Characterization of the switch of kinetoplast DNA minicircle dominance during development and reversion of drug resistance in Leishmania

Real-time PCR applications for diagnosis of leishmaniasis

Leishmaniasis is a vector-borne disease caused by many Leishmania species, which can infect both humans and other mammals. Leishmaniasis is a complex disease, with heterogeneous clinical manifestations ranging from asymptomatic infections to lesions at cutaneous sites (cutaneous leishmaniasis), mucosal sites (mucocutaneous leishmaniasis) or in visceral organs (visceral leishmaniasis), depending on the species and host characteristics. Often, symptoms are inconclusive and leishmaniasis can be confused with other co-endemic diseases. Moreover, co-infections (mainly with HIV in humans) can produce atypical clinical presentations. A correct diagnosis is crucial to apply the appropriate treatment and the use of molecular techniques in diagnosis of leishmaniasis has become increasingly relevant due to their remarkable sensitivity, specificity and possible application to a variety of clinical samples. Among them, real-time PCR (qPCR)-based approaches have become increasingly popular in the last years not only for detection and quantification of Leishmania species but also for species identification. However, despite qPCR-based methods having proven to be very effective in the diagnosis of leishmaniasis, a standardized method does not exist. This review summarizes the qPCR-based methods in the diagnosis of leishmaniasis focusing on the recent developments and applications in this field.

High-throughput sequencing of kDNA amplicons for the analysis of Leishmania minicircles and identification of Neotropical species

Leishmania kinetoplast DNA contains thousands of small circular molecules referred to as kinetoplast DNA (kDNA) minicercles. kDNA minicircles are the preferred targets for sensitive Leishmania detection, because they are present in high copy number and contain conserved sequence blocks in which polymerase chain reaction (PCR) primers can be designed. On the other hand, the heterogenic nature of minicircle networks has hampered the use of this peculiar genomic region for strain typing. The characterization of Leishmania minicirculomes used to require isolation and cloning steps prior to sequencing. Here, we show that high-throughput sequencing of single minicircle PCR products allows bypassing these laborious laboratory tasks. The 120 bp long minicircle conserved region was amplified by PCR from 18 Leishmania strains representative of the major species complexes found in the Neotropics. High-throughput sequencing of PCR products enabled recovering significant numbers of distinct minicircle sequences from each strain, reflecting minicircle class diversity. Minicircle sequence analysis revealed patterns that are congruent with current hypothesis of Leishmania relationships. Then, we show that a barcoding-like approach based on minicircle sequence comparisons may allow reliable identifications of Leishmania spp. This work opens up promising perspectives for the study of kDNA minicercles and a variety of applications in Leishmania research.

Genotypic profiles of Leishmania (Viannia) braziliensis strains from cutaneous leishmaniasis patients and their relationship with the response to meglumine antimoniate treatment: a pilot study

Background: Forty-four strains isolated from a cohort of cutaneous leishmaniasis (CL) patients who did or did not respond to one course of treatment with meglumine antimoniate were investigated to explore genetic polymorphisms in parasite kinetoplast DNA minicircles. Leishmania (Viannia) braziliensis strains isolated from responder (R) and non-responder (NR) patients who acquired infection in Rio de Janeiro or in other Brazilian states were studied using low-stringency single-specific primer polymerase chain reaction (LSSP-PCR) to identify genetic polymorphisms. Results: Polymorphisms were observed in parasites recovered from patient lesions. No association was found between a specific genotype and R or NR patients. Phenetic analysis grouped the genotypes into three main clusters, with similarity indices varying from 0.72 to 1.00. Although no specific genotype association was detected, at least one group of L. (V.) braziliensis genotypes that circulates in Rio de Janeiro was discriminated in clusters I and III, showing phenotypes of good and poor responses to treatment, respectively. Cluster I comprised parasite profiles recovered from R patients from Rio de Janeiro and in cluster III, NR samples were prevalent. Cluster II comprised 24 isolates, with 21 from Rio de Janeiro and three from other states, equally distributed between R and NR patients. Additionally, we found that parasites sharing all common genetic characteristics acted differently in response to treatment. Conclusions: These results are of clinical-epidemiological importance since they demonstrate that populations of L. (V.) braziliensis that exhibit high levels of genetic similarity also display different phenotypes associated with meglumine antimoniate responses in cutaneous leishmaniasis patients.

