G Quadruplex in Plants: A Ubiquitous Regulatory Element and Its Biological Relevance

Characterization of G-quadruplexes in the Helicobacter pylori genome and assessment of therapeutic potential of G4 ligands

Helicobacter pylori, a leading human pathogen associated with duodenal ulcer and gastric cancer, presents a significant threat to human health due to increasing antibiotic resistance rates. This study investigates G-quadruplexes (G4s), which are non-canonical secondary structures form in G-rich regions within the H. pylori genome. Extensive research on G4s in eukaryotes has revealed their role in epigenetically regulating cellular processes like gene transcription, DNA replication, and oncogene expression. However, understanding of G4-mediated gene regulation in other organisms, especially bacterial pathogens, remains limited. Although G4 motifs have been extensively studied in a few bacterial species such as Mycobacterium, Streptococci, and Helicobacter, research on G4 motifs in other bacterial species is still sparse. Like in other organisms such as archaea, mammals, and viruses, G4s in H. pylori display a non-random distribution primarily situated within open reading frames of various protein-coding genes. The occurrence of G4s in functional regions of the genome and their conservation across different species indicates that their placement is not random, suggesting an evolutionary pressure to maintain these sequences at specific genomic sites. Moreover, G-quadruplexes show enrichment in specific gene classes, suggesting their potential involvement in regulating the expression of genes related to cell wall/membrane/envelope biogenesis, amino acid transport, and metabolism. This indicates a probable regulatory role for G4s in controlling the expression of genes essential for H. pylori survival and virulence. Biophysical techniques such as Circular Dichroism spectroscopy and Nuclear Magnetic Resonance were used to characterize G4 motifs within selected H. pylori genes. The study revealed that G-quadruplex ligand inhibited the growth of H. pylori, with minimal inhibitory concentrations in the low micromolar range. This suggests that targeting G4 structures could offer a promising approach for developing novel anti-H. pylori drugs.

Characterization of G-quadruplex structures in genes involved in survival and pathogenesis of Acinetobacter baumannii as a potential drug target

Acinetobacter baumannii is a notorious pathogen that commonly thrives in hospital environments and is responsible for numerous nosocomial infections in humans. The burgeoning multi-drug resistance leaves relatively minimal options for treating the bacterial infection, posing a significant problem and prompting the identification of new approaches for tackling the same. This motivated us to focus on non-canonical nucleic acid structures, mainly G-quadruplexes, as drug targets. G-quadruplexes have recently been gaining attention due to their involvement in multiple bacterial and viral pathogenesis. Herein, we sought to explore conserved putative G-quadruplex motifs in A. baumannii. In silico analysis revealed the presence of eight conserved motifs in genes involved in bacterial survival and pathogenesis. The biophysical and biomolecular analysis confirmed stable G-quadruplex formation by the motifs and showed a high binding affinity with the well-reported G-quadruplex binding ligand, BRACO-19. BRACO-19 exposure also decreased the growth of bacteria and downregulated the expression of G-quadruplex-harboring genes. The biofilm-forming ability of the bacteria was also affected by BRACO-19 addition. Taking all these observations into account, we have shown here for the first time the potential of G-quadruplex structures as a promising drug target in Acinetobacter baumannii, for addressing the challenges posed by this infamous pathogen.

The Characterization of G-Quadruplexes in Tobacco Genome and Their Function under Abiotic Stress

Tobacco is an ideal model plant in scientific research. G-quadruplex is a guanine-rich DNA structure, which regulates transcription and translation. In this study, the prevalence and potential function of G-quadruplexes in tobacco were systematically analyzed. In tobacco genomes, there were 2,924,271,002 G-quadruplexes in the nuclear genome, 430,597 in the mitochondrial genome, and 155,943 in the chloroplast genome. The density of the G-quadruplex in the organelle genome was higher than that in the nuclear genome. G-quadruplexes were abundant in the transcription regulatory region of the genome, and a difference in G-quadruplex density in two DNA strands was also observed. The promoter of 60.4% genes contained at least one G-quadruplex. Compared with up-regulated differentially expressed genes (DEGs), the G-quadruplex density in down-regulated DEGs was generally higher under drought stress and salt stress. The G-quadruplex formed by simple sequence repeat (SSR) and its flanking sequence in the promoter region of the NtBBX (Nitab4.5_0002943g0010) gene might enhance the drought tolerance of tobacco. This study lays a solid foundation for further research on G-quadruplex function in tobacco and other plants.

The Epigenomic Features and Potential Functions of PEG- and PDS-Favorable DNA G-Quadruplexes in Rice

A G-quadruplex (G4) is a typical non-B DNA structure and involved in various DNA-templated events in eukaryotic genomes. PEG and PDS chemicals have been widely applied for promoting the folding of in vivo or in vitro G4s. However, how PEG and PDS preferentially affect a subset of G4 formation genome-wide is still largely unknown. We here conducted a BG4-based IP-seq in vitro under K++PEG or K++PDS conditions in the rice genome. We found that PEG-favored IP-G4s+ have distinct sequence features, distinct genomic distributions and distinct associations with TEGs, non-TEGs and subtypes of TEs compared to PDS-favored ones. Strikingly, PEG-specific IP-G4s+ are associated with euchromatin with less enrichment levels of DNA methylation but with more enriched active histone marks, while PDS-specific IP-G4s+ are associated with heterochromatin with higher enrichment levels of DNA methylation and repressive marks. Moreover, we found that genes with PEG-specific IP-G4s+ are more expressed than those with PDS-specific IP-G4s+, suggesting that PEG/PDS-specific IP-G4s+ alone or coordinating with epigenetic marks are involved in the regulation of the differential expression of related genes, therefore functioning in distinct biological processes. Thus, our study provides new insights into differential impacts of PEG and PDS on G4 formation, thereby advancing our understanding of G4 biology.

