A nomenclature for restriction enzymes, DNA methyltransferases, homing endonucleases and their genes

OUP accepted manuscript

Epigenetic DNA methylation plays an important role in bacteria by influencing gene expression and allowing discrimination between self-DNA and intruders such as phages and plasmids. Restriction–modification (RM) systems use a methyltransferase (MTase) to modify a specific sequence motif, thus protecting host DNA from cleavage by a cognate restriction endonuclease (REase) while leaving invading DNA vulnerable. Other REases occur solitarily and cleave methylated DNA. REases and RM systems are frequently mobile, influencing horizontal gene transfer by altering the compatibility of the host for foreign DNA uptake. However, whether mobile defence systems affect pre-existing host defences remains obscure. Here, we reveal an epigenetic conflict between an RM system (PcaRCI) and a methylation-dependent REase (PcaRCII) in the plant pathogen Pectobacterium carotovorum RC5297. The PcaRCI RM system provides potent protection against unmethylated plasmids and phages, but its methylation motif is targeted by the methylation-dependent PcaRCII. This potentially lethal co-existence is enabled through epigenetic silencing of the PcaRCII-encoding gene via promoter methylation by the PcaRCI MTase. Comparative genome analyses suggest that the PcaRCII-encoding gene was already present and was silenced upon establishment of the PcaRCI system. These findings provide a striking example for selfishness of RM systems and intracellular competition between different defences.

Characterization of the Type I Restriction Modification System Broadly Conserved among Group A Streptococci

Although prokaryotic DNA methylation investigations have long focused on immunity against exogenous DNA, it has been recently recognized that DNA methylation impacts gene expression and phase variation in Streptococcus pneumoniae and Streptococcus suis. A comprehensive analysis of DNA methylation is lacking for beta-hemolytic streptococci, and thus we sought to examine DNA methylation in the major human pathogen group A Streptococcus (GAS). Using a database of 224 GAS genomes encompassing 80 emm types, we found that nearly all GAS strains encode a type I restriction modification (RM) system that lacks the hsdS′ alleles responsible for impacting gene expression in S. pneumoniae and S. suis. The GAS type I system is located on the core chromosome, while sporadically present type II orphan methyltransferases were identified on prophages. By combining single-molecule real-time (SMRT) analyses of 10 distinct emm types along with phylogenomics of 224 strains, we were able to assign 13 methylation patterns to the GAS population. Inactivation of the type I RM system, occurring either naturally through phage insertion or through laboratory-induced gene deletion, abrogated DNA methylation detectable via either SMRT or MinION sequencing. Contrary to a previous report, inactivation of the type I system did not impact transcript levels of the gene (mga) encoding the key multigene activator protein (Mga) or Mga-regulated genes. Inactivation of the type I system significantly increased plasmid transformation rates. These data delineate the breadth of the core chromosomal type I RM system in the GAS population and clarify its role in immunity rather than impacting Mga regulon expression.

IMPORTANCE The advent of whole-genome approaches capable of detecting DNA methylation has markedly expanded appreciation of the diverse roles of epigenetic modification in prokaryotic physiology. For example, recent studies have suggested that DNA methylation impacts gene expression in some streptococci. The data described herein are from the first systematic analysis of DNA methylation in a beta-hemolytic streptococcus and one of the few analyses to comprehensively characterize DNA methylation across hundreds of strains of the same bacterial species. We clarify that DNA methylation in group A Streptococcus (GAS) is primarily due to a type I restriction modification (RM) system present in the core genome and does not impact mga-regulated virulence gene expression, but does impact immunity against exogenous DNA. The identification of the DNA motifs recognized by each type I RM system may assist with optimizing methods for GAS genetic manipulation and help us understand how bacterial pathogens acquire exogenous DNA elements.

Advanced biotechnology using methyltransferase and its applications in bacteria: a mini review

Since prokaryotic restriction-modification (RM) systems protect the host by cleaving foreign DNA by restriction endonucleases, it is difficult to introduce engineered plasmid DNAs into newly isolated microorganisms whose RM system is not discovered. The prokaryotes also possess methyltransferases to protect their own DNA from the endonucleases. As those methyltransferases can be utilized to methylate engineered plasmid DNAs before transformation and to enhance the stability within the cells, the study on methyltransferases in newly isolated bacteria is essential for genetic engineering. Here, we introduce the mechanism of the RM system, specifically the methyltransferases and their biotechnological applications. These biotechnological strategies could facilitate plasmid DNA-based genetic engineering in bacteria strains that strongly defend against foreign DNA.

Evaluation of the Properties of the DNA Methyltransferase from Aeropyrum pernix K1

Little is known regarding the DNA methyltransferases (MTases) in hyperthermophilic archaea. In this study, we focus on an MTase from Aeropyrum pernix K1, a hyperthermophilic archaeon that is found in hydrothermal vents and whose optimum growth temperature is 90°C to 95°C. From genomic sequence analysis, A. pernix K1 has been predicted to have a restriction-modification system (R-M system). The restriction endonuclease from A. pernix K1 (known as ApeKI from New England BioLabs Inc. [catalog code R06435]) has been described previously, but the properties of the MTase from A. pernix K1 (M.ApeKI) have not yet been clarified. Thus, we demonstrated the properties of M.ApeKI. In this study, M.ApeKI was expressed in Escherichia coli strain JM109 and affinity purified using its His tag. The recognition sequence of M.ApeKI was determined by methylation activity and bisulfite sequencing (BS-seq). High-performance liquid chromatography (HPLC) was used to detect the position of the methyl group in methylated cytosine. As a result, it was clarified that M.ApeKI adds the methyl group at the C-5 position of the second cytosine in 5'-GCWGC-3'. Moreover, we also determined that the MTase optimum temperature was over 70°C and that it is strongly tolerant to high temperatures. M.ApeKI is the first highly thermostable DNA (cytosine-5)-methyltransferase to be evaluated by experimental evidence. IMPORTANCE In general, thermophilic bacteria with optimum growth temperatures over or equal to 60°C have been predicted to include only N⁴-methylcytosine or N⁶-methyladenine as methylated bases in their DNA, because 5-methylcytosine is susceptible to deamination by heat. However, from this study, A. pernix K1, with an optimum growth temperature at 95°C, was demonstrated to produce a DNA (cytosine-5)-methyltransferase. Thus, A. pernix K1 presumably has 5-methylcytosine in its DNA and may produce an original repair system for the expected C-to-T mutations. M.ApeKI was demonstrated to be tolerant to high temperatures; thus, we expect that M.ApeKI may be valuable for the development of a novel analysis system or epigenetic editing tool.

