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July 19, 2019

Improved reference genome of Aedes aegypti informs arbovirus vector control.

Female Aedes aegypti mosquitoes infect more than 400 million people each year with dangerous viral pathogens including dengue, yellow fever, Zika and chikungunya. Progress in understanding the biology of mosquitoes and developing the tools to fight them has been slowed by the lack of a high-quality genome assembly. Here we combine diverse technologies to produce the markedly improved, fully re-annotated AaegL5 genome assembly, and demonstrate how it accelerates mosquito science. We anchored physical and cytogenetic maps, doubled the number of known chemosensory ionotropic receptors that guide mosquitoes to human hosts and egg-laying sites, provided further insight into the size and composition of the sex-determining M locus, and revealed copy-number variation among glutathione S-transferase genes that are important for insecticide resistance. Using high-resolution quantitative trait locus and population genomic analyses, we mapped new candidates for dengue vector competence and insecticide resistance. AaegL5 will catalyse new biological insights and intervention strategies to fight this deadly disease vector.

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July 7, 2019

Biochemical characterization of a Naegleria TET-like oxygenase and its application in single molecule sequencing of 5-methylcytosine.

Modified DNA bases in mammalian genomes, such as 5-methylcytosine ((5m)C) and its oxidized forms, are implicated in important epigenetic regulation processes. In human or mouse, successive enzymatic conversion of (5m)C to its oxidized forms is carried out by the ten-eleven translocation (TET) proteins. Previously we reported the structure of a TET-like (5m)C oxygenase (NgTET1) from Naegleria gruberi, a single-celled protist evolutionarily distant from vertebrates. Here we show that NgTET1 is a 5-methylpyrimidine oxygenase, with activity on both (5m)C (major activity) and thymidine (T) (minor activity) in all DNA forms tested, and provide unprecedented evidence for the formation of 5-formyluridine ((5f)U) and 5-carboxyuridine ((5ca)U) in vitro. Mutagenesis studies reveal a delicate balance between choice of (5m)C or T as the preferred substrate. Furthermore, our results suggest substrate preference by NgTET1 to (5m)CpG and TpG dinucleotide sites in DNA. Intriguingly, NgTET1 displays higher T-oxidation activity in vitro than mammalian TET1, supporting a closer evolutionary relationship between NgTET1 and the base J-binding proteins from trypanosomes. Finally, we demonstrate that NgTET1 can be readily used as a tool in (5m)C sequencing technologies such as single molecule, real-time sequencing to map (5m)C in bacterial genomes at base resolution.

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July 7, 2019

Complete genome sequence of a novel bacterium within the family Rhodocyclaceae that degrades polycyclic aromatic hydrocarbons.

A polycyclic aromatic hydrocarbon-degrading bacterium designated strain Ca6, a member of the family Rhodocyclaceae and a representative of the uncharacterized pyrene group 1 (PG1), was isolated and its genome sequenced. The presence of several genes suspected to be associated with PG1 was confirmed, and additional genes for aromatic compound metabolism were detected. Copyright © 2015 Singleton et al.

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July 7, 2019

Covalent modification of bacteriophage T4 DNA inhibits CRISPR-Cas9.

The genomic DNAs of tailed bacteriophages are commonly modified by the attachment of chemical groups. Some forms of DNA modification are known to protect phage DNA from cleavage by restriction enzymes, but others are of unknown function. Recently, the CRISPR-Cas nuclease complexes were shown to mediate bacterial adaptive immunity by RNA-guided target recognition, raising the question of whether phage DNA modifications may also block attack by CRISPR-Cas9. We investigated phage T4 as a model system, where cytosine is replaced with glucosyl-hydroxymethylcytosine (glc-HMC). We first quantified the extent and distribution of covalent modifications in T4 DNA by single-molecule DNA sequencing and enzymatic probing. We then designed CRISPR spacer sequences targeting T4 and found that wild-type T4 containing glc-HMC was insensitive to attack by CRISPR-Cas9 but mutants with unmodified cytosine were sensitive. Phage with HMC showed only intermediate sensitivity. While this work was in progress, another group reported examples of heavily engineered CRISRP-Cas9 complexes that could, in fact, overcome the effects of T4 DNA modification, indicating that modifications can inhibit but do not always fully block attack.Bacteria were recently found to have a form of adaptive immunity, the CRISPR-Cas systems, which use nucleic acid pairing to recognize and cleave genomic DNA of invaders such as bacteriophage. Historic work with tailed phages has shown that phage DNA is often modified by covalent attachment of large chemical groups. Here we demonstrate that DNA modification in phage T4 inhibits attack by the CRISPR-Cas9 system. This finding provides insight into mechanisms of host-virus competition and also a new set of tools that may be useful in modulating the activity of CRISPR-Cas9 in genome engineering applications. Copyright © 2015 Bryson et al.

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July 7, 2019

Novel recA-independent horizontal gene transfer in Escherichia coli K-12.