Weighted gene co-expression network analysis of nitrogen (N)-responsive genes and the putative role of G-quadruplexes in N use efficiency (NUE) in rice

Rice is an important target to improve crop nitrogen (N) use efficiency (NUE), and the identification and shortlisting of the candidate genes are still in progress. We analyzed data from 16 published N-responsive transcriptomes/microarrays to identify, eight datasets that contained the maximum number of 3020 common genes, referred to as N-responsive genes. These include different classes of transcription factors, transporters, miRNA targets, kinases and events of post-translational modifications. A Weighted gene co-expression network analysis (WGCNA) with all the 3020 N-responsive genes revealed 15 co-expression modules and their annotated biological roles. Protein-protein interaction network analysis of the main module revealed the hub genes and their functional annotation revealed their involvement in the ubiquitin process. Further, the occurrences of G-quadruplex sequences were examined, which are known to play important roles in epigenetic regulation but are hitherto unknown in N-response/NUE. Out of the 3020 N-responsive genes studied, 2298 contained G-quadruplex sequences. We compared these N-responsive genes containing G-quadruplex sequences with the 3601 genes we previously identified as NUE-related (for being both N-responsive and yield-associated). This analysis revealed 389 (17%) NUE-related genes containing G-quadruplex sequences. These genes may be involved in the epigenetic regulation of NUE, while the rest of the 83% (1811) genes may regulate NUE through genetic mechanisms and/or other epigenetic means besides G-quadruplexes. A few potentially important genes/processes identified as associated with NUE were experimentally validated in a pair of rice genotypes contrasting for NUE. The results from the WGCNA and G4 sequence analysis of N-responsive genes helped identify and shortlist six genes as candidates to improve NUE. Further, the hitherto unavailable segregation of genetic and epigenetic gene targets could aid in informed interventions through genetic and epigenetic means of crop improvement.

Comprehensive insights into the structures and dynamics of plant telomeric G-quadruplexes

Telomeres, which are located at the ends of eukaryotic chromosomes, are crucial for genomic maintenance. Most telomeric DNA is composed of tandemly repeated guanine (G)-rich sequences, which form G-quadruplexes (G4s). The structures and dynamics of telomeric G4s are essential for telomere functioning and helpful for G4-based biosensing. However, they are far from being understood, especially for plants. In this contribution, the folding, environment-induced G4 dynamics, and protein-catalyzed unfolding of plant telomeric G4s were comprehensively studied. It was found that diverse plant telomeric sequences from land plants to green algae could fold into G4 structures. In addition, 5'-proximal ssDNA but not 3'-proximal ssDNA drove conversion of anti-parallel G4 structures to parallel structures, and both 5' and 3' ssDNA decreased the stability of G4s in dilute solution. Furthermore, molecular crowding promoted the formation of parallel structures for three-layer but not for two-layer G4s, and increased the stability of all selected G4s. Finally, AtRecQ2 helicase resolved the stable parallel structure of typical plant telomeric G4 in crowded solution, but ssDNA binding protein AtRPA did not. Furthermore, AtRecQ2 unwound the structure more efficiently in the presence of AtRPA. The results may expand our understanding on the structures and dynamics of plant telomeric G4s.

Analysis of G-Quadruplex-Forming Sequences in Drought Stress-Responsive Genes, and Synthesis Genes of Phenolic Compounds in Arabidopsis thaliana

Sequences of nucleic acids with the potential to form four-stranded G-quadruplex structures are intensively studied mainly in the context of human diseases, pathogens, or extremophile organisms; nonetheless, the knowledge about their occurrence and putative role in plants is still limited. This work is focused on G-quadruplex-forming sites in two gene sets of interest: drought stress-responsive genes, and genes related to the production/biosynthesis of phenolic compounds in the model plant organism Arabidopsis thaliana. In addition, 20 housekeeping genes were analyzed as well, where the constitutive gene expression was expected (with no need for precise regulation depending on internal or external factors). The results have shown that none of the tested gene sets differed significantly in the content of G-quadruplex-forming sites, however, the highest frequency of G-quadruplex-forming sites was found in the 5'-UTR regions of phenolic compounds' biosynthesis genes, which indicates the possibility of their regulation at the mRNA level. In addition, mainly within the introns and 1000 bp flanks downstream gene regions, G-quadruplex-forming sites were highly underrepresented. Finally, cluster analysis allowed us to observe similarities between particular genes in terms of their PQS characteristics. We believe that the original approach used in this study may become useful for further and more comprehensive bioinformatic studies in the field of G-quadruplex genomics.

Beyond the Primary Structure of Nucleic Acids: Potential Roles of Epigenetics and Noncanonical Structures in the Regulations of Plant Growth and Stress Responses

Epigenetics deals with changes in gene expression that are not caused by modifications in the primary sequence of nucleic acids. These changes beyond primary structures of nucleic acids not only include DNA/RNA methylation, but also other reversible conversions, together with histone modifications or RNA interference. In addition, under particular conditions (such as specific ion concentrations or protein-induced stabilization), the right-handed double-stranded DNA helix (B-DNA) can form noncanonical structures commonly described as "non-B DNA" structures. These structures comprise, for example, cruciforms, i-motifs, triplexes, and G-quadruplexes. Their formation often leads to significant differences in replication and transcription rates. Noncanonical RNA structures have also been documented to play important roles in translation regulation and the biology of noncoding RNAs. In human and animal studies, the frequency and dynamics of noncanonical DNA and RNA structures are intensively investigated, especially in the field of cancer research and neurodegenerative diseases. In contrast, noncanonical DNA and RNA structures in plants have been on the fringes of interest for a long time and only a few studies deal with their formation, regulation, and physiological importance for plant stress responses. Herein, we present a review focused on the main fields of epigenetics in plants and their possible roles in stress responses and signaling, with special attention dedicated to noncanonical DNA and RNA structures.