Molecular approaches for a better understanding of the epidemiology and population genetics ofLeishmania

Molecular approaches are being used increasingly for epidemiological studies of visceral and cutaneous leishmaniases. Several molecular markers resolving genetic differences betweenLeishmaniaparasites at species and strain levels have been developed to address key epidemiological and population genetic questions. The current gold standard, multilocus enzyme typing (MLEE), needs cultured parasites and lacks discriminatory power. PCR assays identifying species directly with clinical samples have proven useful in numerous field studies. Multilocus sequence typing (MLST) is potentially the most powerful phylogenetic approach and will, most probably, replace MLEE in the future. Multilocus microsatellite typing (MLMT) is able to discriminate below the zymodeme level and seems to be the best candidate for becoming the gold standard for distinction of strains. Population genetic studies by MLMT revealed geographical and hierarchic population structure inL. tropica, L. majorand theL. donovanicomplex. The existence of hybrids and gene flow betweenLeishmaniapopulations suggests that sexual recombination is more frequent than previously thought. However, typing and analytical tools need to be further improved. Accessible databases should be created and sustained for integrating data obtained by different researchers. This would allow for global analyses and help to avoid biases in analyses due to small sample sizes.

kDNA minicircle signatures of Leishmania (Viannia) braziliensis in oral and nasal mucosa from mucosal leishmaniasis patients

Polymerase chain reaction (PCR) and low-stringency single-specific primer PCR (LSSP-PCR) analyses were used to detect Leishmania (Viannia) braziliensis DNA and investigate kDNA signatures of parasite populations present in oral and nasal mucosa lesions from mucosal leishmaniasis patients. A total of 25 samples from 22 patients were processed by specific PCR/hybridization assays. Parasite DNA was detected in all samples analyzed. The intraspecific polymorphism of the variable region of L. (V.) braziliensis kDNA minicircles was also investigated by LSSP-PCR. Similar kDNA signatures were observed in parasites recovered from nasal and oral mucosa lesions of the same patient. In contrast, genetically divergent profiles were detected in lesions from patients biopsied at different times within a period of 1 year. This is the first work to report genetic typing of L. (V.) braziliensis directly from human oral and nasal mucosal lesions.

Identification of macrosialin (CD68) on the surface of host macrophages as the receptor for the intercellular adhesive molecule (ICAM-L) of Leishmania amazonensis

The intercellular adhesive molecule, ICAM-L, of Leishmania amazonensis is known to block the attachment as well as internalisation of Leishmania for infection in host macrophages. We employed monoclonal antibodies (mAb) to the surface molecules of a macrophage to block the attachment of ICAM-L to the macrophage surface and identified that CD68 macrosialin is likely the receptor molecule on the macrophage for ICAM-L. We then demonstrated physical interaction between ICAM-L and macrosialin by co-immunoprecipitation of macrosialin with ICAM-L or vice versa. Finally, macrosialin is expressed in macrosialin-negative murine fibroblast cell line NCTC clone 2555 and demonstrates that both ICAM-L and promastigotes of L. amazonensis can bind to the CD68 transfectant. We thus conclude that CD68 macrosialin is the receptor on host macrophages for ICAM-L. Also, involvement of ICAM-L-macrosialin interaction in other Leishmania species and other mammalian macrophages were demonstrated, indicating the biological relevance of this ligand-receptor interaction.

