose 5'-phosphate (5'-dRP)-lyase activity that can act on ssDNA, nicked molecules and DNA molecules without ends, suggesting a potential role in BER during spore outgrowth. Coordination with BsuLigD makes possible the efficient joining of DNA ends with near terminal abasic sites. The role of this new enzymatic activity of Ku and its potential importance in the NHEJ pathway is discussed. The presence of an AP-lyase activity also in the homolog protein from the distantly related bacterium Pseudomonas aeruginosa allows us to expand our results to other bacterial Ku proteins.

Enzyme-adenylate structure of a bacterial ATP-dependent DNA ligase with a minimized DNA-binding surface

The enzyme–adenylate structure of a bacterial ATP-dependent DNA ligase (ADL), which does not have any additional DNA-binding domains, is similar to minimal viral ADLs that comprise only the core catalytic domains. The bacterial ADL also lacks the unstructured loops which are involved in DNA binding in the viral ADLs, implying that it must instead use short well structured motifs of the core domains to engage its substrate.

DNA ligases are a structurally diverse class of enzymes which share a common catalytic core and seal breaks in the phosphodiester backbone of double-stranded DNA via an adenylated intermediate. Here, the structure and activity of a recombinantly produced ATP-dependent DNA ligase from the bacterium Psychromonas sp. strain SP041 is described. This minimal-type ligase, like its close homologues, is able to ligate singly nicked double-stranded DNA with high efficiency and to join cohesive-ended and blunt-ended substrates to a more limited extent. The 1.65 Å resolution crystal structure of the enzyme–adenylate complex reveals no unstructured loops or segments, and suggests that this enzyme binds the DNA without requiring full encirclement of the DNA duplex. This is in contrast to previously characterized minimal DNA ligases from viruses, which use flexible loop regions for DNA interaction. The Psychromonas sp. enzyme is the first structure available for the minimal type of bacterial DNA ligases and is the smallest DNA ligase to be crystallized to date.

The Ku heterodimer: function in DNA repair and beyond

Ku is an abundant, highly conserved DNA binding protein found in both prokaryotes and eukaryotes that plays essential roles in the maintenance of genome integrity. In eukaryotes, Ku is a heterodimer comprised of two subunits, Ku70 and Ku80, that is best characterized for its central role as the initial DNA end binding factor in the "classical" non-homologous end joining (C-NHEJ) pathway, the main DNA double-strand break (DSB) repair pathway in mammals. Ku binds double-stranded DNA ends with high affinity in a sequence-independent manner through a central ring formed by the intertwined strands of the Ku70 and Ku80 subunits. At the break, Ku directly and indirectly interacts with several C-NHEJ factors and processing enzymes, serving as the scaffold for the entire DNA repair complex. There is also evidence that Ku is involved in signaling to the DNA damage response (DDR) machinery to modulate the activation of cell cycle checkpoints and the activation of apoptosis. Interestingly, Ku is also associated with telomeres, where, paradoxically to its DNA end-joining functions, it protects the telomere ends from being recognized as DSBs, thereby preventing their recombination and degradation. Ku, together with the silent information regulator (Sir) complex is also required for transcriptional silencing through telomere position effect (TPE). How Ku associates with telomeres, whether it is through direct DNA binding, or through protein-protein interactions with other telomere bound factors remains to be determined. Ku is central to the protection of organisms through its participation in C-NHEJ to repair DSBs generated during V(D)J recombination, a process that is indispensable for the establishment of the immune response. Ku also functions to prevent tumorigenesis and senescence since Ku-deficient mice show increased cancer incidence and early onset of aging. Overall, Ku function is critical to the maintenance of genomic integrity and to proper cellular and organismal development.

Bacterial genome instability

Bacterial genomes are remarkably stable from one generation to the next but are plastic on an evolutionary time scale, substantially shaped by horizontal gene transfer, genome rearrangement, and the activities of mobile DNA elements. This implies the existence of a delicate balance between the maintenance of genome stability and the tolerance of genome instability. In this review, we describe the specialized genetic elements and the endogenous processes that contribute to genome instability. We then discuss the consequences of genome instability at the physiological level, where cells have harnessed instability to mediate phase and antigenic variation, and at the evolutionary level, where horizontal gene transfer has played an important role. Indeed, this ability to share DNA sequences has played a major part in the evolution of life on Earth. The evolutionary plasticity of bacterial genomes, coupled with the vast numbers of bacteria on the planet, substantially limits our ability to control disease.

Mycobacterium smegmatis Ku binds DNA without free ends

Ku is central to the non-homologous end-joining pathway of double-strand-break repair in all three major domains of life, with eukaryotic homologues being associated with more diversified roles compared with prokaryotic and archaeal homologues. Ku has a conserved central 'ring-shaped' core domain. While prokaryotic homologues lack the N- and C-terminal domains that impart functional diversity to eukaryotic Ku, analyses of Ku from certain prokaryotes such as Pseudomonas aeruginosa and Mycobacterium smegmatis have revealed the presence of distinct C-terminal extensions that modulate DNA-binding properties. We report in the present paper that the lysine-rich C-terminal extension of M. smegmatis Ku contacts the core protein domain as evidenced by an increase in DNA-binding affinity and a decrease in thermal stability and intrinsic tryptophan fluorescence upon its deletion. Ku deleted for this C-terminus requires free DNA ends for binding, but translocates to internal DNA sites. In contrast, full-length Ku can directly bind DNA without free ends, suggesting that this property is conferred by its C-terminus. Such binding to internal DNA sites may facilitate recruitment to sites of DNA damage. The results of the present study also suggest that extensions beyond the shared core domain may have independently evolved to expand Ku function.