Exploring the Relationship between G-Quadruplex Nucleic Acids and Plants: From Plant G-Quadruplex Function to Phytochemical G4 Ligands with Pharmaceutic Potential

G-quadruplex (G4) oligonucleotides are higher-order DNA and RNA secondary structures of enormous relevance due to their implication in several biological processes and pathological states in different organisms. Strategies aiming at modulating human G4 structures and their interrelated functions are first-line approaches in modern research aiming at finding new potential anticancer treatments or G4-based aptamers for various biomedical and biotechnological applications. Plants offer a cornucopia of phytocompounds that, in many cases, are effective in binding and modulating the thermal stability of G4s and, on the other hand, contain almost unexplored G4 motifs in their genome that could inspire new biotechnological strategies. Herein, we describe some G4 structures found in plants, summarizing the existing knowledge of their functions and biological role. Moreover, we review some of the most promising G4 ligands isolated from vegetal sources and report on the known relationships between such phytochemicals and G4-mediated biological processes that make them potential leads in the pharmaceutical sector.

Alternative RNA Conformations: Companion or Combatant

RNA molecules, in one form or another, are involved in almost all aspects of cell physiology, as well as in disease development. The diversity of the functional roles of RNA comes from its intrinsic ability to adopt complex secondary and tertiary structures, rivaling the diversity of proteins. The RNA molecules form dynamic ensembles of many interconverting conformations at a timescale of seconds, which is a key for understanding how they execute their cellular functions. Given the crucial role of RNAs in various cellular processes, we need to understand the RNA molecules from a structural perspective. Central to this review are studies aimed at revealing the regulatory role of conformational equilibria in RNA in humans to understand genetic diseases such as cancer and neurodegenerative diseases, as well as in pathogens such as bacteria and viruses so as to understand the progression of infectious diseases. Furthermore, we also summarize the prior studies on the use of RNA structures as platforms for the rational design of small molecules for therapeutic applications.

Potential G-quadruplexes within the Promoter Nuclease Hypersensitive Sites of the Heat-responsive Genes in Rice

G-quadruplexes (G4s) have been shown to be involved in the regulation of multiple cellular processes. Exploring putative G4-forming sequences (PQSs) in heat-responsive genes of rice and their folding structures under different conditions will help to understand the mechanism in response to heat stress. In this work, we discovered a prevalence of PQSs in nuclease hypersensitive sites within the promoters of heat-responsive genes. Moreover, 50% of the searched G3 PQSs ((G3+L1-7)3+G3+) locate in heat shock transcription factors. Circular dichroism spectroscopy, thermal difference spectroscopy, and UV melting analysis demonstrated the representative PQSs could adopt stable G4s at physiological temperature and potassium concentration. These PQSs were able to stall Klenow Fragment (KF) DNA polymerase by the formation of G4s. However, the G4s with Tm values around 50 - 60 oC could be increasingly unwound by KF with the increase of temperatures from 25 to 50 oC, implying these G4s could sense the changes in temperature by structural switch. This work offers fresh clue to understand the potential of G4-involved functions of PQSs and the molecular events in plants in the response to heat stress.

The Newly Sequenced Genome of Pisum sativum Is Replete with Potential G-Quadruplex-Forming Sequences-Implications for Evolution and Biological Regulation

G-quadruplexes (G4s) have been long considered rare and physiologically unimportant in vitro curiosities, but recent methodological advances have proved their presence and functions in vivo. Moreover, in addition to their functional relevance in bacteria and animals, including humans, their importance has been recently demonstrated in evolutionarily distinct plant species. In this study, we analyzed the genome of Pisum sativum (garden pea, or the so-called green pea), a unique member of the Fabaceae family. Our results showed that this genome contained putative G4 sequences (PQSs). Interestingly, these PQSs were located nonrandomly in the nuclear genome. We also found PQSs in mitochondrial (mt) and chloroplast (cp) DNA, and we experimentally confirmed G4 formation for sequences found in these two organelles. The frequency of PQSs for nuclear DNA was 0.42 PQSs per thousand base pairs (kbp), in the same range as for cpDNA (0.53/kbp), but significantly lower than what was found for mitochondrial DNA (1.58/kbp). In the nuclear genome, PQSs were mainly associated with regulatory regions, including 5'UTRs, and upstream of the rRNA region. In contrast to genomic DNA, PQSs were located around RNA genes in cpDNA and mtDNA. Interestingly, PQSs were also associated with specific transposable elements such as TIR and LTR and around them, pointing to their role in their spreading in nuclear DNA. The nonrandom localization of PQSs uncovered their evolutionary and functional significance in the Pisum sativum genome.

Epigenomic Features and Potential Functions of K+ and Na+ Favorable DNA G-Quadruplexes in Rice

DNA G-quadruplexes (G4s) are non-canonical four-stranded DNA structures involved in various biological processes in eukaryotes. Molecularly crowded solutions and monovalent cations have been reported to stabilize in vitro and in vivo G4 formation. However, how K⁺ and Na⁺ affect G4 formation genome-wide is still unclear in plants. Here, we conducted BG4-DNA-IP-seq, DNA immunoprecipitation with anti-BG4 antibody coupled with sequencing, under K⁺ and Na⁺ + PEG conditions in vitro. We found that K⁺-specific IP-G4s had a longer peak size, more GC and PQS content, and distinct AT and GC skews compared to Na⁺-specific IP-G4s. Moreover, K⁺- and Na⁺-specific IP-G4s exhibited differential subgenomic enrichment and distinct putative functional motifs for the binding of certain trans-factors. More importantly, we found that K⁺-specific IP-G4s were more associated with active marks, such as active histone marks, and low DNA methylation levels, as compared to Na⁺-specific IP-G4s; thus, K⁺-specific IP-G4s in combination with active chromatin features facilitate the expression of overlapping genes. In addition, K⁺- and Na⁺-specific IP-G4 overlapping genes exhibited differential GO (gene ontology) terms, suggesting they may have distinct biological relevance in rice. Thus, our study, for the first time, explores the effects of K⁺ and Na⁺ on global G4 formation in vitro, thereby providing valuable resources for functional G4 studies in rice. It will provide certain G4 loci for the biotechnological engineering of rice in the future.