Two distinct arsenite-resistant variants of Leishmania amazonensis take different routes to achieve resistance as revealed by comparative transcriptomics

Genome-wide search for the genes involved in arsenite resistance in two distinct variants A and A' of Leishmania amazonensis revealed that the two variants used two different mechanisms to achieve resistance, even though these two variants were derived from the same clone and selected against arsenite under the same conditions. In variant A, the variant with DNA amplification, the biochemical pathways for detoxification of oxidative stress, the energy generation system to support the biochemical and physiological needs of the variant for DNA and protein synthesis and the arsenite translocating system to dispose arsenite are among the primary biochemical events that are upregulated under the arsenite stress to gain resistance. In variant A', the variant without DNA amplification, the upregulation of aquaglyceroporin (AQP) gene and the high level of resistance to arsenate point to the direction that the resistance gained by the variant is due to arsenate which is probably oxidized from arsenite in the arsenite solution used for selection and the maintenance of the cell culture. As a result of the AQP upregulation for arsenite disposal, a different set of biochemical pathways for detoxification of oxidative stress, energy generation and cellular signaling are upregulated to sustain the growth of the variant to gain resistance to arsenate. From current evidences, reactive oxygen species (ROS) overproduced by the parasite soon after exposure to arsenite appear to play an instrumental role in both variants to initiate the subsequent biochemical events that allow the same clone of L. amazonensis to take two totally different routes to diverge into two different variants.

Divergence of trypanothione-dependent tryparedoxin cascade into cytosolic and mitochondrial pathways in arsenite-resistant variants of Leishmania amazonensis

Evidences are presented in the in vivo study that overexpression of tryparedoxin peroxidases (TXNPxs) diverged into cytosolic pathway in arsenite-resistant variant A and mitochondrial pathway in variant A' of Leishmania amazonensis is due to the upregulation of the corresponding upstream tryparedoxins (TXNs) in the cytosol as well as the mitochondrion respectively. Evidences are also presented that exposure of L. amazonensis to arsenite in the early hours led to the production of reactive oxygen species (ROS), which in turn induced the overexpression of the genes of both cytosolic and mitochondrial trypanothione-dependent tryparedoxin pathway due probably to physiological and functional needs. The sequence of events leading to the upregulation indicates that cytosolic tryparedoxin pathway is upregulated earlier than that of mitochondrial tryparedoxin pathway. Based on the kinetics of gene upregulation of the cytosolic pathway is different from that of mitochondrial pathway, and cTXNPx and mTXNPx differentially detoxify H(2)O(2) and of t-butyl hydroperoxide respectively, it is postulated that during arsenite selection, different ROS species may have been overproduced in either variants A or A', leading to the divergence of the trypanothione-dependent tryparedoxin pathways in these variants.

Induction of apoptosis-like cell death by pentamidine and doxorubicin through differential inhibition of topoisomerase II in arsenite-resistant L. donovani

The current study has been undertaken to investigate the sensitivity of the topoisomerase II (topo II) of wild type (Ld-Wt) and arsenite-resistant (Ld-As20) L. donovani to an anti-leishmanial agent pentamidine and an anti-cancer drug doxorubicin. We demonstrate that the cross resistance to pentamidine and doxorubicin in Ld-As20, was in part implicated through differential inhibition of topo II in Ld-Wt and Ld-As20. Further, the treatment of promastigotes at drug concentrations inhibiting 50% of topo II activity inflicted a regulated cell death sharing several apoptotic features like externalization of phosphatidylserine, loss of mitochondrial membrane potential, cytochrome C release into the cytosol, activation of cellular proteases and DNA fragmentation. The cytotoxic potential of pentamidine and doxorubicin in L. donovani has been shown to be mediated through topoisomerase II inhibition and results in inciting programmed cell death process.

Loop 1 structure of the leishmanial ICAM-L molecule is crucial for parasite binding and infection of host macrophages

The binding of each intercellular adhesive molecule (ICAM) molecule fragment from Leishmania amazonensis (ICAM-L) to host macrophages was investigated using an indirect immunofluorescent sandwich technique, based on the observation that ICAM-L can block the uptake of L. amazonensis on the macrophage surface and all prepared ICAM-L fragments can react with rabbit anti-ICAM-L antiserum. The ICAM-L fragments lacking the loop 1 (LI) structure failed to bind to macrophages, and the disruption of the LI structure by mercaptoethanol led to the failure of binding. The fragments containing the LI structure functioned similarly to ICAM-L, by temporarily retarding host cell growth and cell cycle progression, and inhibiting the Leishmania infection of host macrophages. These results suggest that LI constitutes the main determinant of the ICAM-L molecule in binding to, and infection of, host macrophages.

