Single-molecule analysis of DNA replication reveals novel features in the divergent eukaryotes Leishmania and Trypanosoma brucei versus mammalian cells

The genetic landscape of origins of replication in P. falciparum

Various origin mapping approaches have enabled genome-wide identification of origins of replication (ORI) in model organisms, but only a few studies have focused on divergent organisms. By employing three complementary approaches we provide a high-resolution map of ORIs in Plasmodium falciparum, the deadliest human malaria parasite. We profiled the distribution of origin of recognition complex (ORC) binding sites by ChIP-seq of two PfORC subunits and mapped active ORIs using NFS and SNS-seq. We show that ORIs lack sequence specificity but are not randomly distributed, and group in clusters. Licensing is biased towards regions of higher GC content and associated with G-quadruplex forming sequences (G4FS). While strong transcription likely enhances firing, active origins are depleted from transcription start sites. Instead, most accumulate in transcriptionally active gene bodies. Single molecule analysis of nanopore reads containing multiple initiation events, which could have only come from individual nuclei, showed a relationship between the replication fork pace and the distance to the nearest origin. While some similarities were drawn with the canonic eukaryote model, the distribution of ORIs in P. falciparum is likely shaped by unique genomic features such as extreme AT-richness-a product of evolutionary pressure imposed by the parasitic lifestyle.

Life in plastic, it's fantastic! How Leishmania exploit genome instability to shape gene expression

Leishmania are kinetoplastid pathogens that cause leishmaniasis, a debilitating and potentially life-threatening infection if untreated. Unusually, Leishmania regulate their gene expression largely post-transcriptionally due to the arrangement of their coding genes into polycistronic transcription units that may contain 100s of functionally unrelated genes. Yet, Leishmania are capable of rapid and responsive changes in gene expression to challenging environments, often instead correlating with dynamic changes in their genome composition, ranging from chromosome and gene copy number variations to the generation of extrachromosomal DNA and the accumulation of point mutations. Typically, such events indicate genome instability in other eukaryotes, coinciding with genetic abnormalities, but for Leishmania, exploiting these products of genome instability can provide selectable substrates to catalyse necessary gene expression changes by modifying gene copy number. Unorthodox DNA replication, DNA repair, replication stress factors and DNA repeats are recognised in Leishmania as contributors to this intrinsic instability, but how Leishmania regulate genome plasticity to enhance fitness whilst limiting toxic under- or over-expression of co-amplified and co-transcribed genes is unclear. Herein, we focus on fresh, and detailed insights that improve our understanding of genome plasticity in Leishmania. Furthermore, we discuss emerging models and factors that potentially circumvent regulatory issues arising from polycistronic transcription. Lastly, we highlight key gaps in our understanding of Leishmania genome plasticity and discuss future studies to define, in higher resolution, these complex regulatory interactions.

Dynamics of DNA Replication during Male Gametogenesis in the Malaria Parasite Plasmodium Falciparum

Malaria parasites undergo a single phase of sexual reproduction in their complex lifecycle. It involves specialised, sexually committed cells called gametocytes, which develop rapidly into mature gametes and mate upon entering the mosquito midgut. Gamete development is unique, involving unprecedentedly fast replication to produce male gametes. Within ~15 minutes a male gametocyte replicates its ~23 Mb genome three times to produce 8 genomes, segregates these into newly-assembled flagellated gametes and releases them to seek female gametes. Here, for the first time, we use fluorescent labelling of de novo DNA synthesis to follow this process at the whole-cell and single-molecule levels. We make several novel observations, including characterising the origin recognition complex protein Orc1 for the first time in gametocytes, finding that cytokinesis is uncoupled from DNA replication (implying a lack of cell cycle checkpoints), and that the single-molecule dynamics of DNA replication are entirely different from the dynamics in asexual schizogony.

Automatic DNA replication tract measurement to assess replication and repair dynamics at the single-molecule level

MOTIVATION

DNA fibre assay has a potential application in genomic medicine, cancer and stem cell research at the single-molecule level. A major challenge for the clinical and research implementation of DNA fibre assays is the slow speed in which manual analysis takes place as it limits the clinical actionability. While automatic detection of DNA fibres speeds up this process considerably, current publicly available software have limited features in terms of their user interface for manual correction of results, which in turn limit their accuracy and ability to account for atypical structures that may be important in diagnosis or investigative studies. We recognize that core improvements can be made to the GUI to allow for direct interaction with automatic results to preserve accuracy as well as enhance the versatility of automatic DNA fibre detection for use in variety of situations.