In bacteria, mechanisms that incorporate DNA into a genome without strand-transfer proteins such as RecA play a major role in generating novelty by horizontal gene transfer. We describe a new illegitimate recombination event in Escherichia coli K-12: RecA-independent homologous replacements, with very large (megabase-length) donor patches replacing recipient DNA. A previously uncharacterized gene (yjiP) increases the frequency of RecA-independent replacement recombination. To show this, we used conjugal DNA transfer, combining a classical conjugation donor, HfrH, with modern genome engineering methods and whole genome sequencing analysis to enable interrogation of genetic dependence of integration mechanisms and characterization of recombination products. As in classical experiments, genomic DNA transfer begins at a unique position in the donor, entering the recipient via conjugation; antibiotic resistance markers are then used to select recombinant progeny. Different configurations of this system were used to compare known mechanisms for stable DNA incorporation, including homologous recombination, F’-plasmid formation, and genome duplication. A genome island of interest known as the immigration control region was specifically replaced in a minority of recombinants, at a frequency of 3 X 10-12 CFU/recipient per hour.

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July 7, 2019

The genome of the Saprophytic fungus Verticillium tricorpus reveals a complex effector repertoire resembling that of its pathogenic relatives.

Vascular wilts caused by Verticillium spp. are destructive plant diseases affecting hundreds of hosts. Only a few Verticillium spp. are causal agents of vascular wilt diseases, of which V. dahliae is the most notorious pathogen, and several V. dahliae genomes are available. In contrast, V. tricorpus is mainly known as a saprophyte and causal agent of opportunistic infections. Based on a hybrid approach that combines second and third generation sequencing, a near-gapless V. tricorpus genome assembly was obtained. With comparative genomics, we sought to identify genomic features in V. dahliae that confer the ability to cause vascular wilt disease. Unexpectedly, both species encode similar effector repertoires and share a genomic structure with genes encoding secreted proteins clustered in genomic islands. Intriguingly, V. tricorpus contains significantly fewer repetitive elements and an extended spectrum of secreted carbohydrate- active enzymes when compared with V. dahliae. In conclusion, we highlight the technical advances of a hybrid sequencing and assembly approach and show that the saprophyte V. tricorpus shares many hallmark features with the pathogen V. dahliae.

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July 7, 2019

Contiguity: Contig adjacency graph construction and visualisation

Contiguity is interactive software for the visualization and manipulation of de novo genome assemblies. 14 Contiguity creates and displays information on contig adjacency which is contextualized by the 15 simultaneous display of a comparison between assembled contigs and reference sequence. Where 16 scaffolders allow unambiguous connections between contigs to be resolved into a single scaffold, 17 Contiguity allows the user to create all potential scaffolds in ambiguous regions of the genome. This 18 enables the resolution of novel sequence or structural variants from the assembly. In addition, 19 Contiguity provides a sequencing and assembly agnostic approach for the creation of contig adjacency 20 graphs. To maximize the number of contig adjacencies determined, Contiguity combines information 21 from read pair mappings, sequence overlap and De Bruijn graph exploration. We demonstrate how 22 highly sensitive graphs can be achieved using this method. Contig adjacency graphs allow the user to 23 visualize potential arrangements of contigs in unresolvable areas of the genome. By combining 24 adjacency information with comparative genomics, Contiguity provides an intuitive approach for 25 exploring and improving sequence assemblies. It is also useful in guiding manual closure of long read 26 sequence assemblies. Contiguity is an open source application, implemented using Python and the 27 Tkinter GUI package that can run on any Unix, OSX and Windows operating system. It has been 28 designed and optimized for bacterial assemblies. Contiguity is available at 29 http://mjsull.github.io/Contiguity .

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July 7, 2019

Twenty years of bacterial genome sequencing.

Twenty years ago, the publication of the first bacterial genome sequence, from Haemophilus influenzae, shook the world of bacteriology. In this Timeline, we review the first two decades of bacterial genome sequencing, which have been marked by three revolutions: whole-genome shotgun sequencing, high-throughput sequencing and single-molecule long-read sequencing. We summarize the social history of sequencing and its impact on our understanding of the biology, diversity and evolution of bacteria, while also highlighting spin-offs and translational impact in the clinic. We look forward to a ‘sequencing singularity’, where sequencing becomes the method of choice for as-yet unthinkable applications in bacteriology and beyond.

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July 7, 2019

Methylome diversification through changes in DNA methyltransferase sequence specificity.