Application of kDNA as a molecular marker to analyse Leishmania infantum diversity in Portugal

Around the Mediterranean basin Leishmania infantum is an important parasite causing canine leishmaniasis and visceral and cutaneous clinical forms in both immunocompetent and immunocompromised humans. Efficient monitoring and evaluation of epidemiology with discriminatory molecular markers are required. We investigated the genetic diversity of L. infantum in Portugal by polymerase chain amplification and restriction fragment length polymorphism analysis of kinetoplastid DNA, as molecular marker. We analysed 120 Portuguese isolates of L. infantum plus 16 other non-Portuguese isolates (as a reference group) from humans, dogs and sand flies. The Portuguese population showed a high degree of polymorphism with a total of 13 profiles identified. The predominant profile was A, which was only detected in the Portuguese samples. The kinetoplastid DNA PCR-RFLP assay described here was suitable for use directly with biological samples and the profiles obtained were stable during long-term growth in vitro and in laboratory animals.

Radically different maxicircle classes within the same kinetoplast: an artefact or a novel feature of the kinetoplast genome?

We discuss here some results which suggest that radically different maxicircle classes coexist within the same kinetoplast. These data, although tentative and incomplete, may provide a new outlook on the kinetoplast genome structure and expression.

Mechanism of metalloid-induced death in Leishmania spp.: role of iron, reactive oxygen species, Ca2+, and glutathione

There is growing evidence that metalloid-induced cell death in protozoan parasites is due to oxidative injury; however, the biochemical changes related to this event are not fully understood. Leishmania spp. demonstrated cross-resistance to two related metalloids, arsenic and antimony, and both metalloids induced cell death accompanied by cell shrinkage and DNA fragmentation that was preceded by an increase in reactive oxygen species. Both drugs caused mitochondrial dysfunction in terms of loss of membrane potential and a drop in ATP levels. Arsenic treatment resulted in an elevation of intracellular Ca2+ levels that did not occur with antimony exposure. Cellular glutathione level was reduced after antimony treatment but arsenic did not affect glutathione. Inhibition of Ca2+ influx during arsenic treatment reduced cell death, whereas supplementation of glutathione during antimony treatment rescued cell loss. Under iron-depleted conditions, the cytotoxic effects of arsenic and antimony did not occur and cell survival increased; in contrast, the presence of excess iron resulted in higher cell death. Therefore, this study provides a new possibility that iron can potentiate parasite death induced by metalloids like arsenic and antimony. In addition, an important observation is that the two similar metalloids produce toxicity by very different mechanisms.

Distinct overexpression of cytosolic and mitochondrial tryparedoxin peroxidases results in preferential detoxification of different oxidants in arsenite-resistant Leishmania amazonensis with and without DNA amplification

A cytosolic (cTXNPx) and a mitochondrial (mTXNPx) tryparedoxin peroxidase genes, cloned from wildtype Leishmania amazonensis clone 2-23 are homologous in nucleic acid and amino acid sequences to the respective genes described for L. infantum and L. chagasi. Surprisingly, as shown in the results of transcription assays, protein determination and fluorescent antibody detection in situ, cTXNPx is distinctly overexpressed in the cytoplasm of arsenite-resistant A variant with DNA amplification, whereas mTXNPx is distinctly overexpressed in the mitochondrion of arsenite-resistant A' variant without DNA amplification, although A and A' are arsenite-resistant variants derived from the same wildtype clone of L. amazonensis, and selected against arsenite under the same conditions. Since the tunicamycin-resistant variant (T) derived from the same W(2-23) clone and the hydroxyurea-resistant (Hu(2-6)) variant derived from clone W(2-6) do not show overexpression of these two genes, it is suggested that the distinct overexpression of cTXNPx and mTXNPx genes in arsenite-resistant A and A' variants is linked to arsenite selection process. These two genes in A and A' variants, and cTXNPx(+) and mTXNPx(+) transfectants are similar to the respective genes described for L. infantum and L. chagasi in terms of antioxidant activities against H2O2 and t-butyl hydroperoxide, in which cTXNPx is more resistant to H2O2, and mTXNPx is more resistant to t-butyl hydroperoxide than the wildtype. Both genes, however, are cross-resistant to NO as compared to the control wildtype. In the transfectants carrying cTXNPx and mTXNPx in inverted orientation, these two genes are expressed in a level lower than that in wildtype. The decreased expression was followed by increased sensitivity of these transfectants to the oxidants. This possibly is due to the formation of antisense mRNA in these transfectants that causes a specific downregulation of the respective genes.