RESULTS

To address the unmet needs of diverse DNA fibre analysis investigations, we propose DNA Stranding, an open-source software that is able to perform accurate fibre length quantification (13.22% mean relative error) and fibre pattern recognition (R > 0.93) with up to six fibre patterns supported. With the graphical interface, we developed, user can conduct semi-automatic analyses which benefits from the advantages of both automatic and manual processes to improve workflow efficiency without compromising accuracy.

AVAILABILITY AND IMPLEMENTATION

The software package is available at https://github.com/lgole/DNAStranding.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

Estimation of the Minimum Number of Replication Origins Per Chromosome in any Organism

Eukaryote nuclear genomes predominantly replicate through multiple replication origins. The number of replication origins activated per chromosome during the S-phase duration may vary according to many factors, but the predominant one is replication stress. Several studies have applied different approaches to estimate the number and map the positions of the replication origins in various organisms. However, without a parameter to restrict the minimum of necessary origins, less sensitive techniques may suggest conflicting results. The estimation of the minimum number of replication origins (MO) per chromosome is an innovative method that allows the establishment of a threshold, which serves as a parameter for genomic approaches that map origins. For this, the MO can be easily obtained through a formula that requires as parameters: chromosome size, S-phase duration, and replication rate. The chromosome size for any organism can be acquired in genomic databanks (such as NCBI), the S-phase duration can be estimated by monitoring DNA replication, and the replication rate is obtained through the DNA combing approach. The estimation of MO is a simple, quick, and easy method that provides a new methodological framework to assist studies of mapping replication origins in any organism.

Read, Write, Adapt: Challenges and Opportunities during Kinetoplastid Genome Replication

The genomes of all organisms are read throughout their growth and development, generating new copies during cell division and encoding the cellular activities dictated by the genome’s content. However, genomes are not invariant information stores but are purposefully altered in minor and major ways, adapting cellular behaviour and driving evolution. Kinetoplastids are eukaryotic microbes that display a wide range of such read–write genome activities, in many cases affecting critical aspects of their biology, such as host adaptation. Here we discuss the range of read–write genome changes found in two well-studied kinetoplastid parasites, Trypanosoma brucei and Leishmania, focusing on recent work that suggests such adaptive genome variation is linked to novel strategies the parasites use to replicate their unconventional genomes.

Polycistronic transcription dominates and shapes kinetoplastid genomes, inevitably leading to clashes with DNA replication. By harnessing the resultant DNA damage for adaptation, kinetoplastids have huge potential for dynamic read–write genome variation.

Major origins of DNA replication are confined to the boundaries of polycistronic transcription units in the Trypanosoma brucei and Leishmania genomes, putatively limiting DNA damage. Subtelomeres may lack this arrangement, generating read–write hotspots.

In T. brucei, early replication of the highly transcribed subtelomeric variant surface glycoprotein (VSG) expression site may ensure replication-transcription clashes within this site to trigger DNA recombination, an event critical for antigenic variation.

Leishmania genomes show extensive aneuploidy and copy number variation. Notably, DNA replication requires recombination factors and relies on post-S phase replication of subtelomeres.

Evolution of compartmentalised DNA replication programmes underpin important aspects of genome biology in kinetoplastids, illustrating the consolidation of genome maintenance strategies to promote genome plasticity.

Genome duplication in Leishmania major relies on persistent subtelomeric DNA replication

DNA replication is needed to duplicate a cell’s genome in S phase and segregate it during cell division. Previous work in Leishmania detected DNA replication initiation at just a single region in each chromosome, an organisation predicted to be insufficient for complete genome duplication within S phase. Here, we show that acetylated histone H3 (AcH3), base J and a kinetochore factor co-localise in each chromosome at only a single locus, which corresponds with previously mapped DNA replication initiation regions and is demarcated by localised G/T skew and G4 patterns. In addition, we describe previously undetected subtelomeric DNA replication in G2/M and G1-phase-enriched cells. Finally, we show that subtelomeric DNA replication, unlike chromosome-internal DNA replication, is sensitive to hydroxyurea and dependent on 9-1-1 activity. These findings indicate that Leishmania’s genome duplication programme employs subtelomeric DNA replication initiation, possibly extending beyond S phase, to support predominantly chromosome-internal DNA replication initiation within S phase.