Chromosome organization affects genome evolution in Sulfolobus archaea

In all organisms, the DNA sequence and the structural organization of chromosomes affect gene expression. The extremely thermophilic crenarchaeon Sulfolobus has one circular chromosome with three origins of replication. We previously revealed that this chromosome has defined A and B compartments that have high and low gene expression, respectively. As well as higher levels of gene expression, the A compartment contains the origins of replication. To evaluate the impact of three-dimensional organization on genome evolution, we characterized the effect of replication origins and compartmentalization on primary sequence evolution in eleven Sulfolobus species. Using single-nucleotide polymorphism analyses, we found that distance from an origin of replication was associated with increased mutation rates in the B but not in the A compartment. The enhanced polymorphisms distal to replication origins suggest that replication termination may have a causal role in their generation. Further mutational analyses revealed that the sequences in the A compartment are less likely to be mutated, and that there is stronger purifying selection than in the B compartment. Finally, we applied the Assay for Transposase-Accessible Chromatin using sequencing (ATAC-seq) to show that the B compartment is less accessible than the A compartment. Taken together, our data suggest that compartmentalization of chromosomal DNA can influence chromosome evolution in Sulfolobus. We propose that the A compartment serves as a haven for stable maintenance of gene sequences, while sequences in the B compartment can be diversified.

Quantification of 8-oxoG in Plant Telomeres

Chemical modifications in DNA impact gene regulation and chromatin structure. DNA oxidation, for example, alters gene expression, DNA synthesis and cell cycle progression. Modification of telomeric DNA by oxidation is emerging as a marker of genotoxic damage and is associated with reduced genome integrity and changes in telomere length and telomerase activity. 8-oxoguanine (8-oxoG) is the most studied and common outcome of oxidative damage in DNA. The G-rich nature of telomeric DNA is proposed to make it a hotspot for oxidation, but because telomeres make up only a tiny fraction of the genome, it has been difficult to directly test this hypothesis by studying dynamic DNA modifications specific to this region in vivo. Here, we present a new, robust method to differentially enrich telomeric DNA in solution, coupled with downstream methods for determination of chemical modification. Specifically, we measure 8-oxoG in Arabidopsis thaliana telomeres under normal and oxidative stress conditions. We show that telomere length is unchanged in response to oxidative stress in three different wild-type accessions. Furthermore, we report that while telomeric DNA comprises only 0.02–0.07% of the total genome, telomeres contribute between 0.2 and 15% of the total 8-oxoG. That is, plant telomeres accumulate 8-oxoG at levels approximately 100-fold higher than the rest of the genome under standard growth conditions. Moreover, they are the primary targets of further damage upon oxidative stress. Interestingly, the accumulation of 8-oxoG in the chromosome body seems to be inversely proportional to telomere length. These findings support the hypothesis that telomeres are hotspots of 8-oxoG and may function as sentinels of oxidative stress in plants.

Epigenomic features of DNA G-quadruplexes and their roles in regulating rice gene transcription

A DNA G-quadruplex (G4) is a non-canonical four-stranded nucleic acid structure involved in many biological processes in mammals. The current knowledge on plant DNA G4s, however, is limited; whether and how DNA G4s impact gene expression in plants is still largely unknown. Here, we applied a protocol referred to as BG4-DNA-IP-seq followed by a comprehensive characterization of DNA G4s in rice (Oryza sativa L.); we next integrated dG4s (experimentally detectable G4s) with existing omics data and found that dG4s exhibited differential DNA methylation between transposable element (TE) and non-TE genes. dG4 regions displayed genic-dependent enrichment of epigenomic signatures; finally, we showed that these sites displayed a positive association with expression of DNA G4-containing genes when located at promoters, and a negative association when located in the gene body, suggesting localization-dependent promotional/repressive roles of DNA G4s in regulating gene transcription. This study reveals interrelations between DNA G4s and epigenomic signatures, as well as implicates DNA G4s in modulating gene transcription in rice. Our study provides valuable resources for the functional characterization or bioengineering of some of key DNA G4s in rice.

The Ribosomal DNA Loci of the Ancient Monocot Pistia stratiotes L. (Araceae) Contain Different Variants of the 35S and 5S Ribosomal RNA Gene Units

The freshwater plant water lettuce (Pistia stratiotes L.) grows in warm climatic zones and is used for phytoremediation and biomass production. P. stratiotes belongs to the Araceae, an ecologically and structurally diverse early monocot family, but the phylogenetic relationships among Araceae members are poorly understood. Ribosomal DNAs (rDNAs), including the 35S and 5S rDNA, encode the RNA components of ribosomes and are widely used in phylogenetic and evolutionary studies of various plant taxa. Here, we comprehensively characterized the chromosomal locations and molecular organization of 35S and 5S rDNA genes in water lettuce using karyological and molecular methods. Fluorescence in situ hybridization revealed a single location for the 35S and 5S rDNA loci, each on a different pair of the species' 28 chromosomes. Molecular cloning and nucleotide sequencing of 35S rDNA of P. stratiotes, the first representative Araceae sensu stricto in which such a study was performed, displayed typical structural characteristics. The full-length repeat showed high sequence conservation of the regions producing the 18S, 5.8S, and 25S rRNAs and divergence of the internal transcribed spacers ITS1 and ITS2 as well as the large intergenic spacer (IGS). Alignments of the deduced sequence of 18S rDNA with the sequences available for other Araceae and representatives of other clades were used for phylogenetic analysis. Examination of 11 IGS sequences revealed significant intra-genomic length variability due to variation in subrepeat number, with four types of units detected within the 35S rDNA locus of the P. stratiotes genome (estimated size 407 Mb/1C). Similarly, the 5S rDNA locus harbors gene units comprising a conserved 119-bp sequence encoding 5S rRNA and two types of non-transcribed spacer (NTS) sequences. Type I was classified into four subtypes, which apparently originated via progressive loss of subrepeats within the duplicated NTS region containing the 3' part of the 5S rRNA gene. The minor Type II NTS is shorter than Type I and differs in nucleotide composition. Some DNA clones containing two or three consecutive 5S rDNA repeats harbored 5S rDNA genes with different types of NTSs, confirming the mosaic composition of the 5S rDNA locus.