PCR-RFLP to identify Leishmania (Viannia) braziliensis and L. (Leishmania) amazonensis causing American cutaneous leishmaniasis

A PCR-RFLP based method was developed to diagnose and identify the Leishmania species causing American cutaneous leishmaniasis (ACL) in a panel of clinical samples obtained from an endemic region of Brazil. The comparison of the results obtained by PCR-RFLP and PCR-hybridization in the identification of Leishmania (Viannia) braziliensis and L. (Leishmania) amazonensis were highly concordant (kappa=91.5%). The PCR-RFLP method was reliable, fast and easy to conduct on biopsies and presents potential value of utmost importance for the diagnosis and identification of Leishmania in clinical specimens, infected reservoirs and vectors.

Natural and induced dyskinetoplastic trypanosomatids: how to live without mitochondrial DNA

Salivarian trypanosomes are the causative agents of several diseases of major social and economic impact. The most infamous parasites of this group are the African subspecies of the Trypanosoma brucei group, which cause sleeping sickness in humans and nagana in cattle. In terms of geographical distribution, however, Trypanosoma equiperdum and Trypanosoma evansi have been far more successful, causing disease in livestock in Africa, Asia, and South America. In these latter forms the mitochondrial DNA network, the kinetoplast, is altered or even completely lost. These natural dyskinetoplastic forms can be mimicked in bloodstream form T. brucei by inducing the loss of kinetoplast DNA (kDNA) with intercalating dyes. Dyskinetoplastic T. brucei are incapable of completing their usual developmental cycle in the insect vector, due to their inability to perform oxidative phosphorylation. Nevertheless, they are usually as virulent for their mammalian hosts as parasites with intact kDNA, thus questioning the therapeutic value of attempts to target mitochondrial gene expression with specific drugs. Recent experiments, however, have challenged this view. This review summarises the data available on dyskinetoplasty in trypanosomes and revisits the roles the mitochondrion and its genome play during the life cycle of T. brucei.

Homologous minicircles in Leishmania donovani

Leishmania minicircular deoxyribonucleic acid (DNA) is arranged into different classes according to sequence. These classes differ substantially in sequence, despite species- and genus-specific regions, and are present in widely different copy numbers within and between Leishmania strains. Homologous minicircles have been identified in different species of Leishmania by comparing sequences of known minicircles. However, it is possible to select for minicircles of the same class by amplifying Leishmania DNA with polymerase chain reaction primers from the conserved and variable regions. This approach was used with 2 different minicircle classes in the L. donovani complex. In all isolates tested it was possible to amplify minicircles of the selected class.