Replication origin location might contribute to genetic variability in Trypanosoma cruzi

Background

DNA replication in trypanosomatids operates in a uniquely challenging environment, since most of their genomes are constitutively transcribed. Trypanosoma cruzi, the etiological agent of Chagas disease, presents high variability in both chromosomes size and copy number among strains, though the underlying mechanisms are unknown.

Results

Here we have mapped sites of DNA replication initiation across the T. cruzi genome using Marker Frequency Analysis, which has previously only been deployed in two related trypanosomatids. The putative origins identified in T. cruzi show a notable enrichment of GC content, a preferential position at subtelomeric regions, coinciding with genes transcribed towards the telomeres, and a pronounced enrichment within coding DNA sequences, most notably in genes from the Dispersed Gene Family 1 (DGF-1).

Conclusions

These findings suggest a scenario where collisions between DNA replication and transcription are frequent, leading to increased genetic variability, as seen by the increase SNP levels at chromosome subtelomeres and in DGF-1 genes containing putative origins.

Comparative Analysis of the Minimum Number of Replication Origins in Trypanosomatids and Yeasts

Single-celled eukaryote genomes predominantly replicate through multiple origins. Although origin usage during the S-phase has been elucidated in some of these organisms, few studies have comparatively approached this dynamic. Here, we developed a user-friendly website able to calculate the length of the cell cycle phases for any organism. Next, using a formula developed by our group, we showed a comparative analysis among the minimum number of replication origins (MO) required to duplicate an entire chromosome within the S-phase duration in trypanosomatids (Trypanosoma cruzi, Leishmania major, and Trypanosoma brucei) and yeasts (Saccharomyces cerevisiae and Schizosaccharomyces pombe). Using the data obtained by our analysis, it was possible to predict the MO required in a situation of replication stress. Also, our findings allow establishing a threshold for the number of origins, which serves as a parameter for genome approaches that map origins. Moreover, our data suggest that when compared to yeasts, trypanosomatids use much more origins than the minimum needed. This is the first time a comparative analysis of the minimum number of origins has been successfully applied. These data may provide new insight into the understanding of the replication mechanism and a new methodological framework for studying single-celled eukaryote genomes.

Transcription activity contributes to the firing of non-constitutive origins in African trypanosomes helping to maintain robustness in S-phase duration

The co-synthesis of DNA and RNA potentially generates conflicts between replication and transcription, which can lead to genomic instability. In trypanosomatids, eukaryotic parasites that perform polycistronic transcription, this phenomenon and its consequences are still little studied. Here, we showed that the number of constitutive origins mapped in the Trypanosoma brucei genome is less than the minimum required to complete replication within S-phase duration. By the development of a mechanistic model of DNA replication considering replication-transcription conflicts and using immunofluorescence assays and DNA combing approaches, we demonstrated that the activation of non-constitutive (backup) origins are indispensable for replication to be completed within S-phase period. Together, our findings suggest that transcription activity during S phase generates R-loops, which contributes to the emergence of DNA lesions, leading to the firing of backup origins that help maintain robustness in S-phase duration. The usage of this increased pool of origins, contributing to the maintenance of DNA replication, seems to be of paramount importance for the survival of this parasite that affects million people around the world.

Leishmania amazonensis ferric iron reductase (LFR1) is a bifunctional enzyme: Unveiling a NADPH oxidase activity