Epigenetic modifications such as DNA methylation have large effects on gene expression and genome maintenance. Helicobacter pylori, a human gastric pathogen, has a large number of DNA methyltransferase genes, with different strains having unique repertoires. Previous genome comparisons suggested that these methyltransferases often change DNA sequence specificity through domain movement–the movement between and within genes of coding sequences of target recognition domains. Using single-molecule real-time sequencing technology, which detects N6-methyladenines and N4-methylcytosines with single-base resolution, we studied methylated DNA sites throughout the H. pylori genome for several closely related strains. Overall, the methylome was highly variable among closely related strains. Hypermethylated regions were found, for example, in rpoB gene for RNA polymerase. We identified DNA sequence motifs for methylation and then assigned each of them to a specific homology group of the target recognition domains in the specificity-determining genes for Type I and other restriction-modification systems. These results supported proposed mechanisms for sequence-specificity changes in DNA methyltransferases. Knocking out one of the Type I specificity genes led to transcriptome changes, which suggested its role in gene expression. These results are consistent with the concept of evolution driven by DNA methylation, in which changes in the methylome lead to changes in the transcriptome and potentially to changes in phenotype, providing targets for natural or artificial selection.

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July 7, 2019

Whole-genome assembly of Klebsiella pneumoniae coproducing NDM-1 and OXA-232 carbapenemases using Single-Molecule, Real-Time Sequencing.

The whole-genome sequence of a carbapenem-resistant Klebsiella pneumoniae strain, PittNDM01, which coproduces NDM-1 and OXA-232 carbapenemases, was determined in this study. The use of single-molecule, real-time (SMRT) sequencing provided a closed genome in a single sequencing run. K. pneumoniae PittNDM01 has a single chromosome of 5,348,284 bp and four plasmids: pPKPN1 (283,371 bp), pPKPN2 (103,694 bp), pPKPN3 (70,814 bp), and pPKPN4 (6,141 bp). The contents of the chromosome were similar to that of the K. pneumoniae reference genome strain MGH 78578, with the exception of a large inversion spanning 23.3% of the chromosome. In contrast, three of the four plasmids are unique. The plasmid pPKPN1, an IncHI1B-like plasmid, carries the blaNDM-1, armA, and qnrB1 genes, along with tellurium and mercury resistance operons. blaNDM-1 is carried on a unique structure in which Tn125 is further bracketed by IS26 downstream of a class 1 integron. The IncFIA-like plasmid pPKPN3 also carries an array of resistance elements, including blaCTX-M-15 and a mercury resistance operon. The ColE-type plasmid pPKPN4 carrying blaOXA-232 is identical to a plasmid previously reported from France. SMRT sequencing was useful in resolving the complex bacterial genomic structures in the de novo assemblies. Copyright © 2014, American Society for Microbiology. All Rights Reserved.

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July 7, 2019

Get your high-quality low-cost genome sequence.

The study of whole-genome sequences has become essential for almost all branches of biological research. Next-generation sequencing (NGS) has revolutionized the scalability, speed, and resolution of sequencing and brought genomic science within reach of academic laboratories that study non-model organisms. Here, we show that a high-quality draft genome of a eukaryote can be obtained at relatively low cost by exploiting a hybrid combination of sequencing strategies. Copyright © 2014 Elsevier Ltd. All rights reserved.

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July 7, 2019

A precise chloroplast genome of Nelumbo nucifera (Nelumbonaceae) evaluated with Sanger, Illumina MiSeq, and PacBio RS II sequencing platforms: insight into the plastid evolution of basal eudicots.

BackgroundThe chloroplast genome is important for plant development and plant evolution. Nelumbo nucifera is one member of relict plants surviving from the late Cretaceous. Recently, a new sequencing platform PacBio RS II, known as `SMRT (Single Molecule, Real-Time) sequencing¿, has been developed. Using the SMRT sequencing to investigate the chloroplast genome of N. nucifera will help to elucidate the plastid evolution of basal eudicots.ResultsThe sizes of the de novo assembled complete chloroplast genome of N. nucifera were 163,307 bp, 163,747 bp and 163,600 bp with average depths of coverage of 7×, 712× and 105× sequenced by Sanger, Illumina MiSeq and PacBio RS II, respectively. The precise chloroplast genome of N. nucifera was obtained from PacBio RS II data proofread by Illumina MiSeq reads, with a quadripartite structure containing a large single copy region (91,846 bp) and a small single copy region (19,626 bp) separated by two inverted repeat regions (26,064 bp). The genome contains 113 different genes, including four distinct rRNAs, 30 distinct tRNAs and 79 distinct peptide-coding genes. A phylogenetic analysis of 133 taxa from 56 orders indicated that Nelumbo with an age of 177 million years is a sister clade to Platanus, which belongs to the basal eudicots. Basal eudicots began to emerge during the early Jurassic with estimated divergence times at 197 million years using MCMCTree. IR expansions/contractions within the basal eudicots seem to have occurred independently.ConclusionsBecause of long reads and lack of bias in coverage of AT-rich regions, PacBio RS II showed a great promise for highly accurate `finished¿ genomes, especially for a de novo assembly of genomes. N. nucifera is one member of basal eudicots, however, evolutionary analyses of IR structural variations of N. nucifera and other basal eudicots suggested that IR expansions/contractions occurred independently in these basal eudicots or were caused by independent insertions and deletions. The precise chloroplast genome of N. nucifera will present new information for structural variation of chloroplast genomes and provide new insight into the evolution of basal eudicots at the primary sequence and structural level.

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