Unknown DNA Research Articles

Differential strand separation at critical temperature: A minimally disruptive enrichment method for low-abundance unknown DNA mutations

Detection of low-level DNA variations in the presence of wild-type DNA is important in several fields of medicine, including cancer, prenatal diagnosis and infectious diseases. PCR-based methods to enrich mutations during amplification have limited multiplexing capability, are mostly restricted to known mutations and are prone to polymerase or mis-priming errors. Here, we present Differential Strand Separation at Critical Temperature (DISSECT), a method that enriches unknown mutations of targeted DNA sequences purely based on thermal denaturation of DNA heteroduplexes without the need for enzymatic reactions. Target DNA is pre-amplified in a multiplex reaction and hybridized onto complementary probes immobilized on magnetic beads that correspond to wild-type DNA sequences. Presence of any mutation on the target DNA forms heteroduplexes that are subsequently denatured from the beads at a critical temperature and selectively separated from wild-type DNA. We demonstrate multiplexed enrichment by 100- to 400-fold for KRAS and TP53 mutations at multiple positions of the targeted sequence using two to four successive cycles of DISSECT. Cancer and plasma-circulating DNA samples containing traces of mutations undergo mutation enrichment allowing detection via Sanger sequencing or high-resolution melting. The simplicity, scalability and reliability of DISSECT make it a powerful method for mutation enrichment that integrates well with existing downstream detection methods.

A Support Vector Machine based method to distinguish proteobacterial proteins from eukaryotic plant proteins

BackgroundMembers of the phylum Proteobacteria are most prominent among bacteria causing plant diseases that result in a diminution of the quantity and quality of food produced by agriculture. To ameliorate these losses, there is a need to identify infections in early stages. Recent developments in next generation nucleic acid sequencing and mass spectrometry open the door to screening plants by the sequences of their macromolecules. Such an approach requires the ability to recognize the organismal origin of unknown DNA or peptide fragments. There are many ways to approach this problem but none have emerged as the best protocol. Here we attempt a systematic way to determine organismal origins of peptides by using a machine learning algorithm. The algorithm that we implement is a Support Vector Machine (SVM).ResultThe amino acid compositions of proteobacterial proteins were found to be different from those of plant proteins. We developed an SVM model based on amino acid and dipeptide compositions to distinguish between a proteobacterial protein and a plant protein. The amino acid composition (AAC) based SVM model had an accuracy of 92.44% with 0.85 Matthews correlation coefficient (MCC) while the dipeptide composition (DC) based SVM model had a maximum accuracy of 94.67% and 0.89 MCC. We also developed SVM models based on a hybrid approach (AAC and DC), which gave a maximum accuracy 94.86% and a 0.90 MCC. The models were tested on unseen or untrained datasets to assess their validity.ConclusionThe results indicate that the SVM based on the AAC and DC hybrid approach can be used to distinguish proteobacterial from plant protein sequences.

Dissecting DNA damage response pathways by analysing protein localization and abundance changes during DNA replication stress

Re-localization of proteins is a hallmark of the DNA damage response. We use high-throughput microscopic screening of the yeast GFP fusion collection to develop a systems-level view of protein re-organization following drug-induced DNA replication stress. Changes in protein localization and abundance reveal drug-specific patterns of functional enrichments. Classification of proteins by sub-cellular destination allows the identification of pathways that respond to replication stress. We analyzed pairwise combinations of GFP fusions and gene deletion mutants to define and order two novel DNA damage responses. In the first, Cmr1 forms subnuclear foci that are regulated by the histone deacetylase Hos2 and are distinct from the typical Rad52 repair foci. In a second example, we find that the checkpoint kinases Mec1/Tel1 and the translation regulator Asc1 regulate P-body formation. This method identifies response pathways that were not detected in genetic and protein interaction screens, and can be readily applied to any form of chemical or genetic stress to reveal cellular response pathways.