Leishmania amazonensis is one of leishmaniasis' causative agents, a disease that has no cure and leads to the appearance of cutaneous lesions. Recently, our group showed that heme activates a Na⁺/K⁺ ATPase in these parasites through a signaling cascade involving hydrogen peroxide (H₂O₂) generation. Heme has a pro-oxidant activity and signaling capacity, but the mechanism by which this molecule increases H₂O₂ levels in L. amazonensis has not been elucidated. Here we investigated the source of H₂O₂ stimulated by heme, ruling out the participation of mitochondria and raising the possibility of a role for a NADPH oxidase (Nox) activity. Despite the absence of a classical Nox sequence in trypanosomatid genomes, L. amazonensis expresses a surface ferric iron reductase (LFR1). Interestingly, Nox enzymes are thought to have evolved from ferric iron reductases because they share same core domain and are very similar in structure. The main difference is that Nox catalyses electron flow from NADPH to oxygen, generating reactive oxygen species (ROS), while ferric iron reductase promotes electron flow to ferric iron, generating ferrous iron. Using L. amazonensis overexpressing or knockout for LFR1 and heterologous expression of LFR1 in mammalian embryonic kidney (HEK 293) cells, we show that this enzyme is bifunctional, being able to generate both ferrous iron and H₂O₂. It was previously described that protozoans knockout for LFR1 have their differentiation to virulent forms (amastigote and metacyclic promastigote) impaired. In this work, we observed that LFR1 overexpression stimulates protozoan differentiation to amastigote forms, reinforcing the importance of this enzyme in L. amazonensis life cycle regulation. Thus, we not only identified a new source of ROS production in Leishmania, but also described, for the first time, an enzyme with both ferric iron reductase and Nox activities.

Coordination of the Cell Cycle in Trypanosomes

Trypanosomes have complex life cycles within which there are both proliferative and differentiation cell divisions. The coordination of the cell cycle to achieve these different divisions is critical for the parasite to infect both host and vector. From studying the regulation of the proliferative cell cycle of the Trypanosoma brucei procyclic life cycle stage, three subcycles emerge that control the duplication and segregation of ( a) the nucleus, ( b) the kinetoplast, and ( c) a set of cytoskeletal structures. We discuss how the clear dependency relationships within these subcycles, and the potential for cross talk between them, are likely required for overall cell cycle coordination. Finally, we look at the implications this interdependence has for proliferative and differentiation divisions through the T. brucei life cycle and in related parasitic trypanosomatid species.

Genome-wide function of MCM-BP in Trypanosoma brucei DNA replication and transcription

In Trypanosoma brucei, genes are arranged in Polycistronic Transcription Units (PTUs), which are demarcated by transcription start and stop sites. Transcription start sites are also binding sites of Origin Recognition Complex 1 (ORC1). This spatial coincidence implies that transcription and replication in trypanosomes must occur in a highly ordered and cooperative manner. Interestingly, a previously published genetic screen identified the T. brucei MCM-BP, which interacts with subunits of MCM helicase, as a protein whose downregulation results in the loss of transcriptional silencing at subtelomeric loci. Here, I show that TbMCM-BP is required for DNA replication and transcription. TbMCM-BP depletion causes a significant reduction of replicating cells in S phase and genome-wide impairments of replication origin activation. Moreover, levels of sense and antisense transcripts increase at boundaries of PTUs in the absence of TbMCM-BP. TbMCM-BP is also important for transcriptional repression of the specialized subtelomeric PTUs, the Bloodstream-form Expression-Sites (BESs), which house the major antigenic determinant (the Variant Surface Glycoprotein, VSG gene) as well as TbORC1 binding sites. Overall, this study reveals that TbMCM-BP, a replication initiation protein, also guides the initiation, termination and directionality of transcription.

Asymmetric histone inheritance via strand-specific incorporation and biased replication fork movement

Many stem cells undergo asymmetric division to produce a self-renewing stem cell and a differentiating daughter cell. Here we show that, similarly to H3, histone H4 is inherited asymmetrically in Drosophila melanogaster male germline stem cells undergoing asymmetric division. In contrast, both H2A and H2B are inherited symmetrically. By combining super-resolution microscopy and chromatin fiber analyses with proximity ligation assays on intact nuclei, we find that old H3 is preferentially incorporated by the leading strand, whereas newly synthesized H3 is enriched on the lagging strand. Using a sequential nucleoside analog incorporation assay, we detect a high incidence of unidirectional replication fork movement in testes-derived chromatin and DNA fibers. Biased fork movement coupled with a strand preference in histone incorporation would explain how asymmetric old and new H3 and H4 are established during replication. These results suggest a role for DNA replication in patterning epigenetic information in asymmetrically dividing cells in multicellular organisms.