A Type I Restriction Modification System Influences Genomic Evolution Driven by Horizontal Gene Transfer in Paenibacillus polymyxa

Considered a “Generally Recognized As Safe” (GRAS) bacterium, the plant growth–promoting rhizobacterium Paenibacillus polymyxa has been widely applied in agriculture and animal husbandry. It also produces valuable compounds that are used in medicine and industry. Our previous work showed the presence of restriction modification (RM) system in P. polymyxa ATCC 842. Here, we further analyzed its genome and methylome by using SMRT sequencing, which revealed the presence of a larger number of genes, as well as a plasmid documented as a genomic region in a previous report. A number of mobile genetic elements (MGEs), including 78 insertion sequences, six genomic islands, and six prophages, were identified in the genome. A putative lysozyme-encoding gene from prophage P6 was shown to express lysin which caused cell lysis. Analysis of the methylome and genome uncovered a pair of reverse-complementary DNA methylation motifs which were widespread in the genome, as well as genes potentially encoding their cognate type I restriction-modification system PpoAI. Further genetic analysis confirmed the function of PpoAI as a RM system in modifying and restricting DNA. The average frequency of the DNA methylation motifs in MGEs was lower than that in the genome, implicating a role of PpoAI in restricting MGEs during genomic evolution of P. polymyxa. Finally, comparative analysis of R, M, and S subunits of PpoAI showed that homologs of the PpoAI system were widely distributed in species belonging to other classes of Firmicute, implicating a role of the ancestor of PpoAI in the genomic evolution of species beyond Paenibacillus.

DNA methylation by three Type I restriction modification systems of Escherichia coli does not influence gene regulation of the host bacterium

DNA methylation is a common epigenetic mark that influences transcriptional regulation, and therefore cellular phenotype, across all domains of life. In particular, both orphan methyltransferases and those from phasevariable restriction modification systems (RMSs) have been co-opted to regulate virulence epigenetically in many bacteria. We now show that three distinct non-phasevariable Type I RMSs in Escherichia coli have no measurable impact on gene expression, in vivo virulence, or any of 1190 in vitro growth phenotypes. We demonstrated this using both Type I RMS knockout mutants as well as heterologous installation of Type I RMSs into two E. coli strains. These data provide three clear and currently rare examples of restriction modification systems that have no impact on their host organism’s gene regulation. This leads to the possibility that other such nonregulatory methylation systems may exist, broadening our view of the potential role that RMSs may play in bacterial evolution.

Resveratrol inhibits MUC5AC expression by regulating SPDEF in lung cancer cells

BACKGROUND

MUC5AC was recently identified to play important roles in the proliferation and metastasis of malignant mucinous lung tumor cells. Resveratrol (Res), a natural compound with anticancer effects in lung cancer cells, has been reported to inhibit mucin production in airway epithelial cells. This study aimed to investigate the inhibitory effect of Res on MUC5AC expression in lung mucinous adenocarcinoma cells and the potential mechanisms.

METHODS

Mucus-producing A549 human lung carcinoma cells were used to test the effects of Res on SPDEF and MUC5AC expression. Gene and protein expression was assessed by real-time quantitative PCR (qPCR), immunofluorescence and western blotting assays. SPDEF lentivirus was used to upregulate SPDEF expression levels in mucus-producing A549 human lung carcinoma cells. Cell proliferation was assessed by Cell Counting Kit-8 (CCK-8) assay.

RESULTS

Res decreased MUC5AC expression in an SPDEF-dependent manner in mucus-producing A549 human lung carcinoma cells, and this change was accompanied by decreased ERK expression and AKT pathway activation. Moreover, SPDEF was found to be overexpressed in lung adenocarcinoma (LUAD), especially in mucinous adenocarcinoma. In-vitro functional assays showed that overexpression of SPDEF reduced the chemosensitivity of A549 cells to cisplatin (DDP). In addition, Res treatment increased A549 cell chemosensitivity to DDP by inhibiting the SPDEF-MUC5AC axis.

CONCLUSION

Our results indicate that the SPDEF-MUC5AC axis is associated with DDP sensitivity, and that Res decreases SPDEF and MUC5AC expression by inhibiting ERK and AKT signaling in A549 cells, which provides a potential pharmacotherapy for the prevention and therapeutic management of mucinous adenocarcinoma.

Beyond Restriction Modification: Epigenomic Roles of DNA Methylation in Prokaryotes

The amount of bacterial and archaeal genome sequence and methylome data has greatly increased over the last decade, enabling new insights into the functional roles of DNA methylation in these organisms. Methyltransferases (MTases), the enzymes responsible for DNA methylation, are exchanged between prokaryotes through horizontal gene transfer and can function either as part of restriction-modification systems or in apparent isolation as single (orphan) genes. The patterns of DNA methylation they confer on the host chromosome can have significant effects on gene expression, DNA replication, and other cellular processes. Some processes require very stable patterns of methylation, resulting in conservation of persistent MTases in a particular lineage. Other processes require patterns that are more dynamic yet more predictable than what is afforded by horizontal gene transfer and gene loss, resulting in phase-variable or recombination-driven MTase alleles. In this review, we discuss what is currently known about the functions of DNA methylation in prokaryotes in light of these evolutionary patterns. Expected final online publication date for the Annual Review of Microbiology, Volume 75 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Characterization of BisI Homologs

BisI is a sequence-specific and 5-methylcytosine (m5C)-dependent restriction endonuclease (REase), that cleaves the modified DNA sequence Gm5CNGC (G indicates that the cytosine opposite to G is modified). We expressed and purified a number of BisI homologs from sequenced bacterial genomes and used Illumina sequencing to determine the Pam7902I (Esp638I-like) cleavage sites in phage Xp12 DNA. One BisI homolog KpnW2I is EcoBLMcrX-like, cleaving GCNGC/RCNGY/RCNRC sites with m5C. We also cloned and expressed three BisI homologs from metagenome sequences derived from thermophilic sources. One enzyme EsaTMI is active at 37 to 65°C. EsaHLI cleaves GCNGC sites with three to four m5C and is active up to 50°C. In addition, we determined the number and position of m5C in BisI sites for efficient cleavage. BisI cleavage efficiency of GCNGC site is as following: Gm5CNGC (two internal m5C) > Gm5CNGC (one internal m5C) > GCNGm5C (one external m5C) > > GCNGC (unmodified). Three or four m5C in GCNGC site also supports BisI cleavage although partial inhibition was observed on duplex oligos with four m5C. BisI can be used to partially cleave a desired GCNGC site targeted with a complementary oligonucleotide (hemi-methylated). The m5C-dependent BisI variants will be useful for epigenetic research.

Genomic insights on DNase production in Streptococcus agalactiae ST17 and ST19 strains

Streptococcus agalactiae evasion from the human defense mechanisms has been linked to the production of DNases. These were proposed to contribute to the hypervirulence of S. agalactiae ST17/capsular-type III strains, mostly associated with neonatal meningitis. We performed a comparative genomic analysis between ST17 and ST19 human strains with different cell tropism and distinct DNase production phenotypes. All S. agalactiae ST17 strains, with the exception of 2211-04, were found to display DNase activity, while the opposite scenario was observed for ST19, where 1203-05 was the only DNase(+) strain. The analysis of the genetic variability of the seven genes putatively encoding secreted DNases in S. agalactiae revealed an exclusive amino acid change in the predicted signal peptide of GBS0661 (NucA) of the ST17 DNase(-), and an exclusive amino acid change alteration in GBS0609 of the ST19 DNase(+) strain. Further core-genome analysis identified some specificities (SNVs or indels) differentiating the DNase(-) ST17 2211-04 and the DNase(+) ST19 1203-05 from the remaining strains of each ST. The pan-genomic analysis evidenced an intact phage without homology in S. agalactiae and a transposon homologous to TnGBS2.3 in ST17 DNase(-) 2211-04; the transposon was also found in one ST17 DNase(+) strain, yet with a different site of insertion. A group of nine accessory genes were identified among all ST17 DNase(+) strains, including the Eco47II family restriction endonuclease and the C-5 cytosine-specific DNA methylase. None of these loci was found in any DNase(-) strain, which may suggest that these proteins might contribute to the lack of DNase activity. In summary, we provide novel insights on the genetic diversity between DNase(+) and DNase(-) strains, and identified genetic traits, namely specific mutations affecting predicted DNases (NucA and GBS0609) and differences in the accessory genome, that need further investigation as they may justify distinct DNase-related virulence phenotypes in S. agalactiae.