Major Achievements in the Design of Quadruplex-Interactive Small Molecules

Organic small molecules that can recognize and bind to G-quadruplex and i-Motif nucleic acids have great potential as selective drugs or as tools in drug target discovery programs, or even in the development of nanodevices for medical diagnosis. Hundreds of quadruplex-interactive small molecules have been reported, and the challenges in their design vary with the intended application. Herein, we survey the major achievements on the therapeutic potential of such quadruplex ligands, their mode of binding, effects upon interaction with quadruplexes, and consider the opportunities and challenges for their exploitation in drug discovery.

The Dynamics of NO3- and NH4+ Uptake in Duckweed Are Coordinated with the Expression of Major Nitrogen Assimilation Genes

Duckweed plants play important roles in aquatic ecosystems worldwide. They rapidly accumulate biomass and have potential uses in bioremediation of water polluted by fertilizer runoff or other chemicals. Here we studied the assimilation of two major sources of inorganic nitrogen, nitrate (NO3- ) and ammonium (NH4+), in six duckweed species: Spirodela polyrhiza, Landoltia punctata, Lemna aequinoctialis, Lemna turionifera, Lemna minor, and Wolffia globosa. All six duckweed species preferred NH4+ over NO3- and started using NO3- only when NH4+ was depleted. Using the available genome sequence, we analyzed the molecular structure and expression of eight key nitrogen assimilation genes in S. polyrhiza. The expression of genes encoding nitrate reductase and nitrite reductase increased about 10-fold when NO3- was supplied and decreased when NH4+ was supplied. NO3- and NH4+ induced the glutamine synthetase (GS) genes GS1;2 and the GS2 by 2- to 5-fold, respectively, but repressed GS1;1 and GS1;3. NH4+ and NO3- upregulated the genes encoding ferredoxin- and NADH-dependent glutamate synthases (Fd-GOGAT and NADH-GOGAT). A survey of nitrogen assimilation gene promoters suggested complex regulation, with major roles for NRE-like and GAATC/GATTC cis-elements, TATA-based enhancers, GA/CTn repeats, and G-quadruplex structures. These results will inform efforts to improve bioremediation and nitrogen use efficiency.

G-Quadruplex in Gene Encoding Large Subunit of Plant RNA Polymerase II: A Billion-Year-Old Story

G-quadruplexes have long been perceived as rare and physiologically unimportant nucleic acid structures. However, several studies have revealed their importance in molecular processes, suggesting their possible role in replication and gene expression regulation. Pathways involving G-quadruplexes are intensively studied, especially in the context of human diseases, while their involvement in gene expression regulation in plants remains largely unexplored. Here, we conducted a bioinformatic study and performed a complex circular dichroism measurement to identify a stable G-quadruplex in the gene RPB1, coding for the RNA polymerase II large subunit. We found that this G-quadruplex-forming locus is highly evolutionarily conserved amongst plants sensu lato (Archaeplastida) that share a common ancestor more than one billion years old. Finally, we discussed a new hypothesis regarding G-quadruplexes interacting with UV light in plants to potentially form an additional layer of the regulatory network.

Mosaic Arrangement of the 5S rDNA in the Aquatic Plant Landoltia punctata (Lemnaceae)

Duckweeds are a group of monocotyledonous aquatic plants in the Araceae superfamily, represented by 37 species divided into five genera. Duckweeds are the fastest growing flowering plants and are distributed around the globe; moreover, these plants have multiple applications, including biomass production, wastewater remediation, and making pharmaceutical proteins. Dotted duckweed (Landoltia punctata), the sole species in genus Landoltia, is one of the most resilient duckweed species. The ribosomal DNA (rDNA) encodes the RNA components of ribosomes and represents a significant part of plant genomes but has not been comprehensively studied in duckweeds. Here, we characterized the 5S rDNA genes in L. punctata by cloning and sequencing 25 PCR fragments containing the 5S rDNA repeats. No length variation was detected in the 5S rDNA gene sequence, whereas the nontranscribed spacer (NTS) varied from 151 to 524 bp. The NTS variants were grouped into two major classes, which differed both in nucleotide sequence and the type and arrangement of the spacer subrepeats. The dominant class I NTS, with a characteristic 12-bp TC-rich sequence present in 3-18 copies, was classified into four subclasses, whereas the minor class II NTS, with shorter, 9-bp nucleotide repeats, was represented by two identical sequences. In addition to these diverse subrepeats, class I and class II NTSs differed in their representation of cis-elements and the patterns of predicted G-quadruplex structures, which may influence the transcription of the 5S rDNA. Similar to related duckweed species in the genus Spirodela, L. punctata has a relatively low rDNA copy number, but in contrast to Spirodela and the majority of other plants, the arrangement of the 5S rDNA units demonstrated an unusual, heterogeneous pattern in L. punctata, as revealed by analyzing clones containing double 5S rDNA neighboring units. Our findings may further stimulate the research on the evolution of the plant rDNA and discussion of the molecular forces driving homogenization of rDNA repeats in concerted evolution.

G-Quadruplex Structures in Bacteria: Biological Relevance and Potential as an Antimicrobial Target

DNA strands consisting of multiple runs of guanines can adopt a noncanonical, four-stranded DNA secondary structure known as G-quadruplex or G4 DNA. G4 DNA is thought to play an important role in transcriptional and translational regulation of genes, DNA replication, genome stability, and oncogene expression in eukaryotic genomes. In other organisms, including several bacterial pathogens and some plant species, the biological roles of G4 DNA and G4 RNA are starting to be explored. Recent investigations showed that G4 DNA and G4 RNA are generally conserved across plant species. In silico analyses of several bacterial genomes identified putative guanine-rich, G4 DNA-forming sequences in promoter regions. The sequences were particularly abundant in certain gene classes, suggesting that these highly diverse structures can be employed to regulate the expression of genes involved in secondary metabolite synthesis and signal transduction. Furthermore, in the pathogen Mycobacterium tuberculosis, the distribution of G4 motifs and their potential role in the regulation of gene transcription advocate for the use of G4 ligands to develop novel antitubercular therapies. In this review, we discuss the various roles of G4 structures in bacterial DNA and the application of G4 DNA as inhibitors or therapeutic agents to address bacterial pathogens.