Analysis of DNA Exchange Using Thymidine Analogs (ADExTA) in Trypanosoma cruzi

Trypanosoma cruzi is a protozoan parasite belonging to the Trypanosomatidae family. Although the trypanosomatids multiply predominantly by clonal generation, the presence of DNA exchange in some of them has been puzzling researchers over the years, mainly because it may represent a novel form that these organisms use to gain variability. Analysis of DNA Exchange using Thymidine Analogs (ADExTA) is a method that allows the in vitro detection and measurement of rates of DNA exchange, particularly in trypanosomatid cells, in a rapid and simple manner by indirect immunofluorescence assay (IFA). The method can be used to detect DNA exchange within one trypanosomatid lineage or among different lineages by paired analysis. The principle of this assay is based on the incorporation of two distinguishable halogenated thymidine analogs called 5'-chloro-2'-deoxyuridine (CldU) and 5'-iodo-2'-deoxyuridine (IdU) during DNA replication. After mixing the two cell cultures that had been previously incorporated with CldU and IdU separately, the presence of these unusual deoxynucleosides in the genome can be detected by specific antibodies. For this, a DNA denaturation step is required to expose the sites of thymidine analogs incorporated. Subsequently, a secondary reaction using fluorochrome-labeled antibodies will generate distinct signals under fluorescence analysis. By using this method, DNA exchange verification (i.e., the presence of both CldU and IdU in the same cell) is possible using a standard fluorescence microscope. It typically takes 2-3 days from the thymidine analogs incorporation to results. Of note, ADExTA is relatively cheap and does not require transfections or harsh genetic manipulation. These features represent an advantage when compared to other time-consuming protocols that demand DNA manipulation to introduce distinct drug-resistance markers in different cells for posterior selection.

The Dynamics of Replication in Trypanosoma cruzi Parasites by Single-Molecule Analysis

Here, we investigated the features of replication in Trypanosoma cruzi epimastigotes based on fork speed progression, which is influenced by distinct features such as DNA polymerase rate, susceptibility to DNA damage and repair, secondary structures, transcription and chromatin state. Although T. cruzi exhibits a mean fork speed (2.05 ± 0.10 kb/min) very similar to other trypanosomatids, we found that the majority of DNA molecules replicated more slowly, with a frequency distribution approximately 1 kb/min. This frequency distribution analysis provides more information about the replication profile of this organism.

Gene and Chromosomal Copy Number Variations as an Adaptive Mechanism Towards a Parasitic Lifestyle in Trypanosomatids

Trypanosomatids are a group of kinetoplastid parasites including some of great public health importance, causing debilitating and life-long lasting diseases that affect more than 24 million people worldwide. Among the trypanosomatids, Trypanosoma cruzi, Trypanosoma brucei and species from the Leishmania genus are the most well studied parasites, due to their high prevalence in human infections. These parasites have an extreme genomic and phenotypic variability, with a massive expansion in the copy number of species-specific multigene families enrolled in host-parasite interactions that mediate cellular invasion and immune evasion processes. As most trypanosomatids are heteroxenous, and therefore their lifecycles involve the transition between different hosts, these parasites have developed several strategies to ensure a rapid adaptation to changing environments. Among these strategies, a rapid shift in the repertoire of expressed genes, genetic variability and genome plasticity are key mechanisms. Trypanosomatid genomes are organized into large directional gene clusters that are transcribed polycistronically, where genes derived from the same polycistron may have very distinct mRNA levels. This particular mode of transcription implies that the control of gene expression operates mainly at post-transcriptional level. In this sense, gene duplications/losses were already associated with changes in mRNA levels in these parasites. Gene duplications also allow the generation of sequence variability, as the newly formed copy can diverge without loss of function of the original copy. Recently, aneuploidies have been shown to occur in several Leishmania species and T. cruzi strains. Although aneuploidies are usually associated with debilitating phenotypes in superior eukaryotes, recent data shows that it could also provide increased fitness in stress conditions and generate drug resistance in unicellular eukaryotes. In this review, we will focus on gene and chromosomal copy number variations and their relevance to the evolution of trypanosomatid parasites.