Single-molecule mechanical identification and sequencing

High-throughput, low-cost DNA sequencing has emerged as one of the challenges of the postgenomic era. Here we present the proof of concept for a single-molecule platform that allows DNA identification and sequencing. In contrast to most present methods, our scheme is not based on the detection of the fluorescent nucleotides but on DNA hairpin length. By pulling on magnetic beads tethered by a DNA hairpin to the surface, the molecule can be unzipped. In this open state it can hybridize with complementary oligonucleotides, which transiently block the hairpin rezipping when the pulling force is reduced. By measuring from the surface to the bead of a blocked hairpin, one can determine the position of the hybrid along the molecule with nearly single-base precision. Our approach can be used to identify a DNA fragment of known sequence in a mix of various fragments and to sequence an unknown DNA fragment by hybridization or ligation.

Human polyomaviruses identification by logic mining techniques

BackgroundDifferences in genomic sequences are crucial for the classification of viruses into different species. In this work, viral DNA sequences belonging to the human polyomaviruses BKPyV, JCPyV, KIPyV, WUPyV, and MCPyV are analyzed using a logic data mining method in order to identify the nucleotides which are able to distinguish the five different human polyomaviruses.ResultsThe approach presented in this work is successful as it discovers several logic rules that effectively characterize the different five studied polyomaviruses. The individuated logic rules are able to separate precisely one viral type from the other and to assign an unknown DNA sequence to one of the five analyzed polyomaviruses.ConclusionsThe data mining analysis is performed by considering the complete sequences of the viruses and the sequences of the different gene regions separately, obtaining in both cases extremely high correct recognition rates.

A simple, high sensitivity mutation screening using Ampligase mediated T7 endonuclease I and Surveyor nuclease with microfluidic capillary electrophoresis

Mutation and polymorphism detection is of increasing importance for a variety of medical applications, including identification of cancer biomarkers and genotyping for inherited genetic disorders. Among various mutation-screening technologies, enzyme mismatch cleavage (EMC) represents a great potential as an ideal scanning method for its simplicity and high efficiency, where the heteroduplex DNAs are recognized and cleaved into DNA fragments by mismatch-recognizing nucleases. Thereby, the enzymatic cleavage activities of the resolving nucleases play a critical role for the EMC sensitivity. In this study, we utilized the unique features of microfluidic capillary electrophoresis and de novo gene synthesis to explore the enzymatic properties of T7 endonuclease I and Surveyor nuclease for EMC. Homoduplex and HE DNAs with specific mismatches at desired positions were synthesized using PCR (polymerase chain reaction) gene synthesis. The effects of nonspecific cleavage, preference of mismatches, exonuclease activity, incubation time, and DNA loading capability were systematically examined. In addition, the utilization of a thermostable DNA ligase for real-time ligase mediation was investigated. Analysis of the experimental results has led to new insights into the enzymatic cleavage activities of T7 endonuclease I and Surveyor nuclease, and aided in optimizing EMC conditions, which enhance the sensitivity and efficiency in screening of unknown DNA variations.

Isolation of phaC gene from marine bacteria Paracoccus homiensis strain E33 by magnetic beads subtractive hybridization

The isolation of unknown DNA sequences is an important task in molecular biology research. The subtractive hybridization by magnetic beads technique reduces time and cost when the aim of the study is to isolate new genes. We have developed an alternative strategy that combines the subtractive hybridization method with an enrichment microsatellite strategy to isolate a de novo DNA sequence of the phaC gene involved in the polymerization of polyhydroxyalkanoates. A 113-bp fragment was developed as a probe and, based on its melting temperature calculated using the nearest-neighbor model, used to isolate the full-length phaC gene (GenBank Accession number HM026759) of the marine bacteria Paracoccus homiensis strain E33, which was isolated from polluted microbial mats.