Distinction between self and non-self in restriction modification: The mysterious case of type IIL enzymes

Most type II restriction modification systems employ rigorous mechanisms to interrogate methylation in both DNA strands to distinguish between host and invader DNA. Type IIL enzymes like DrdV are an exception to this rule. In this issue of Structure, Shen et al. (2021) address how such enzymes make the distinction successfully.

Nutrient Loading and Viral Memory Drive Accumulation of Restriction Modification Systems in Bloom-Forming Cyanobacteria

The mechanisms driving cyanobacterial harmful algal blooms (HABs) like those caused by Microcystis aeruginosa remain elusive, but improved defense against viral predation has been implicated for success in eutrophic environments. Our genus-level analyses of 139,023 genomes revealed that HAB-forming cyanobacteria carry vastly more restriction modification systems per genome (RMPG) than nearly all other prokaryotic genera, suggesting that viral defense is a cornerstone of their ecological success. In contrast, picocyanobacteria that numerically dominate nutrient-poor systems have the fewest RMPG within the phylum Cyanobacteria. We used classic resource competition models to explore the hypothesis that nutrient enrichments drive ecological selection for high RMPG due to increased host-phage contact rate. These classic models, agnostic to the mechanism of defense, explain how nutrient loading can select for increased RMPG but, importantly, fail to explain the extreme accumulation of these defense systems. However, extreme accumulation of RMPG can be achieved in a novel “memory” model that accounts for a unique activity of restriction modification systems: the accidental methylation of viral DNA by the methyltransferase. The methylated virus “remembers” the RM defenses of its former host and can evade these defenses if they are present in the next host. This viral memory leads to continual RM system devaluation; RMs accumulate extensively because the benefit of each addition is diminished. Our modeling leads to the hypothesis that nutrient loading and virion methylation drive the extreme accumulation of RMPG in HAB-forming cyanobacteria. Finally, our models suggest that hosts with different RMPG values can coexist when hosts have unique sets of RM systems.

A nucleotide-sensing endonuclease from the Gabija bacterial defense system

The arms race between bacteria and phages has led to the development of exquisite bacterial defense systems including a number of uncharacterized systems distinct from the well-known restriction-modification and CRISPR/Cas systems. Here, we report functional analyses of the GajA protein from the newly predicted Gabija system. The GajA protein is revealed as a sequence-specific DNA nicking endonuclease unique in that its activity is strictly regulated by nucleotide concentration. NTP and dNTP at physiological concentrations can fully inhibit the robust DNA cleavage activity of GajA. Interestingly, the nucleotide inhibition is mediated by an ATPase-like domain, which usually hydrolyzes ATP to stimulate the DNA cleavage when associated with other nucleases. These features suggest a mechanism of the Gabija defense in which an endonuclease activity is suppressed under normal conditions, while it is activated by the depletion of NTP and dNTP upon the replication and transcription of invading phages. This work highlights a concise strategy to utilize a DNA nicking endonuclease for phage resistance via nucleotide regulation.

Genome-wide identification of 5-methylcytosine sites in bacterial genomes by high-throughput sequencing of MspJI restriction fragments

Single-molecule Real-Time (SMRT) sequencing can easily identify sites of N6-methyladenine and N4-methylcytosine within DNA sequences, but similar identification of 5-methylcytosine sites is not as straightforward. In prokaryotic DNA, methylation typically occurs within specific sequence contexts, or motifs, that are a property of the methyltransferases that "write" these epigenetic marks. We present here a straightforward, cost-effective alternative to both SMRT and bisulfite sequencing for the determination of prokaryotic 5-methylcytosine methylation motifs. The method, called MFRE-Seq, relies on excision and isolation of fully methylated fragments of predictable size using MspJI-Family Restriction Enzymes (MFREs), which depend on the presence of 5-methylcytosine for cleavage. We demonstrate that MFRE-Seq is compatible with both Illumina and Ion Torrent sequencing platforms and requires only a digestion step and simple column purification of size-selected digest fragments prior to standard library preparation procedures. We applied MFRE-Seq to numerous bacterial and archaeal genomic DNA preparations and successfully confirmed known motifs and identified novel ones. This method should be a useful complement to existing methodologies for studying prokaryotic methylomes and characterizing the contributing methyltransferases.

Crystal structure of restriction endonuclease Kpn2I of CCGG-family

BACKGROUND

Restriction endonucleases belong to prokaryotic restriction-modification systems, that protect host cells from invading DNA. Type II restriction endonucleases recognize short 4-8 bp sequences in the target DNA and cut both DNA strands producing double strand breaks. Type II restriction endonuclease Kpn2I cleaves 5'-T/CCGGA DNA sequence ("/" marks the cleavage position). Analysis of protein sequences suggested that Kpn2I belongs to the CCGG-family, which contains ten enzymes that recognize diverse nucleotides outside the conserved 5'-CCGG core and share similar motifs for the 5'-CCGG recognition and cleavage.

METHODS

We solved a crystal structure of Kpn2I in a DNA-free form at 2.88 Å resolution. From the crystal structure we predicted active center and DNA recognition residues and tested them by mutational analysis. We estimated oligomeric state of Kpn2I by SEC-MALS and performed plasmid DNA cleavage assay to elucidate DNA cleavage mechanism.

RESULTS

Structure comparison confirmed that Kpn2I shares a conserved active site and structural determinants for the 5'-CCGG tetranucleotide recognition with other restriction endonucleases of the CCGG-family. Guided by structural similarity between Kpn2I and the CCGG-family restriction endonucleases PfoI and AgeI, Kpn2I residues involved in the outer base pair recognition were proposed.

CONCLUSIONS

Kpn2I is an orthodox Type IIP restriction endonuclease, which acts as a dimer. Kpn2I shares structural similarity to the CCGG-family restriction endonucleases PfoI, AgeI and PspGI.

GENERAL SIGNIFICANCE

The Kpn2I structure concluded the studies of the CCGG-family, covering detailed structural and biochemical characterization of eleven restriction enzymes and their complexes with DNA.

Microbial Arsenal of Antiviral Defenses - Part I

Bacteriophages or phages are viruses that infect bacterial cells (for the scope of this review we will also consider viruses that infect Archaea). Constant threat of phage infection is a major force that shapes evolution of the microbial genomes. To withstand infection, bacteria had evolved numerous strategies to avoid recognition by phages or to directly interfere with phage propagation inside the cell. Classical molecular biology and genetic engineering have been deeply intertwined with the study of phages and host defenses. Nowadays, owing to the rise of phage therapy, broad application of CRISPR-Cas technologies, and development of bioinformatics approaches that facilitate discovery of new systems, phage biology experiences a revival. This review describes variety of strategies employed by microbes to counter phage infection, with a focus on novel systems discovered in recent years. First chapter covers defense associated with cell surface, role of small molecules, and innate immunity systems relying on DNA modification.