Human Rev1 relies on insert-2 to promote selective binding and accurate replication of stabilized G-quadruplex motifs

We previously reported that human Rev1 (hRev1) bound to a parallel-stranded G-quadruplex (G4) from the c-MYC promoter with high affinity. We have extended those results to include other G4 motifs, finding that hRev1 exhibited stronger affinity for parallel-stranded G4 than either anti-parallel or hybrid folds. Amino acids in the αE helix of insert-2 were identified as being important for G4 binding. Mutating E466 and Y470 to alanine selectively perturbed G4 binding affinity. The E466K mutant restored wild-type G4 binding properties. Using a forward mutagenesis assay, we discovered that loss of hRev1 increased G4 mutation frequency >200-fold compared to the control sequence. Base substitutions and deletions occurred around and within the G4 motif. Pyridostatin (PDS) exacerbated this effect, as the mutation frequency increased >700-fold over control and deletions upstream of the G4 site more than doubled. Mutagenic replication of G4 DNA (±PDS) was partially rescued by wild-type and E466K hRev1. The E466A or Y470A mutants failed to suppress the PDS-induced increase in G4 mutation frequency. These findings have implications for the role of insert-2, a motif conserved in vertebrates but not yeast or plants, in Rev1-mediated suppression of mutagenesis during G4 replication.

Topology-based classification of tetrads and quadruplex structures

Motivation

Quadruplexes attract the attention of researchers from many fields of bio-science. Due to a specific structure, these tertiary motifs are involved in various biological processes. They are also promising therapeutic targets in many strategies of drug development, including anticancer and neurological disease treatment. The uniqueness and diversity of their forms cause that quadruplexes show great potential in novel biological applications. The existing approaches for quadruplex analysis are based on sequence or 3D structure features and address canonical motifs only.

Results

In our study, we analyzed tetrads and quadruplexes contained in nucleic acid molecules deposited in Protein Data Bank. Focusing on their secondary structure topology, we adjusted its graphical diagram and proposed new dot-bracket and arc representations. We defined the novel classification of these motifs. It can handle both canonical and non-canonical cases. Based on this new taxonomy, we implemented a method that automatically recognizes the types of tetrads and quadruplexes occurring as unimolecular structures. Finally, we conducted a statistical analysis of these motifs found in experimentally determined nucleic acid structures in relation to the new classification.

Availability and implementation

https://github.com/tzok/eltetrado/

Supplementary information

Supplementary data are available at Bioinformatics online.

Extra-telomeric impact of telomeres: Emerging molecular connections in pluripotency or stemness

Telomeres comprise specialized nucleic acid-protein complexes that help protect chromosome ends from DNA damage. Moreover, telomeres associate with subtelomeric regions through looping. This results in altered expression of subtelomeric genes. Recent observations further reveal telomere length-dependent gene regulation and epigenetic modifications at sites spread across the genome and distant from telomeres. This regulation is mediated through the telomere-binding protein telomeric repeat-binding factor 2 (TRF2). These observations suggest a role of telomeres in extra-telomeric functions. Most notably, telomeres have a broad impact on pluripotency and differentiation. For example, cardiomyocytes differentiate with higher efficacy from induced pluripotent stem cells having long telomeres, and differentiated cells obtained from human embryonic stem cells with relatively long telomeres have a longer lifespan. Here, we first highlight reports on these two seemingly distinct research areas: the extra-telomeric role of telomere-binding factors and the role of telomeres in pluripotency/stemness. On the basis of the observations reported in these studies, we draw attention to potential molecular connections between extra-telomeric biology and pluripotency. Finally, in the context of the nonlocal influence of telomeres on pluripotency and stemness, we discuss major opportunities for progress in molecular understanding of aging-related disorders and neurodegenerative diseases.

Genome-Wide Discovery of G-Quadruplexes in Wheat: Distribution and Putative Functional Roles

G-quadruplexes are nucleic acid secondary structures formed by a stack of square planar G-quartets. G-quadruplexes were implicated in many biological functions including telomere maintenance, replication, transcription, and translation, in many species including humans and plants. For wheat, however, though it is one of the world's most important staple food, no G-quadruplex studies have been reported to date. Here, we computationally identify putative G4 structures (G4s) in wheat genome for the first time and compare its distribution across the genome against five other genomes (human, maize, Arabidopsis, rice, and sorghum). We identified close to 1 million G4 motifs with a density of 76 G4s/Mb across the whole genome and 93 G4s/Mb over genic regions. Remarkably, G4s were enriched around three regions, two located on the antisense and one on the sense strand at the following positions: 1) the transcription start site (TSS) (antisense), 2) the first coding domain sequence (CDS) (antisense), and 3) the start codon (sense). Functional enrichment analysis revealed that the gene models containing G4 motifs within these peaks were associated with specific gene ontology (GO) terms, such as developmental process, localization, and cellular component organization or biogenesis. We investigated genes encoding MADS-box transcription factors and showed examples of G4 motifs within critical regulatory regions in the VRN-1 genes in wheat. Furthermore, comparison with other plants showed that monocots share a similar distribution of G4s, but Arabidopsis shows a unique G4 distribution. Our study shows for the first time the prevalence and possible functional roles of G4s in wheat.