A method for correcting standard-based real-time PCR DNA quantitation when the standard’s polymerase reaction efficiency is significantly different from that of the unknown’s

Standard-based real-time or quantitative polymerase chain reaction quantitation of an unknown sample's DNA concentration (i.e., [DNA](unk)) assumes that the concentration dependence of the standard and unknown reactions (related to reaction efficiency, E) are equivalent. In our work with background food-borne organisms which can interfere with pathogen detection, we have found that it is generally possible to achieve an acceptable E (1 ± 0.05) for standard solutions by optimizing the PCR conditions, template purity, primer sequence, and amplicon lengths. However, this is frequently not true for the solutions containing unknown amounts of target DNA inasmuch as cell extracts are more chemically complex than the standards which have been amplified (2(30)-fold) as well as undergone a purification process. When significant differences in E occur, it is not possible to accurately estimate unknown target DNA concentration from the standard solution's slope and intercept (from threshold cycle number, or C(T), versus Log[DNA] data). What is needed is a standard-mediated intercept which can be specifically coupled with an unknown solution's PCR concentration dependence. In this work, we develop a simple mathematical procedure to generate a new standard curve with a slope (∂C(T)/∂Log[Dilution](unk)) derived from at least three dilutions of the unknown target DNA solution ([DNA](unk)) and an intercept calculated from the unknown's C(T)s, DNA concentrations interpolated from the standard curve (i.e., the traditional estimate of [DNA](unk)), and ∂C(T)/∂Log[Dilution](unk). We were able to achieve this due to our discovery of the predictable way in which the observed and ideal C(T) versus Log[DNA] slopes and intercepts deviate from one another. This "correction" in the standard-based [DNA](unk) determination is typically 20-60% when the difference in the standard and unknown E is >0.1.

Application of High-Resolution DNA Melting for Genotyping in Lepidopteran Non-Model Species: Ostrinia furnacalis (Crambidae)

Development of an ideal marker system facilitates a better understanding of the genetic diversity in lepidopteran non-model organisms, which have abundant species, but relatively limited genomic resources. Single nucleotide polymorphisms (SNPs) discovered within single-copy genes have proved to be desired markers, but SNP genotyping by current techniques remain laborious and expensive. High resolution melting (HRM) curve analysis represents a simple, rapid and inexpensive genotyping method that is primarily confined to clinical and diagnostic studies. In this study, we evaluated the potential of HRM analysis for SNP genotyping in the lepidopteran non-model species Ostrinia furnacalis (Crambidae). Small amplicon and unlabeled probe assays were developed for the SNPs, which were identified in 30 females of O. furnacalis from 3 different populations by our direct sequencing. Both assays were then applied to genotype 90 unknown female DNA by prior mixing with known wild-type DNA. The genotyping results were compared with those that were obtained using bi-directional sequencing analysis. Our results demonstrated the efficiency and reliability of the HRM assays. HRM has the potential to provide simple, cost-effective genotyping assays and facilitates genotyping studies in any non-model lepidopteran species of interest.

Isolation of centromeric-tandem repetitive DNA sequences by chromatin affinity purification using a HaloTag7-fused centromere-specific histone H3 in tobacco

The centromere is a multi-functional complex comprising centromeric DNA and a number of proteins. To isolate unidentified centromeric DNA sequences, centromere-specific histone H3 variants (CENH3) and chromatin immunoprecipitation (ChIP) have been utilized in some plant species. However, anti-CENH3 antibody for ChIP must be raised in each species because of its species specificity. Production of the antibodies is time-consuming and costly, and it is not easy to produce ChIP-grade antibodies. In this study, we applied a HaloTag7-based chromatin affinity purification system to isolate centromeric DNA sequences in tobacco. This system required no specific antibody, and made it possible to apply a highly stringent wash to remove contaminated DNA. As a result, we succeeded in isolating five tandem repetitive DNA sequences in addition to the centromeric retrotransposons that were previously identified by ChIP. Three of the tandem repeats were centromere-specific sequences located on different chromosomes. These results confirm the validity of the HaloTag7-based chromatin affinity purification system as an alternative method to ChIP for isolating unknown centromeric DNA sequences. The discovery of more than two chromosome-specific centromeric DNA sequences indicates the mosaic structure of tobacco centromeres.