Restriction endonuclease T.Smu451I with new cleavage specificity-neoschizomer of T.AsuI

A specific type II restriction endonuclease T.Smu451I has been purified to electrophoretic homogeneity from the frozen cells of soil bacterium Sphingobacterium multivorum 451 (formerly Flavobacterium multivorum 451), using ultrasonic grinding, nucleic acid removal by streptomycin sulfate, protein precipitation by ammonium sulfate and phosphocellulose P-11, DEAE-Cellulose DE-52, Hepharin-Sepharose CL-6B chromatography, and elucidated several characteristics of T.Smu451I. The molecular weight of the enzyme determined by gel filtration and SDS-polyacrylamide gel electrophoresis was calculated to be 45,000 ± 2000 D (dimer) and 23,000 ± 1000 D (monomer), respectively. The isoelectric point (pI) of T.Smu451I is 5.4. T.Smu451I recognizes pentanucleotide palindromic sequences 5'-GGNC↓C-3' and cleaves between C and C in position shown by arrow to produce 3'-cohesive terminus of trinucleotide. Therefore, T.Smu451I is a neoschizomer of T.AsuI.

An investigation of Burkholderia cepacia complex methylomes via SMRT sequencing and mutant analysis

Bacterial genomes can be methylated at particular motifs by methyltransferases (M). This DNA modification allows restriction endonucleases (R) to discriminate between self and foreign DNA. While the accepted primary function of such restriction modification (RM) systems is to degrade incoming foreign DNA, other roles of RM systems and lone R or M components have been found in genome protection, stability and the regulation of various phenotypes. The Burkholderia cepacia complex (Bcc) is a group of closely related opportunistic pathogens with biotechnological potential. Here, we constructed and analysed mutants lacking various RM components in the clinical Bcc isolate Burkholderia cenocepacia H111 and used SMRT sequencing of single mutants to assign the B. cenocepacia H111 Ms to their cognate motifs. DNA methylation is shown to affect biofilm formation, cell shape, motility, siderophore production and membrane vesicle production. Moreover, DNA methylation had a large effect on the maintenance of the Bcc virulence megaplasmid pC3. Our data also suggest that the gp51 M-encoding gene, which is essential in H111 and is located within a prophage, is required for maintaining the bacteriophage in a lysogenic state, thereby ensuring a constant, low level of phage production within the bacterial population.ImportanceWhile genome sequence determines an organism's proteins, methylation of the nucleotides themselves can confer additional properties. In bacteria, Ms modify specific nucleotide motifs to allow discrimination of 'self' from 'non-self' DNA, e.g. from bacteriophages. Restriction enzymes detect 'non-self' methylation patterns and cut foreign DNA. Furthermore, methylation of promoter regions can influence gene expression and hence affect various phenotypes. In this study, we determined the methylated motifs of four strains from the Burkholderia cepacia complex of opportunistic pathogens. We deleted all genes encoding the restriction and modification components in one of these strains, Burkholderia cenocepacia H111. It is shown that DNA methylation affects various phenotypic traits, the most noteworthy being lysogenicity of a bacteriophage and maintenance of a virulence megaplasmid.

Genome-wide methylome analysis of two strains belonging to the hypervirulent Neisseria meningitidis serogroup W ST-11 clonal complex

A rising incidence of meningococcal serogroup W disease has been evident in many countries worldwide. Serogroup W isolates belonging to the sequence type (ST)-11 clonal complex have been associated with atypical symptoms and increased case fatality rates. The continued expansion of this clonal complex in the later part of the 2010s has been largely due to a shift from the so-called original UK strain to the 2013 strain. Here we used single-molecule real-time (SMRT) sequencing to determine the methylomes of the two major serogroup W strains belonging to ST-11 clonal complex. Five methylated motifs were identified in this study, and three of the motifs, namely 5'-GATC-3', 5'-GAAGG-3', 5'-GCGCGC-3', were found in all 13 isolates investigated. The results showed no strain-specific motifs or difference in active restriction modification systems between the two strains. Two phase variable methylases were identified and the enrichment or depletion of the methylation motifs generated by these methylases varied between the two strains. Results from this work give further insight into the low diversity of methylomes in highly related strains and encourage further research to decipher the role of regions with under- or overrepresented methylation motifs.

Genetic and metabolic engineering challenges of C1-gas fermenting acetogenic chassis organisms

Unabated mining and utilisation of petroleum and petroleum resources and their conversion to essential fuels and chemicals have drastic environmental consequences, contributing to global warming and climate change. In addition, fossil fuels are finite resources, with a fast-approaching shortage. Accordingly, research efforts are increasingly focusing on developing sustainable alternatives for chemicals and fuels production. In this context, bioprocesses, relying on microorganisms, have gained particular interest. For example, acetogens use the Wood-Ljungdahl pathway to grow on single carbon C1-gases (CO2 and CO) as their sole carbon source and produce valuable products such as acetate or ethanol. These autotrophs can, therefore, be exploited for large-scale fermentation processes to produce industrially relevant chemicals from abundant greenhouse gases. In addition, genetic tools have recently been developed to improve these chassis organisms through synthetic biology approaches. This review will focus on the challenges of genetically and metabolically modifying acetogens. It will first discuss the physical and biochemical obstacles complicating successful DNA transfer in these organisms. Current genetic tools developed for several acetogens, crucial for strain engineering to consolidate and expand their catalogue of products, will then be described. Recent tool applications for metabolic engineering purposes to allow redirection of metabolic fluxes or production of non-native compounds will lastly be covered.

DNA adenine methylase, not the PstI restriction-modification system, regulates virulence gene expression in Shiga toxin-producing Escherichia coli

We previously reported a distinct methylome between the two Shiga toxin-producing Escherichia coli (STEC) O145:H28 strains linked to the 2010 U.S. lettuce-associated outbreak (RM13514) and the 2007 Belgium ice cream-associated outbreak (RM13516), respectively. This difference was thought to be attributed to a prophage encoded type II restriction-modification system (PstI R-M) in RM13514. Here, we characterized this PstI R-M system in comparison to DNA adenine methylase (Dam), a highly conserved enzyme in γ proteobacteria, by functional genomics. Deficiency in Dam led to a differential expression of over 1000 genes in RM13514, whereas deficiency in PstI R-M only impacted a few genes transcriptionally. Dam regulated genes involved in diverse functions, whereas PstI R-M regulated genes mostly encoding transporters and adhesins. Dam regulated a large number of genes located on prophages, pathogenicity islands, and plasmids, including Shiga toxin genes, type III secretion system (TTSS) genes, and enterohemolysin genes. Production of Stx2 in dam mutant was significantly higher than in RM13514, supporting a role of Dam in maintaining lysogeny of Stx2-prophage. However, following mitomycin C treatment, Stx2 in RM13514 was significantly higher than that of dam or PstI R-M deletion mutant, implying that both Dam and PstI R-M contributed to maximum Stx2 production.