A comprehensive evaluation of a typical plant telomeric G-quadruplex (G4) DNA reveals the dynamics of G4 formation, rearrangement, and unfolding

Telomeres are specific nucleoprotein structures that are located at the ends of linear eukaryotic chromosomes and play crucial roles in genomic stability. Telomere DNA consists of simple repeats of a short G-rich sequence: TTAGGG in mammals and TTTAGGG in most plants. In recent years, the mammalian telomeric G-rich repeats have been shown to form G-quadruplex (G4) structures, which are crucial for modulating telomere functions. Surprisingly, even though plant telomeres are essential for plant growth, development, and environmental adaptions, only few reports exist on plant telomeric G4 DNA (pTG4). Here, using bulk and single-molecule assays, including CD spectroscopy, and single-molecule FRET approaches, we comprehensively characterized the structure and dynamics of a typical plant telomeric sequence, d[GGG(TTTAGGG)3]. We found that this sequence can fold into mixed G4s in potassium, including parallel and antiparallel structures. We also directly detected intermediate dynamic transitions, including G-hairpin, parallel G-triplex, and antiparallel G-triplex structures. Moreover, we observed that pTG4 is unfolded by the AtRecQ2 helicase but not by AtRecQ3. The results of our work shed light on our understanding about the existence, topological structures, stability, intermediates, unwinding, and functions of pTG4.

Tight DNA-protein complexes isolated from barley seedlings are rich in potential guanine quadruplex sequences

Background

The concept of chromatin domains attached to the nuclear matrix is being revisited, with nucleus described as a set of topologically associating domains. The significance of the tightly bound to DNA proteins (TBP), a protein group that remains attached to DNA after its deproteinization should be also revisited, as the existence of these interactions is in good agreement with the concept of the topologically associating domain. The work aimed to characterize the DNA component of TBP isolated from barley seedlings.

Methods

The tight DNA-protein complexes from the first leaves, coleoptiles, and roots of barley seedlings were isolated by purification with chromatography on nitrocellulose or exhaustive digestion of DNA with DNase I. Cloning and transformation were performed using pMOSBBlue Blunt Ended Cloning Kit. Inserts were amplified by PCR, and sequencing was performed on the MegaBace 1000 Sequencing System. The BLAST search was performed using sequence databases at NCBI, CR-EST, and TREP and Ensembl Plants databases. Comparison to MAR/SAR sequences was performed using http://smartdb.bioinf.med.uni-goettingen.de/cgi-bin/SMARtDB/smar.cgi database. The prediction of G quadruplexes (GQ) was performed with the aid of R-studio library pqsfinder. CD spectra were recorded on a Chirascan CS/3D spectrometer.

Results

Although the barley genome is AT-rich (43% of GC pairs), most DNA fragments associated with TBP were GC-rich (up to 70% in some fractions). Both fractionation procedures yielded a high proportion of CT-motif sequences presented predominantly by the 16-bp CC(TCTCCC)₂ TC fragment present in clones derived from the TBP-bound DNA and absent in free DNA. BLAST analysis revealed alignment with different barley repeats. Some clones, however, aligned with both nuclear and chloroplast structural genes. Alignments with MAR/SAR motifs were very few. The analysis produced by the pqsfinder program revealed numerous potential quadruplex-forming sites in the TBP-bound sequences. A set of oligonucleotides containing sites of possible GQs were designed and ordered. Three of them represented the minus strand of the CT-repeat. Two were derived from sequences of two clones of nitrocellulose retained fraction from leaves and contained GC-rich motifs different from the CT motif. Circular dichroism spectroscopy revealed profound changes in spectra when oligonucleotides were incubated with 100 mM KCl. There was either an increase of positive band in the area of 260 nm or the formation of a positive band at 290 nm. In the former case, changes are typical for parallel G-quadruplexes and, in the latter, 3 + 1 structures.

Discussion

The G-quadruplexes anchor proteins are probably involved in the maintenance of the topologically associated domain structure.

Identification and characterization of two conserved G-quadruplex forming motifs in the Nipah virus genome and their interaction with G-quadruplex specific ligands

The G-quadruplex (GQ) motifs are considered as potential drug-target sites for several human pathogenic viruses such as Zika, Hepatitis, Ebola, and Human Herpesviruses. The recent outbreaks of Nipah virus (NiV) in India, the highly fatal emerging zoonotic virus is a potential threat to global health security as no anti-viral drug or vaccine in currently available. Therefore, here in the present study, we sought to assess the ability of the putative G-quadruplex forming sequences in the NiV genome to form G-quadruplex structures and act as targets for anti-viral compounds. Bioinformatics analysis underpinned by various biophysical and biochemical techniques (such as NMR, CD, EMSA, DMS footprinting assay) confirmed the presence of two highly conserved G-quadruplex forming sequences (HGQs) in the G and L genes of NiV. These genes encode the cell attachment glycoprotein and RNA-dependent RNA polymerase, respectively and are essential for the virus entry and replication within the host cell. It remains possible that stabilization of these HGQs by the known G-quadruplex binding ligands like TMPyP4 and Braco-19 represents a promising strategy to inhibit the expression of the HGQ harboring genes and thereby stop the viral entry and replication inside the host cell. Accordingly, we report for the first time, that HGQs in Nipah virus genome are targets for G-quadruplex specific ligands; therefore, could serve as potential targets for anti-viral therapy.

The Interplay between G-quadruplex and Transcription

G4 DNA is a non-canonical DNA structure consisting of a stacked array of Gquartets held together by base pairing between guanine bases. The formation of G4 DNA requires a cluster of guanine-runs within a strand of DNA. Even though the chemistry of this remarkable DNA structure has been under investigation for decades, evidence supporting the biological relevance of G4 DNA has only begun to emerge and point to very important and conserved biological functions. This review will specifically focus on the interplay between transcription and G4 DNA and discuss two alternative but interconnected perspectives. The first part of the review will describe the evidence substantiating the intriguing idea that a shift in DNA structural conformation could be another layer of non-genetic or epigenetic regulator of gene expression and thereby an important determinant of cell fate. The second part will describe the recent genetic studies showing that those genomic loci containing G4 DNA-forming guanine-rich sequences are potential hotspots of genome instability and that the level and orientation of transcription is critical in the materialization of genome instability associated with these sequences.