Maternal gametic transmission of translocations or inversions of human chromosome 11p15.5 results in regional DNA hypermethylation and downregulation of CDKN1C expression

Beckwith–Wiedemann syndrome (BWS) is an overgrowth syndrome associated with genetic or epigenetic alterations in one of two imprinted domains on chromosome 11p15.5. Rarely, chromosomal translocations or inversions of chromosome 11p15.5 are associated with BWS but the molecular pathophysiology in such cases is not understood. In our series of 3 translocation and 2 inversion patients with BWS, the chromosome 11p15.5 breakpoints map within the centromeric imprinted domain, 2. We hypothesized that either microdeletions/microduplications adjacent to the breakpoints could disrupt genomic sequences important for imprinted gene regulation. An alternate hypothesis was that epigenetic alterations of as yet unknown regulatory DNA sequences, result in the BWS phenotype. A high resolution Nimblegen custom microarray was designed representing all non-repetitive sequences in the telomeric 33 Mb of the short arm of human chromosome 11. For the BWS-associated chromosome 11p15.5 translocations and inversions, we found no evidence of microdeletions/microduplications. DNA methylation was also tested on this microarray using the HpaII tiny fragment enrichment by ligation-mediated PCR (HELP) assay. This high-resolution DNA methylation microarray analysis revealed a gain of DNA methylation in the translocation/inversion patients affecting the p-ter segment of chromosome 11p15, including both imprinted domains. BWS patients that inherited a maternal translocation or inversion also demonstrated reduced expression of the growth suppressing imprinted gene, CDKN1C in Domain 2. In summary, our data demonstrate that translocations and inversions involving imprinted domain 2 on chromosome 11p15.5, alter regional DNA methylation patterns and imprinted gene expression in cis, suggesting that these epigenetic alterations are generated by an alteration in “chromatin context”.

AT-rich DNA trapped in the cytoplasm

AT-rich DNA induces the expression of type I IFNs through an unknown cytoplasmic DNA sensor.

Identification of Gene Expression Elements in Histomonas meleagridis Using Splinkerette PCR, a Variation of Ligated Adaptor PCR

Histomonas meleagridis is the causative agent of blackhead disease in gallinaceous birds. Limited genetic information exists for this organism, with the majority of sequence information coming from the coding regions of genes. No information is available for intergenic regions that contain DNA elements required for the regulation of gene expression. In this study, we demonstrate that splinkerette PCR, a variation of ligated adaptor PCR, can be used to identify regions of unknown sequence that lie upstream and downstream of known genomic sequences. Using this technique, we identified upstream sequences of 2 β-tubulin genes. Sequence analysis identified the 5' coding portions of the β-tubulin genes, the intergenic regions, and 2 different open reading frames encoding for a putative serine/threonine phosphatase and a putative ras-related protein, racG. We predict that these intergenic regions contain polyadenylation and cleavage signals for the 2 open reading frames and initiator elements for the β-tubulin genes. Our research demonstrates the use of splinkerette PCR as a valuable tool to identify unknown DNA sequences. In addition, the identification of the regulatory elements necessary for gene transcription in H. meleagridis will provide tools for future studies on its gene expression.

Genomic Selection Identifies Vertebrate Transcription Factor Fezf2 Binding Sites and Target Genes

Identification of transcription factor targets is critical to understanding gene regulatory networks. Here, we uncover transcription factor binding sites and target genes employing systematic evolution of ligands by exponential enrichment (SELEX). Instead of selecting randomly synthesized DNA oligonucleotides as in most SELEX studies, we utilized zebrafish genomic DNA to isolate fragments bound by Fezf2, an evolutionarily conserved gene critical for vertebrate forebrain development. This is, to our knowledge, the first time that SELEX is applied to a vertebrate genome. Computational analysis of bound genomic fragments predicted a core consensus binding site, which identified response elements that mediated Fezf2-dependent transcription both in vitro and in vivo. Fezf2-bound fragments were enriched for conserved sequences. Surprisingly, ∼20% of these fragments overlapped well annotated protein-coding exons. Through loss of function, gain of function, and chromatin immunoprecipitation, we further identified and validated eomesa/tbr2 and lhx2b as biologically relevant target genes of Fezf2. Mutations in eomesa/tbr2 cause microcephaly in humans, whereas lhx2b is a critical regulator of cell fate and axonal targeting in the developing forebrain. These results demonstrate the feasibility of employing genomic SELEX to identify vertebrate transcription factor binding sites and target genes and reveal Fezf2 as a transcription activator and a candidate for evaluation in human microcephaly.