Conservation and Variation in Strategies for DNA Replication of Kinetoplastid Nuclear Genomes

Introduction:

Understanding how the nuclear genome of kinetoplastid parasites is replicated received experimental stimulus from sequencing of the Leishmania major, Trypanosoma brucei and Trypanosoma cruzi genomes around 10 years ago. Gene annotations suggested key players in DNA replication initiation could not be found in these organisms, despite considerable conservation amongst characterised eukaryotes. Initial studies that indicated trypanosomatids might possess an archaeal-like Origin Recognition Complex (ORC), composed of only a single factor termed ORC1/CDC6, have been supplanted by the more recent identification of an ORC in T. brucei. However, the constituent subunits of T. brucei ORC are highly diverged relative to other eukaryotic ORCs and the activity of the complex appears subject to novel, positive regulation. The availability of whole genome sequences has also allowed the deployment of genome-wide strategies to map DNA replication dynamics, to date in T. brucei and Leishmania. ORC1/CDC6 binding and function in T. brucei displays pronounced overlap with the unconventional organisation of gene expression in the genome. Moreover, mapping of sites of replication initiation suggests pronounced differences in replication dynamics in Leishmania relative to T. brucei.

Conclusion:

Here we discuss what implications these emerging data may have for parasite and eukaryotic biology of DNA replication.

Nuclear DNA Replication in Trypanosomatids: There Are No Easy Methods for Solving Difficult Problems

In trypanosomatids, etiological agents of devastating diseases, replication is robust and finely controlled to maintain genome stability and function in stressful environments. However, these parasites encode several replication protein components and complexes that show potentially variant composition compared with model eukaryotes. This review focuses on the advances made in recent years regarding the differences and peculiarities of the replication machinery in trypanosomatids, including how such divergence might affect DNA replication dynamics and the replication stress response. Comparing the DNA replication machinery and processes of parasites and their hosts may provide a foundation for the identification of targets that can be used in the development of chemotherapies to assist in the eradication of diseases caused by these pathogens.

In trypanosomatids, DNA replication is tightly controlled by protein complexes that diverge from those of model eukaryotes.

There is no consensus for the number of replication origins used by trypanosomatids; how their replication dynamics compares with that of model organisms is the subject of debate.

The DNA replication rate in trypanosomatids is similar to, but slightly higher than, that of model eukaryotes, which may be related to chromatin structure and function.

Recent data suggest that the origin recognition complex in trypanosomatids closely resembles the multisubunit eukaryotic model.

The absence of fundamental replication-associated proteins in trypanosomatids suggests that new signaling pathways may be present in these parasites to direct DNA replication and the replicative stress response.

Replication fork slowing and stalling are distinct, checkpoint-independent consequences of replicating damaged DNA

In response to DNA damage during S phase, cells slow DNA replication. This slowing is orchestrated by the intra-S checkpoint and involves inhibition of origin firing and reduction of replication fork speed. Slowing of replication allows for tolerance of DNA damage and suppresses genomic instability. Although the mechanisms of origin inhibition by the intra-S checkpoint are understood, major questions remain about how the checkpoint regulates replication forks: Does the checkpoint regulate the rate of fork progression? Does the checkpoint affect all forks, or only those encountering damage? Does the checkpoint facilitate the replication of polymerase-blocking lesions? To address these questions, we have analyzed the checkpoint in the fission yeast Schizosaccharomyces pombe using a single-molecule DNA combing assay, which allows us to unambiguously separate the contribution of origin and fork regulation towards replication slowing, and allows us to investigate the behavior of individual forks. Moreover, we have interrogated the role of forks interacting with individual sites of damage by using three damaging agents-MMS, 4NQO and bleomycin-that cause similar levels of replication slowing with very different frequency of DNA lesions. We find that the checkpoint slows replication by inhibiting origin firing, but not by decreasing fork rates. However, the checkpoint appears to facilitate replication of damaged templates, allowing forks to more quickly pass lesions. Finally, using a novel analytic approach, we rigorously identify fork stalling events in our combing data and show that they play a previously unappreciated role in shaping replication kinetics in response to DNA damage.