Biochemical and molecular characterization of a restriction endonuclease Tvu2HI from Thermoactinomyces vulgaris 2H and study of its R-M system

A bacterial strain 2H isolated from soil and identified as Thermoactinomyces vulgaris produce a potent Type II restriction endonuclease activity that has been extracted by a PEG/dextran aqueous two-phase system. Optimal temperature for the restriction endonuclease activity was 55-65°C. Specific DNA cleavage was obtained at pH range 7-10 and 10-20mM MgCl₂. Restriction cleavage analysis followed by sequencing confirms GG^CC as the recognition sequence. This enzyme, named Tvu2HI, is a thermostable isoschizomer of the mesophilic prototype restriction endonuclease HaeIII. Sequencing of the complete Thermoactinomyces vulgaris 2H genome revealed the presence of two adjacent ORFs coding for the restriction endonuclease Tvu2HI and the corresponding methyltransferase; an ORF coding for a putative Vsr nicking enzyme was found close to those coding for the Tvu2HI restriction-modification system. Phylogenetic analysis based on sequence alignment suggests a common origin of Tvu2HI R-M system with HaeIII-like R-M systems. This is the first investigation dealing with a Type II restriction endonuclease identified in a natural isolate of the genus Thermoactinomyces.

Structure-Based Deep Mining Reveals First-Time Annotations for 46 Percent of the Dark Annotation Space of the 9,671-Member Superproteome of the Nucleocytoplasmic Large DNA Viruses

We conducted an exhaustive search for three-dimensional structural homologs to the proteins of 20 key phylogenetically distinct nucleocytoplasmic DNA viruses (NCLDV). Structural matches covered 429 known protein domain superfamilies, with the most highly represented being ankyrin repeat, P-loop NTPase, F-box, protein kinase, and membrane occupation and recognition nexus (MORN) repeat. Domain superfamily diversity correlated with genome size, but a diversity of around 200 superfamilies appeared to correlate with an abrupt switch to paralogization. Extensive structural homology was found across the range of eukaryotic RNA polymerase II subunits and their associated basal transcription factors, with the coordinated gain and loss of clusters of subunits on a virus-by-virus basis. The total number of predicted endonucleases across the 20 NCLDV was nearly quadrupled from 36 to 132, covering much of the structural and functional diversity of endonucleases throughout the biosphere in DNA restriction, repair, and homing. Unexpected findings included capsid protein-transcription factor chimeras; endonuclease chimeras; enzymes for detoxification; antimicrobial peptides and toxin-antitoxin systems associated with symbiosis, immunity, and addiction; and novel proteins for membrane abscission and protein turnover.IMPORTANCE We extended the known annotation space for the NCLDV by 46%, revealing high-probability structural matches for fully 45% of the 9,671 query proteins and confirming up to 98% of existing annotations per virus. The most prevalent protein families included ankyrin repeat- and MORN repeat-containing proteins, many of which included an F-box, suggesting extensive host cell modulation among the NCLDV. Regression suggested a minimum requirement for around 36 protein structural superfamilies for a viable NCLDV, and beyond around 200 superfamilies, genome expansion by the acquisition of new functions was abruptly replaced by paralogization. We found homologs to herpesvirus surface glycoprotein gB in cytoplasmic viruses. This study provided the first prediction of an endonuclease in 10 of the 20 viruses examined; the first report in a virus of a phenolic acid decarboxylase, proteasomal subunit, or cysteine knot (defensin) protein; and the first report of a prokaryotic-type ribosomal protein in a eukaryotic virus.

Identification of Uncharacterized Components of Prokaryotic Immune Systems and Their Diverse Eukaryotic Reformulations

Nucleotide-activated effector deployment, prototyped by interferon-dependent immunity, is a common mechanistic theme shared by immune systems of several animals and prokaryotes. Prokaryotic versions include CRISPR-Cas with the CRISPR polymerase domain, their minimal variants, and systems with second messenger oligonucleotide or dinucleotide synthetase (SMODS). Cyclic or linear oligonucleotide signals in these systems help set a threshold for the activation of potentially deleterious downstream effectors in response to invader detection. We establish such a regulatory mechanism to be a more general principle of immune systems, which can also operate independently of such messengers. Using sensitive sequence analysis and comparative genomics, we identify 12 new prokaryotic immune systems, which we unify by this principle of threshold-dependent effector activation. These display regulatory mechanisms paralleling physiological signaling based on 3'-5' cyclic mononucleotides, NAD⁺-derived messengers, two- and one-component signaling that includes histidine kinase-based signaling, and proteolytic activation. Furthermore, these systems allowed the identification of multiple new sensory signal sensory components, such as a tetratricopeptide repeat (TPR) scaffold predicted to recognize NAD⁺-derived signals, unreported versions of the STING domain, prokaryotic YEATS domains, and a predicted nucleotide sensor related to receiver domains. We also identify previously unrecognized invader detection components and effector components, such as prokaryotic versions of the Wnt domain. Finally, we show that there have been multiple acquisitions of unidentified STING domains in eukaryotes, while the TPR scaffold was incorporated into the animal immunity/apoptosis signal-regulating kinase (ASK) signalosome.IMPORTANCE Both prokaryotic and eukaryotic immune systems face the dangers of premature activation of effectors and degradation of self-molecules in the absence of an invader. To mitigate this, they have evolved threshold-setting regulatory mechanisms for the triggering of effectors only upon the detection of a sufficiently strong invader signal. This work defines general templates for such regulation in effector-based immune systems. Using this, we identify several previously uncharacterized prokaryotic immune mechanisms that accomplish the regulation of downstream effector deployment by using nucleotide, NAD⁺-derived, two-component, and one-component signals paralleling physiological homeostasis. This study has also helped identify several previously unknown sensor and effector modules in these systems. Our findings also augment the growing evidence for the emergence of key animal immunity and chromatin regulatory components from prokaryotic progenitors.

N6-methyladenine and epigenetic immunity of Deinococcus radiodurans

DNA methylation is ubiquitously found in all three domains of life. This epigenetic modification on adenine or cytosine residues serves to regulate gene expression or to defend against invading DNA in bacteria. Here, we report the significance of N6-methyladenine (6mA) to epigenetic immunity in Deinococcus radiodurans. Putative protein encoded by DR_2267 ORF (Dam2_DR) contributed 35% of genomic 6mA in D. radiodurans but did not influence gene expression or radiation resistance. Dam2_DR was characterized to be a functional S-adenosyl methionine (SAM)-dependent N6-adenine DNA methyltransferase (MTase) but with no endonuclease activity. Adenine methylation from Dam2_DR or Dam1_DR (N6-adenine MTase encoded by DR_0643) improved DNA uptake during natural transformation. To the contrary, methylation from Escherichia coli N6-adenine MTase (Dam_EC that methylates adenine in GATC sequence) on donor plasmid drastically reduced DNA uptake in D. radiodurans, even in presence of Dam2_DR or Dam1_DR methylated adenines. With these results, we conclude that self-type N6-adenine methylation on donor DNA had a protective effect in absence of additional foreign methylation, a separate methylation-dependent Restriction Modification (R-M) system effectively identifies and limits uptake of G6mATC sequence containing donor DNA. This is the first report demonstrating presence of epigenetic immunity in D. radiodurans.