First detection and analysis of a fish circovirus

Circoviruses are present worldwide in birds and pigs but their occurrence in fish has not yet been reported. Recently, increased mortality was observed in barbel fry (Barbus barbus) in Hungary. This paper reports the detection of previously unknown circular viral DNA genomes in barbels by the use of a circovirus-specific wide-range nested PCR. The analysis of two complete genomes (Barbel circovirus, BaCV1 and BaCV2) indicated that they belonged into a new genetic group within the family Circoviridae, distinct from known circoviruses and circovirus-like genomes. Their genome size was 1957 bases and contained two major ORFs similar to the capsid and replication-associated protein genes of circoviruses. A connection between the presence of the virus and clinical manifestations of the infection could not be proved.

Tomato LeTHIC is an Fe-Requiring HMP-P Synthase Involved in Thiamine Synthesis and Regulated by Multiple Factors

Thiamine is a key primary metabolite which is necessary for the viability of all organisms. It is a dietary requirement for mammals because only prokaryotes, fungi and plants are thiamine prototrophs. In contrast to the well documented biosynthetic mechanism in bacteria, much remains to be deciphered in plants. In this work, a tomato thiamine-auxotrophic (thiamineless, tl) mutant was characterized. The tl mutant occurs due to inactivation of LeTHIC transcription as a result of insertion of a large unknown DNA fragment in its 5'-untranslated region. Expression of wild-type LeTHIC in tl plants was able to complement the mutant to wild type. LeTHIC possessed the same function as E.cTHIC [an Escherichia coli 4-amino-5-hydroxymethyl-2-methylpyrimidine phosphate (HMP-P) synthase involved in synthesis of the pyrimidine moiety of thiamine] because expression of LeTHIC rescued THIC-deficient strains of E. coli under culture conditions without thiamine supplementation, suggesting that plants employ a bacteria-like route of pyrimidine moiety synthesis. LeTHIC is an Fe-S cluster protein localized in chloroplasts, and Fe is required for maintenance of its enzyme activity because Fe deficiency resulted in a significant reduction of thiamine content in tomato leaves. Further, we also showed that the expression of LeTHIC is tightly regulated at the transcriptional and post-transcriptional level by multiple factors, such as light, Fe status and thiamine pyrophosphate (TPP)-riboswitch. The results clearly demonstrated that a feedback regulation mechanism is involved in synthesis of the pyrimidine moiety for controlling thiamine synthesis in tomato. Our results provide a new insight into understanding the molecular mechanism of thiamine biosynthesis in plants.

Target-Mediated Protection of Endogenous MicroRNAs in C. elegans

MicroRNAs (miRNAs) are tightly regulated through transcriptional and posttranscriptional mechanisms, including degradation by nucleases. Here, we report that in C. elegans, target mRNAs can protect their cognate miRNAs from degradation invivo. We show that the let-7(n2853) mutation destabilizes the mature let-7 miRNA by impairing this protection. Moreover, presence of a cognate target or depletion of the xrn-1 (XRN1) or xrn-2 (XRN2/Rat1p) exoribonucleases enforces accumulation of certain miRNA passenger (miR(∗)) strands. Thus, following biased miRNA strand loading into Argonaute, elimination of nonfunctional RNAs can further refine miRNA strand selection. Conversely, by aligning the levels of miRNAs with those of their targets, the opposing activities of mature miRNA degradation and target-mediated miRNA protection (TMMP) may enable dynamic expression of either mature strand ofa pre-miRNA, and evolution of miRNAs. Thus, it seems that mRNAs are more than inert targets and function with miRNAs in a network of mutual regulation.