Molecular, cellular and functional characterizations of a novel ICAM-like molecule of the immunoglobulin superfamily from Leishmania mexicana amazonensis

A molecule with two immunoglobulin (Ig) domains cloned from Leishmania mexicana amazonensis was characterized to have a sequence homology to the Ig domains of an ICAM-like molecule telencephalin, cloned from the brain of mammals, as well as to the variable domains of human immunoglobulin lambda light chain. The molecule therefore appears to be an ICAM-like molecule as well as a member of the immunoglobulin superfamily. We thus named it ICAM-L for Leishmania ICAM. The gene was coamplified with the ribonucleotide reductase M(2) subunit gene responsible for hydroxyurea resistance from hydroxyurea (Hu)-resistant Leishmania variants. As expected, an increase of the ICAM-L protein as well as an increase of the specific ICAM-L transcript of 2.1 kb was detected in the Hu-resistant variants with increasing doses of the drug used for resistance selection. Structurally, ICAM-L is more similar to the secretory adhesive molecules, such as 1Bgp and the link protein of the immunoglobulin superfamily, in that it lacks a transmembrane region and a GPI anchor sequence. Although ICAM-L was mainly localized in the nucleus of the parasite by confocal microscopy, however, detailed studies by electron microscopy and FACS analysis indicated that the protein was also localized on the surface of the parasite. The surface localization of the protein was furthered strengthened by the observations that anti-ICAM-L or ICAM-L itself can significantly block the binding of the parasite to macrophages. The blocking of the attachment of parasite to macrophages may indicate that ICAM-L functions as an intercellular adhesive molecule.

Interference of the simian virus 40 origin of replication by the cytomegalovirus immediate early gene enhancer: evidence for competition of active regulatory chromatin conformation in a single domain

Replication origins are often found closely associated with transcription regulatory elements in both prokaryotic and eukaryotic cells. To examine the relationship between these two elements, we studied the effect of a strong promoter-enhancer on simian virus 40 (SV40) DNA replication. The human cytomegalovirus (CMV) immediate early gene enhancer-promoter was found to exert a strong inhibitory effect on SV40 origin-based plasmid replication in Cos-1 cells in a position- and dose-dependent manner. Deletion analysis indicated that the effect was exerted by sequences located in the enhancer portion of the CMV sequence, thus excluding the mechanism of origin occlusion by transcription. Insertion of extra copies of the SV40 origin only partially alleviated the inhibition. Analysis of nuclease-sensitive cleavage sites of chromatin containing the transfected plasmids indicate that the chromatin was cleaved at one of the regulatory sites in the plasmids containing more than one regulatory site, suggesting that only one nuclease-hypersensitive site existed per chromatin. A positive correlation was found between the degree of inhibition of DNA replication and the decrease of P1 cleavage frequency at the SV40 origin. The CMV enhancer was also found to exhibit an inhibitory effect on the CMV enhancer-promoter driving chloramphenicol acetyltransferase expression in a dose-dependent manner. Together these results suggest that inhibition of SV40 origin-based DNA replication by the CMV enhancer is due to intramolecular competition for the formation of active chromatin structure.

Expression and cellular localization of ribonucleotide reductase small subunit M2 protein in hydroxyurea-resistant Leishmania mexicana amazonensis

We raised a specific antiserum against the recombinant M2 subunit protein of ribonucleotide reductase of Leishmania mexicana amazonensis in rabbit. This antiserum was used to study the expression and cellular location of the M2 protein in wildtype as well as hydroxyurea-resistant variants (HuR) of the parasite. The protein increased with increasing dose of the drug used for selection of resistance. The increase in protein level was accompanied by an increase in the copy numbers of mRNA of the M2 gene in the variants. In contrast to mammalian cells, the M2 protein of Leishmania is located in the nucleus rather than in the cytoplasm. The number of cells expressing M2 protein is also different in mammalian cells versus Leishmania. In mammalian cells, expression of M2 protein is a strictly S-phase-correlated event and in exponentially growing cells only approximately 50% of the cells are in S-phase and only these cells synthesize M2 protein. In L. m. amazonensis, however, almost all exponentially growing cells are positive for M2 protein. This makes it unlikely that M2 protein expression in Leishmania is S-phase dependent. In view of these findings, a fresh look in the future into the regulatory mechanisms of synthesis and the site of action of RNR in L. m. amazonensis is warranted.