In vitro Type II Restriction of Bacteriophage DNA With Modified Pyrimidines

To counteract host-encoded restriction systems, bacteriophages (phages) incorporate modified bases in their genomes. For example, phages carry in their genomes modified pyrimidines such as 5-hydroxymethyl-cytosine (5hmC) in T4gt deficient in α- and β-glycosyltransferases, glucosylated-5-hydroxymethylcytosine (5gmC) in T4, 5-methylcytosine (5mC) in Xp12, and 5-hydroxymethyldeoxyuridine (5hmdU) in SP8. In this work we sequenced phage Xp12 and SP8 genomes and examined Type II restriction of T4gt, T4, Xp12, and SP8 phage DNAs. T4gt, T4, and Xp12 genomes showed resistance to 81.9% (186 out of 227 enzymes tested), 94.3% (214 out of 227 enzymes tested), and 89.9% (196 out of 218 enzymes tested), respectively, commercially available Type II restriction endonucleases (REases). The SP8 genome, however, was resistant to only ∼8.3% of these enzymes (17 out of 204 enzymes tested). SP8 DNA could be further modified by adenine DNA methyltransferases (MTases) such as M.Dam and M.EcoGII as well as a number of cytosine DNA MTases, such as CpG methylase. The 5hmdU base in SP8 DNA was phosphorylated by treatment with a 5hmdU DNA kinase to achieve ∼20% phosphorylated 5hmdU, resulting resistance or partially resistant to more Type II restriction. This work provides a convenient reference for molecular biologists working with modified pyrimidines and using REases. The genomic sequences of phage Xp12 and SP8 lay the foundation for further studies on genetic pathways for 5mC and 5hmdU DNA base modifications and for comparative phage genomics.

Drivers and sites of diversity in the DNA adenine methylomes of 93 Mycobacterium tuberculosis complex clinical isolates

This study assembles DNA adenine methylomes for 93 Mycobacterium tuberculosis complex (MTBC) isolates from seven lineages paired with fully-annotated, finished, de novo assembled genomes. Integrative analysis yielded four key results. First, methyltransferase allele-methylome mapping corrected methyltransferase variant effects previously obscured by reference-based variant calling. Second, heterogeneity analysis of partially active methyltransferase alleles revealed that intracellular stochastic methylation generates a mosaic of methylomes within isogenic cultures, which we formalize as ‘intercellular mosaic methylation’ (IMM). Mutation-driven IMM was nearly ubiquitous in the globally prominent Beijing sublineage. Third, promoter methylation is widespread and associated with differential expression in the ΔhsdM transcriptome, suggesting promoter HsdM-methylation directly influences transcription. Finally, comparative and functional analyses identified 351 sites hypervariable across isolates and numerous putative regulatory interactions. This multi-omic integration revealed features of methylomic variability in clinical isolates and provides a rational basis for hypothesizing the functions of DNA adenine methylation in MTBC physiology and adaptive evolution.

A positive perspective on DNA methylation: regulatory functions of DNA methylation outside of host defense in Gram-positive bacteria

The presence of post-replicative DNA methylation is pervasive among both prokaryotic and eukaryotic organisms. In bacteria, the study of DNA methylation has largely been in the context of restriction-modification systems, where DNA methylation serves to safeguard the chromosome against restriction endonuclease cleavage intended for invading DNA. There has been a growing recognition that the methyltransferase component of restriction-modification systems can also regulate gene expression, with important contributions to virulence factor gene expression in bacterial pathogens. Outside of restriction-modification systems, DNA methylation from orphan methyltransferases, which lack cognate restriction endonucleases, has been shown to regulate important processes, including DNA replication, DNA mismatch repair, and the regulation of gene expression. The majority of research and review articles have been focused on DNA methylation in the context of Gram-negative bacteria, with emphasis toward Escherichia coli, Caulobacter crescentus, and related Proteobacteria. Here we summarize the epigenetic functions of DNA methylation outside of host defense in Gram-positive bacteria, with a focus on the regulatory effects of both phase variable methyltransferases and DNA methyltransferases from traditional restriction-modification systems.

Types I and V Anti-CRISPR Proteins: From Phage Defense to Eukaryotic Synthetic Gene Circuits

Clustered regularly interspaced short palindromic repeats (CRISPR)-Cas (CRISPR-associated proteins), a prokaryotic RNA-mediated adaptive immune system, has been repurposed for gene editing and synthetic gene circuit construction both in bacterial and eukaryotic cells. In the last years, the emergence of the anti-CRISPR proteins (Acrs), which are natural OFF-switches for CRISPR-Cas, has provided a new means to control CRISPR-Cas activity and promoted a further development of CRISPR-Cas-based biotechnological toolkits. In this review, we focus on type I and type V-A anti-CRISPR proteins. We first narrate Acrs discovery and analyze their inhibitory mechanisms from a structural perspective. Then, we describe their applications in gene editing and transcription regulation. Finally, we discuss the potential future usage-and corresponding possible challenges-of these two kinds of anti-CRISPR proteins in eukaryotic synthetic gene circuits.

Shining Light on Human Gut Bacteriophages

The human gut is a complex environment that contains a multitude of microorganisms that are collectively termed the microbiome. Multiple factors have a role to play in driving the composition of human gut bacterial communities either toward homeostasis or the instability that is associated with many disease states. One of the most important forces are likely to be bacteriophages, bacteria-infecting viruses that constitute by far the largest portion of the human gut virome. Despite this, bacteriophages (phages) are the one of the least studied residents of the gut. This is largely due to the challenges associated with studying these difficult to culture entities. Modern high throughput sequencing technologies have played an important role in improving our understanding of the human gut phageome but much of the generated sequencing data remains uncharacterised. Overcoming this requires database-independent bioinformatic pipelines and even those phages that are successfully characterized only provide limited insight into their associated biological properties, and thus most viral sequences have been characterized as “viral dark matter.” Fundamental to understanding the role of phages in shaping the human gut microbiome, and in turn perhaps influencing human health, is how they interact with their bacterial hosts. An essential aspect is the isolation of novel phage-bacteria host pairs by direct isolation through various screening methods, which can transform in silico phages into a biological reality. However, this is also beset with multiple challenges including culturing difficulties and the use of traditional methods, such as plaquing, which may bias which phage-host pairs that can be successfully isolated. Phage-bacteria interactions may be influenced by many aspects of complex human gut biology which can be difficult to reproduce under laboratory conditions. Here we discuss some of the main findings associated with the human gut phageome to date including composition, our understanding of phage-host interactions, particularly the observed persistence of virulent phages and their hosts, as well as factors that may influence these highly intricate relationships. We also discuss current methodologies and bottlenecks hindering progression in this field and identify potential steps that may be useful in overcoming these hurdles.

Type II Restriction-Modification System from Gardnerella vaginalis ATCC 14018

Intensive horizontal gene transfer may generate diversity and heterogeneity within the genus Gardnerella. Restriction-modification (R-M) systems and CRISPR-Cas are the principal defense tools against foreign DNA in bacteria. Nearly half of the tested Gardnerella spp. isolates harbored the CRISPR-Cas system. Several putative R-M systems of Gardnerella spp. strains were identified in the REBASE database. However, there was no experimental evidence for restriction endonuclease (REase) activity in the isolates. We showed that G. vaginalis strain ATCC 14018 contains the REase R.Gva14018I, which recognizes GGCC and most probably generates blunt ends on cleavage. Bioinformatics evidence and the activity of recombinant methyltransferase M.Gva14018I in vivo indicate that ATCC 14018 possesses a HaeIII-like R-M system. The truncated R.Gva14018I-4 lacking the C-terminal region was expressed in Escherichia coli and displayed wild-type REase specificity. Polyclonal antibodies against R.Gva14018I-4 detected the wild-type REase in the cell lysate of ATCC 14018. The cofactor requirements for activity and bioinformatics analysis indicated that R.Gva14018I belongs to the PD-(D/E)XK family of REases. The REase-like activity was observed in 5 of 31 tested Gardnerella spp. strains, although none of these matched the DNA digestion pattern of R.Gva14018I.