GC Wave Analysis in Promoter Regions via Wavelet Analysis and Support Vector Machine

A large number of genomes have been sequenced and the number is growing rapidly. It is crucial to improve sequence annotation, including promoter prediction. Many aspects of DNA sequences have been examined and used in promoter prediction. In particular, the physical instability correlating GC content in the promoter region has been focus of many studies. To extract the GC signals of a promoter region in a genome sequence, we adopt a scheme combining wavelet analysis and a support vector machine (SVM). In this scheme, we take a simplified way to quantize and extract chemo-physical properties of a DNA sequence. Four types of DNA are converted to binary form with respect to G and C or not. The sequences are expanded to two dimensional spaces, frequency and location, by discrete wavelet transformation (DWT). The fixed length of the promoter and randomly selected DNA segments are prepared as the positive and negative training data, respectively. The two types of data are converted by DWT and learned by a SVM. Then, previously unknown DNA segments are classified as promoter or nonpromoter by the trained SVM.

Identification of Escherichia coli Mismatch-specific Uracil DNA Glycosylase as a Robust Xanthine DNA Glycosylase

The gene for the mismatch-specific uracil DNA glycosylase (MUG) was identified in the Escherichia coli genome as a sequence homolog of the human thymine DNA glycosylase with activity against mismatched uracil base pairs. Examination of cell extracts led us to detect a previously unknown xanthine DNA glycosylase (XDG) activity in E. coli. DNA glycosylase assays with purified enzymes indicated the novel XDG activity is attributable to MUG. Here, we report a biochemical characterization of xanthine DNA glycosylase activity in MUG. The wild type MUG possesses more robust activity against xanthine than uracil and is active against all xanthine-containing DNA (C/X, T/X, G/X, A/X and single-stranded X). Analysis of potentials of mean force indicates that the double-stranded xanthine base pairs have a relatively narrow energetic difference in base flipping, whereas the tendency for uracil base flipping follows the order of C/U > G/U > T/U > A/U. Site-directed mutagenesis performed on conserved motifs revealed that Asn-140 and Ser-23 are important determinants for XDG activity in E. coli MUG. Molecular modeling and molecular dynamics simulations reveal distinct hydrogen-bonding patterns in the active site of E. coli MUG that account for the specificity differences between E. coli MUG and human thymine DNA glycosylase as well as that between the wild type MUG and the Asn-140 and Ser-23 mutants. This study underscores the role of the favorable binding interactions in modulating the specificity of DNA glycosylases.

A Novel and Simple PCR Walking Method for Rapid Acquisition of Long DNA Sequence Flanking a Known Site in Microbial Genome

Acquisition of flanking sequence adjacent to a known DNA site is an important task in microbial genome-related research. In this study, we developed a new method containing two rounds of PCR followed by cloning and sequencing. Firstly, specific primer (SP) is added into the reaction system for primary locus-specific linear amplification, and then a complex long primer (CLP) is added into the cooled reaction system for only one cycle. Amplification products from the first round of PCR are directly purified without electrophoresis, diluted, and used as the templates of the second PCR. Secondly, one long specific primer (LSP) and one long base-fixed primer (LFP) are adopted. The amplicons are purified for cloning and sequencing. The achievement of specific amplification for long flanking region mainly depends on ingenious and precise settings of PCR programs, structure design of CLP primer, adding of CLP primer after specific linear amplification, concentration ratio of CLP and SP primer, applying long primers, etc. Through this method, we successfully performed the long PCR walkings (>1.5Kb) on rpoB gene of Vibrio vulnificus, transposon-like gene of V. alginolyticus, and sto gene of V. cholerae. The method provides a robust and simple strategy for rapid amplification of long unknown DNA fragments from microbes.