Identification and characterization of ARS-like sequences as putative origin(s) of replication in human malaria parasite Plasmodium falciparum

DNA replication is a fundamental process in genome maintenance, and initiates from several genomic sites (origins) in eukaryotes. In Saccharomyces cerevisiae, conserved sequences known as autonomously replicating sequences (ARSs) provide a landing pad for the origin recognition complex (ORC), leading to replication initiation. Although origins from higher eukaryotes share some common sequence features, the definitive genomic organization of these sites remains elusive. The human malaria parasite Plasmodium falciparum undergoes multiple rounds of DNA replication; therefore, control of initiation events is crucial to ensure proper replication. However, the sites of DNA replication initiation and the mechanism by which replication is initiated are poorly understood. Here, we have identified and characterized putative origins in P. falciparum by bioinformatics analyses and experimental approaches. An autocorrelation measure method was initially used to search for regions with marked fluctuation (dips) in the chromosome, which we hypothesized might contain potential origins. Indeed, S. cerevisiae ARS consensus sequences were found in dip regions. Several of these P. falciparum sequences were validated with chromatin immunoprecipitation-quantitative PCR, nascent strand abundance and a plasmid stability assay. Subsequently, the same sequences were used in yeast to confirm their potential as origins in vivo. Our results identify the presence of functional ARSs in P. falciparum and provide meaningful insights into replication origins in these deadly parasites. These data could be useful in designing transgenic vectors with improved stability for transfection in P. falciparum.

Single-molecule analysis reveals that DNA replication dynamics vary across the course of schizogony in the malaria parasite Plasmodium falciparum

The mechanics of DNA replication and cell cycling are well-characterized in model organisms, but less is known about these basic aspects of cell biology in early-diverging Apicomplexan parasites, which do not divide by canonical binary fission but undergo unconventional cycles. Schizogony in the malaria parasite, Plasmodium, generates ~16–24 new nuclei via independent, asynchronous rounds of genome replication prior to cytokinesis and little is known about the control of DNA replication that facilitates this. We have characterised replication dynamics in P. falciparum throughout schizogony, using DNA fibre labelling and combing to visualise replication forks at a single-molecule level. We show that origins are very closely spaced in Plasmodium compared to most model systems, and that replication dynamics vary across the course of schizogony, from faster synthesis rates and more widely-spaced origins through to slower synthesis rates and closer-spaced origins. This is the opposite of the pattern usually seen across S-phase in human cells, when a single genome is replicated. Replication forks also appear to stall at an unusually high rate throughout schizogony. Our work explores Plasmodium DNA replication in unprecedented detail and opens up tremendous scope for analysing cell cycle dynamics and developing interventions targetting this unique aspect of malaria biology.

The genome of Leishmania adleri from a mammalian host highlights chromosome fission in Sauroleishmania

Control of pathogens arising from humans, livestock and wild animals can be enhanced by genome-based investigation. Phylogenetically classifying and optimal construction of these genomes using short sequence reads are key to this process. We examined the mammal-infecting unicellular parasite Leishmania adleri belonging to the lizard-infecting Sauroleishmania subgenus. L. adleri has been associated with cutaneous disease in humans, but can be asymptomatic in wild animals. We sequenced, assembled and investigated the L. adleri genome isolated from an asymptomatic Ethiopian rodent (MARV/ET/75/HO174) and verified it as L. adleri by comparison with other Sauroleishmania species. Chromosome-level scaffolding was achieved by combining reference-guided with de novo assembly followed by extensive improvement steps to produce a final draft genome with contiguity comparable with other references. L. tarentolae and L. major genome annotation was transferred and these gene models were manually verified and improved. This first high-quality draft Leishmania adleri reference genome is also the first Sauroleishmania genome from a non-reptilian host. Comparison of the L. adleri HO174 genome with those of L. tarentolae Parrot-TarII and lizard-infecting L. adleri RLAT/KE/1957/SKINK-7 showed extensive gene amplifications, pervasive aneuploidy, and fission of chromosomes 30 and 36. There was little genetic differentiation between L. adleri extracted from mammals and reptiles, highlighting challenges for leishmaniasis surveillance.

Leishmania DNA Replication Timing: A Stochastic Event?

For eukaryotic genomes, DNA synthesis initiates at multiple discrete regions known as replication origins in a dynamic yet regulated manner to ensure genomic stability. Two recent studies using different approaches reveal few Leishmania origins and that origin firing may proceed in a mainly stochastic manner.