Networking and Specificity-Changing DNA Methyltransferases in Helicobacter pylori

Epigenetic DNA base methylation plays important roles in gene expression regulation. We here describe a gene expression regulation network consisting of many DNA methyltransferases each frequently changing its target sequence-specificity. Our object Helicobacter pylori, a bacterium responsible for most incidence of stomach cancer, carries a large and variable repertoire of sequence-specific DNA methyltransferases. By creating a dozen of single-gene knockout strains for the methyltransferases, we revealed that they form a network controlling methylome, transcriptome and adaptive phenotype sets. The methyltransferases interact with each other in a hierarchical way, sometimes regulated positively by one methyltransferase but negatively with another. Motility, oxidative stress tolerance and DNA damage repair are likewise regulated by multiple methyltransferases. Their regulation sometimes involves translation start and stop codons suggesting coupling of methylation, transcription and translation. The methyltransferases frequently change their sequence-specificity through gene conversion of their target recognition domain and switch their target sets to remodel the network. The emerging picture of a metamorphosing gene regulation network, or firework, consisting of epigenetic systems ever-changing their specificity in search for adaptation, provides a new paradigm in understanding global gene regulation and adaptive evolution.

Optimization of Protein Thermostability and Exploitation of Recognition Behavior to Engineer Altered Protein-DNA Recognition

The redesign of a macromolecular binding interface and corresponding alteration of recognition specificity is a challenging endeavor that remains recalcitrant to computational approaches. This is particularly true for the redesign of DNA binding specificity, which is highly dependent upon bending, hydrogen bonds, electrostatic contacts, and the presence of solvent and counterions throughout the molecular interface. Thus, redesign of protein-DNA binding specificity generally requires iterative rounds of amino acid randomization coupled to selections. Here, we describe the importance of scaffold thermostability for protein engineering, coupled with a strategy that exploits the protein's specificity profile, to redesign the specificity of a pair of meganucleases toward three separate genomic targets. We determine and describe a series of changes in protein sequence, stability, structure, and activity that accumulate during the engineering process, culminating in fully retargeted endonucleases.

The CRISPR-Cas systems were selectively inactivated during evolution of Bacillus cereus group for adaptation to diverse environments

CRISPR-Cas systems are considered as barriers to horizontal gene transfer (HGT). However, the influence of such systems on HGT within species is unclear. Also, little is known about the impact of CRISPR-Cas systems on bacterial evolution at the population level. Here, using Bacillus cereus sensu lato as model, we investigate the interplay between CRISPR-Cas systems and HGT at the population scale. We found that only a small fraction of the strains have CRISPR-Cas systems (13.9% of 1871), and most of such systems are defective based on their gene content analysis. Comparative genomic analysis revealed that the CRISPR-Cas systems are barriers to HGT within this group, since strains harboring active systems contain less mobile genetic elements (MGEs), have lower fraction of unique genes and also display limited environmental distributions than strains without active CRISPR-Cas systems. The introduction of a functional CRISPR-Cas system into a strain lacking the system resulted in reduced adaptability to various stresses and decreased pathogenicity of the transformant strain, indicating that B. cereus group strains could benefit from inactivating such systems. Our work provides a large-scale case to support that the CRISPR-Cas systems are barriers to HGT within species, and that in the B. cereus group the inactivation of CRISPR-Cas systems correlated with acquisition of MGEs that could result in better adaptation to diverse environments.

Structural insights into assembly, operation and inhibition of a type I restriction-modification system

Type I restriction-modification (R-M) systems are widespread in prokaryotic genomes and provide robust protection against foreign DNA. They are multisubunit enzymes with methyltransferase, endonuclease and translocase activities. Despite extensive studies over the past five decades, little is known about the molecular mechanisms of these sophisticated machines. Here, we report the cryo-electron microscopy structures of the representative EcoR124I R-M system in different assemblies (R₂M₂S₁, R₁M₂S₁ and M₂S₁) bound to target DNA and the phage and mobile genetic element-encoded anti-restriction proteins Ocr and ArdA. EcoR124I can precisely regulate different enzymatic activities by adopting distinct conformations. The marked conformational transitions of EcoR124I are dependent on the intrinsic flexibility at both the individual-subunit and assembled-complex levels. Moreover, Ocr and ArdA use a DNA-mimicry strategy to inhibit multiple activities, but do not block the conformational transitions of the complexes. These structural findings, complemented by mutational studies of key intermolecular contacts, provide insights into assembly, operation and inhibition mechanisms of type I R-M systems.

Conserved DNA Methyltransferases: A Window into Fundamental Mechanisms of Epigenetic Regulation in Bacteria

An increasing number of studies have reported that bacterial DNA methylation has important functions beyond the roles in restriction-modification systems, including the ability of affecting clinically relevant phenotypes such as virulence, host colonization, sporulation, biofilm formation, among others. Although insightful, such studies have a largely ad hoc nature and would benefit from a systematic strategy enabling a joint functional characterization of bacterial methylomes by the microbiology community. In this opinion article, we propose that highly conserved DNA methyltransferases (MTases) represent a unique opportunity for bacterial epigenomic studies. These MTases are rather common in bacteria, span various taxonomic scales, and are present in multiple human pathogens. Apart from well-characterized core DNA MTases, like those from Vibrio cholerae, Salmonella enterica, Clostridioides difficile, or Streptococcus pyogenes, multiple highly conserved DNA MTases are also found in numerous human pathogens, including those belonging to the genera Burkholderia and Acinetobacter. We discuss why and how these MTases can be prioritized to enable a community-wide, integrative approach for functional epigenomic studies. Ultimately, we discuss how some highly conserved DNA MTases may emerge as promising targets for the development of novel epigenetic inhibitors for biomedical applications.

Retrons and their applications in genome engineering

Precision genome editing technologies have transformed modern biology. These technologies have arisen from the redirection of natural biological machinery, such as bacteriophage lambda proteins for recombineering and CRISPR nucleases for eliciting site-specific double-strand breaks. Less well-known is a widely distributed class of bacterial retroelements, retrons, that employ specialized reverse transcriptases to produce noncoding intracellular DNAs. Retrons' natural function and mechanism of genetic transmission have remained enigmatic. However, recent studies have harnessed their ability to produce DNA in situ for genome editing and evolution. This review describes retron biology and function in both natural and synthetic contexts. We also highlight areas that require further study to advance retron-based precision genome editing platforms.