Intragenomic heterogeneity of intergenic ribosomal DNA spacers in Cucurbita moschata is determined by DNA minisatellites with variable potential to form non-canonical DNA conformations

The intergenic spacer (IGS) of rDNA is frequently built of long blocks of tandem repeats. To estimate the intragenomic variability of such knotty regions, we employed PacBio sequencing of the Cucurbita moschata genome, in which thousands of rDNA copies are distributed across a number of loci. The rRNA coding regions are highly conserved, indicating intensive interlocus homogenization and/or high selection pressure. However, the IGS exhibits high intragenomic structural diversity. Two repeated blocks, R1 (300-1250 bp) and R2 (290-643 bp), account for most of the IGS variation. They exhibit minisatellite-like features built of multiple periodically spaced short GC-rich sequence motifs with the potential to adopt non-canonical DNA conformations, G-quadruplex-folded and left-handed Z-DNA. The mutual arrangement of these motifs can be used to classify IGS variants into five structural families. Subtle polymorphisms exist within each family due to a variable number of repeats, suggesting the coexistence of an enormous number of IGS variants. The substantial length and structural heterogeneity of IGS minisatellites suggests that the tempo of their divergence exceeds the tempo of the homogenization of rDNA arrays. As frequently occurring among plants, we hypothesize that their instability may influence transcription regulation and/or destabilize rDNA units, possibly spreading them across the genome.

G-Quadruplex DNA and RNA

G-quadruplexes (G4s) have become one of the most exciting nucleic acid secondary structures. A noncanonical, four-stranded structure formed in guanine-rich DNA and RNA sequences, G-quadruplexes can readily form under physiologically relevant conditions and are globularly folded structures. DNA is widely recognized as a double-helical structure essential in genetic information storage. However, only ~3% of the human genome is expressed in protein; RNA and DNA may form noncanonical secondary structures that are functionally important. G-quadruplexes are one such example which have gained considerable attention for their formation and regulatory roles in biologically significant regions, such as human telomeres, oncogene-promoter regions, replication initiation sites, and 5'- and 3'-untranslated region (UTR) of mRNA. They are shown to be a regulatory motif in a number of critical cellular processes including gene transcription, translation, replication, and genomic stability. G-quadruplexes are also found in nonhuman genomes, particularly those of human pathogens. Therefore, G-quadruplexes have emerged as a new class of molecular targets for drug development. In addition, there is considerable interest in the use of G-quadruplexes for biomaterials, biosensors, and biocatalysts. The First International Meeting on Quadruplex DNA was held in 2007, and the G-quadruplex field has been growing dramatically over the last decade. The methods used to study G-quadruplexes have been essential to the rapid progress in our understanding of this exciting nucleic acid secondary structure.

Above and Beyond Watson and Crick: Guanine Quadruplex Structures and Microbes

Advances in understanding mechanisms of nucleic acids have revolutionized molecular biology and medicine, but understanding of nontraditional nucleic acid conformations is less developed. The guanine quadruplex (G4) alternative DNA structure was first described in the 1960s, but the existence of G4 structures (G4-S) and their participation in myriads of biological functions are still underappreciated. Despite many tools to study G4s and many examples of roles for G4s in eukaryotic molecular processes and issues with uncontrolled G4-S formation, there is relatively little knowledge about the roles of G4-S in viral or prokaryotic systems. This review summarizes the state of the art with regard to G4-S in eukaryotes and their potential roles in human disease before discussing the evidence that G4-S have equivalent importance in affecting viral and bacterial life.

Review: Plant G-quadruplex (G4) motifs in DNA and RNA; abundant, intriguing sequences of unknown function

DNA sequences capable of forming G-quadruplex (G4) structures can be predicted and mapped in plant genomes using computerized pattern search programs. Non-telomeric G4 motifs have recently been found to number in the thousands across many plant species and enriched around gene promoters, prompting speculation that they may represent a newly uncovered and ubiquitous family of cis-acting elements. Comparative analysis shows that monocots exhibit five to ten times higher G4 motif density than eudicots, but the significance of this difference has not been determined. The vast scale and complexity of G4 functions, actual or theoretical, are reviewed in relation to the multiple modes of action and myriad genetic functions for which G4s have been implicated in DNA and RNA. Future experimental strategies and opportunities include identifying plant G4-interactomes, resolving the structures of G4s with and without their binding partners, and defining molecular mechanisms through reporter gene, genetic, or genome editing approaches. Given the global importance of plants for food, clothing, medicine, and energy, together with the potential role of G4 motifs as a widely conserved set of DNA sequences that could coordinate gene regulation, future plant G4 research holds great potential for use in plant improvement strategies.

Ground- and Excited-State Interactions of a Psoralen Derivative with Human Telomeric G-Quadruplex DNA

G-quadruplex DNA has been a recent target for anticancer agents, and its binding interactions with small molecules, often used as anticancer drugs, have become an important area of research. Considering that psoralens have long been studied in the context of duplex DNA but that very little is known about their potential as G-quadruplex binders and their excited-state interaction with the latter has not been explored, we have studied herein the binding of a planar water-soluble psoralen derivative, 4'-aminomethyl-4,5',8-trimethylpsoralen (AMT), with the 22-mer human telomeric G-quadruplex-forming sequence, AGGG(TTAGGG)₃, labeled here as (hTel22), and investigated the consequences of photoexcitation of AMT by calorimetric and spectroscopic techniques. The results show an enthalpy-driven 1:1 binding of AMT with hTel22 via end-stacking mode. Fluorescence quenching experiments on 6-fluorescein amidite-labeled oligomers indicate that the binding site is nearer to the 3' end of hTel22 in the diagonal loop region. Femtosecond time-resolved transient absorption measurements indicate electron transfer from the guanine moiety of hTel22 to photoexcited AMT, leading to the formation of a radical pair species (AMT^•-G^•+), which survives for 30 ps and is favored by a parallel/quasi-parallel orientation between the two. The findings reveal psoralens as a prospective class of compounds for the development of anticancer therapeutics by targeting the G-quadruplex DNA.