High Synteny and Sequence Identity between Genomes of Nitrosococcus oceani Strains Isolated from Different Oceanic Gyres Reveals Genome Economization and Autochthonous Clonal Evolution

The ammonia-oxidizing obligate aerobic chemolithoautotrophic gammaproteobacterium, Nitrosococcus oceani, is omnipresent in the world's oceans and as such important to the global nitrogen cycle. We generated and compared high quality draft genome sequences of N. oceani strains isolated from the Northeast (AFC27) and Southeast (AFC132) Pacific Ocean and the coastal waters near Barbados at the interface between the Caribbean Sea and the North Atlantic Ocean (C-27) with the recently published Draft Genome Sequence of N. oceani Strain NS58 (West Pacific Ocean) and the complete genome sequence of N. oceani C-107, the type strain (ATCC 19707) isolated from the open North Atlantic, with the goal to identify indicators for the evolutionary origin of the species. The genomes of strains C-107, NS58, C-27, and AFC27 were highly conserved in content and synteny, and these four genomes contained one nearly sequence-identical plasmid. The genome of strain AFC132 revealed the presence of genetic inventory unknown from other marine ammonia-oxidizing bacteria such as genes encoding NiFe-hydrogenase and a non-ribosomal peptide synthetase (NRPS)-like siderophore biosynthesis module. Comparative genome analysis in context with the literature suggests that AFC132 represents a metabolically more diverse ancestral lineage to the other strains with C-107 and NS58 potentially being the youngest. The results suggest that the N. oceani species evolved by genome economization characterized by the loss of genes encoding catabolic diversity while acquiring a higher redundancy in inventory dedicated to nitrogen catabolism, both of which could have been facilitated by their rich complements of CRISPR/Cas and Restriction Modification systems.

Complete genome and methylome analysis of Neisseria meningitidis associated with increased serogroup Y disease

Invasive meningococcal disease (IMD) due to serogroup Y Neisseria meningitidis emerged in Europe during the 2000s. Draft genomes of serogroup Y isolates in Sweden revealed that although the population structure of these isolates was similar to other serogroup Y isolates internationally, a distinct strain (YI) and more specifically a sublineage (1) of this strain was responsible for the increase of serogroup Y IMD in Sweden. We performed single molecule real-time (SMRT) sequencing on eight serogroup Y isolates from different sublineages to unravel the genetic and epigenetic factors delineating them, in order to understand the serogroup Y emergence. Extensive comparisons between the serogroup Y sublineages of all coding sequences, complex genomic regions, intergenic regions, and methylation motifs revealed small point mutations in genes mainly encoding hypothetical and metabolic proteins, and non-synonymous variants in genes involved in adhesion, iron acquisition, and endotoxin production. The methylation motif CACNNNNNTAC was only found in isolates of sublineage 2. Only seven genes were putatively differentially expressed, and another two genes encoding hypothetical proteins were only present in sublineage 2. These data suggest that the serogroup Y IMD increase in Sweden was most probably due to small changes in genes important for colonization and transmission.

Comparative Genomic Analysis of Rhodococcus equi: An Insight into Genomic Diversity and Genome Evolution

Rhodococcus equi, a member of the Rhodococcus genus, is a gram-positive pathogenic bacterium. Rhodococcus possesses an open pan-genome that constitutes the basis of its high genomic diversity and allows for adaptation to specific niche conditions and the changing host environments. Our analysis further showed that the core genome of R. equi contributes to the pathogenicity and niche adaptation of R. equi. Comparative genomic analysis revealed that the genomes of R. equi shared identical collinearity relationship, and heterogeneity was mainly acquired by means of genomic islands and prophages. Moreover, genomic islands in R. equi were always involved in virulence, resistance, or niche adaptation and possibly working with prophages to cause the majority of genome expansion. These findings provide an insight into the genomic diversity, evolution, and structural variation of R. equi and a valuable resource for functional genomic studies.

Mining the Methylome Reveals Extensive Diversity in Staphylococcus epidermidis Restriction Modification

Staphylococcus epidermidis is a significant opportunistic pathogen of humans. Molecular studies in this species have been hampered by the presence of restriction-modification (RM) systems that limit introduction of foreign DNA. Here, we establish the complete genomes and methylomes for seven clinically significant, genetically diverse S. epidermidis isolates and perform the first systematic genomic analyses of the type I RM systems within both S. epidermidis and Staphylococcus aureus Our analyses revealed marked differences in the gene arrangement, chromosomal location, and movement of type I RM systems between the two species. Unlike S. aureus, S. epidermidis type I RM systems demonstrate extensive diversity even within a single genetic lineage. This is contrary to current assumptions and has important implications for approaching the genetic manipulation of S. epidermidis Using Escherichia coli plasmid artificial modification (PAM) to express S. epidermidis hsdMS, we readily overcame restriction barriers in S. epidermidis and achieved electroporation efficiencies equivalent to those of modification-deficient mutants. With these functional experiments, we demonstrated how genomic data can be used to predict both the functionality of type I RM systems and the potential for a strain to be electroporation proficient. We outline an efficient approach for the genetic manipulation of S. epidermidis strains from diverse genetic backgrounds, including those that have hitherto been intractable. Additionally, we identified S. epidermidis BPH0736, a naturally restriction-defective, clinically significant, multidrug-resistant ST2 isolate, as an ideal candidate for molecular studies.IMPORTANCE Staphylococcus epidermidis is a major cause of hospital-acquired infections, especially those related to implanted medical devices. Understanding how S. epidermidis causes disease and devising ways to combat these infections have been hindered by an inability to genetically manipulate clinically significant hospital-adapted strains. Here, we provide the first comprehensive analyses of the barriers to the uptake of foreign DNA in S. epidermidis and demonstrate that these are distinct from those described for S. aureus Using these insights, we demonstrate an efficient approach for the genetic manipulation of S. epidermidis to enable the study of clinical isolates for the first time.

The interaction of phages and bacteria: the co-evolutionary arms race

Since the dawn of life, bacteria and phages are locked in a constant battle and both are perpetually changing their tactics to overcome each other. Bacteria use various strategies to overcome the invading phages, including adsorption inhibition, restriction-modification (R/E) systems, CRISPR-Cas (clustered regularly interspaced short palindromic repeats-CRISPR-associated proteins) systems, abortive infection (Abi), etc. To counteract, phages employ intelligent tactics for the nullification of bacterial defense systems, such as accessing host receptors, evading R/E systems, and anti-CRISPR proteins. Intense knowledge about the details of these defense pathways is the basis for their broad utilities in various fields of research from microbiology to biotechnology. Hence, in this review, we discuss some strategies used by bacteria to inhibit phage infections as well as phage tactics to circumvent bacterial defense systems. In addition, the application of these strategies will be described as a lesson learned from bacteria and phage combats. The ecological factors that affect the evolution of bacterial immune systems is the other issue represented in this review.

A protein architecture guided screen for modification dependent restriction endonucleases

Modification dependent restriction endonucleases (MDREs) often have separate catalytic and modification dependent domains. We systematically looked for previously uncharacterized fusion proteins featuring a PUA or DUF3427 domain and HNH or PD-(D/E)XK catalytic domain. The enzymes were clustered by similarity of their putative modification sensing domains into several groups. The TspA15I (VcaM4I, CmeDI), ScoA3IV (MsiJI, VcaCI) and YenY4I groups, all featuring a PUA superfamily domain, preferentially cleaved DNA containing 5-methylcytosine or 5-hydroxymethylcytosine. ScoA3V, also featuring a PUA superfamily domain, but of a different clade, exhibited 6-methyladenine stimulated nicking activity. With few exceptions, ORFs for PUA-superfamily domain containing endonucleases were not close to DNA methyltransferase ORFs, strongly supporting modification dependent activity of the endonucleases. DUF3427 domain containing fusion proteins had very little or no endonuclease activity, despite the presence of a putative PD-(D/E)XK catalytic domain. However, their expression potently restricted phage T4gt in Escherichia coli cells. In contrast to the ORFs for PUA domain containing endonucleases, the ORFs for DUF3427 fusion proteins were frequently found in defense islands, often also featuring DNA methyltransferases.