Modification of kinetoplast DNA minicircle composition in pentamidine-resistant Leishmania

Pentamidine, an antiprotozoal drug, was shown to have various cellular and molecular targets depending on the organism. In Leishmania, ultrastructural modifications of kinetoplast and mitochondria have been observed but no data is available on cellular and molecular events involved in development of pentamidine-resistance. The absence of modification of minicircle DNA in pentamidine treated L. donovani and L. amazonensis promastigotes suggested that topoisomerase II activity is not a target. This result was confirmed by quantitation of the enzyme by immunodetection. Southern blot experiments indicated that the kDNA network was altered in resistant clones. Molecular cloning and sequence analysis of kDNA minicircles showed transkinetoplastidy hitherto reported only for arsenite- and tunicamycin-resistant Leishmania. Comparison of wild-type and resistant sequences showed only 32-51% homology. The AT-rich regions, known as binding sites, of the drug occurred less frequently in the resistant clones and their locations were different. These minicircle sequence modifications leading to decreased binding sites for the drug might contribute to pentamidine-resistance in Leishmania.

Leishmania mexicana amazonensis: differential display analysis and cloning of mRNAs from attenuated and infective forms

The virulence of Leishmania mexicana is determined by the concerted action of several parasite molecules. These cells lose their infectivity to host macrophages after prolonged cultivation in axenic growth media. Both virulent and attenuated variants of the parasite cells were cloned. The differential display reverse transcription-polymerase chain reaction technique was employed to understand whether this natural attenuation of the parasite cells is accompanied by differential expression of selected genes in those cells. Twelve different dinucleotide-anchored oligo(dT) antisense primers were used to make cDNAs from poly(A)+ mRNAs isolated from a clonal population of virulent and avirulent cells following a protocol optimized for Leishmania mRNAs. Those cDNAs were subjected to amplifications using each of the three different arbitrary decanucleotide primers and the corresponding anchored oligo(dT) primer. This procedure revealed four virulent-specific cDNA probes and one avirulent-specific cDNA probe. Differential expressions of these genes were confirmed by northern hybridization using the cloned cDNA probes. These results indicate that differential expression of genes may be the key in determining the molecular basis of leishmanial virulence.

Evolution of RNA editing in kinetoplastid protozoa

The editing of RNA in trypanosomatid mitochondria involves the insertion and occasional deletion of uridine residues within coding regions of maxicircle messenger RNA transcripts. The extent to which the transcripts of homologous genes undergo editing differs in different species. In some, entire genes are edited (pan-editing), whereas in others, editing is limited to the 5' termini of editing domains (5' editing). Here we investigate which type of editing is ancestral and which is derived, by analysing RNA editing in the different lineages, using a kinetoplastid phylogeny reconstructed from nuclear small subunit ribosomal RNA sequences. We conclude that the ancestral cryptogenes were pan-edited, and we hypothesize that the 5'-edited homologues were generated by several independent events from partially edited RNAs, in which case editing may be a more primitive mechanism than previously thought.

Selection for arsenite resistance causes reversible changes in minicircle composition and kinetoplast organization in Leishmania mexicana

Certain minor minicircle sequence classes in the kinetoplast DNA (kDNA) networks of arsenite- or tunicamycin-resistant Leishmania mexicana amazonensis variants whose nuclear DNA is amplified appear to be preferentially selected to replicate (S. T. Lee, C. Tarn, and K. P. Chang, Mol. Biochem. Parasitol. 58:187-204, 1993). These sequences replace the predominant wild-type minicircle sequences to become dominant species in the kDNA network. The switch from wild-type-specific to variant-specific minicircles takes place rapidly within the same network, the period of minicircle dominance changes being defined as the transition period. To investigate the structural organization of the kDNA networks during this transition period, we analyzed kDNA from whole arsenite-resistant Leishmania parasites by dot hybridization with sequence-specific DNA probes and by electron-microscopic examination of isolated kDNA networks in vitro. Both analyses concluded that during the switch of dominance the predominant wild-type minicircle class was rapidly lost and that selective replication of variant-specific minicircles subsequently filled the network step by step. There was a time during the transition when few wild-type- or variant-specific minicircles were present, leaving the network almost empty and exposing a species of thick, long, fibrous DNA which seemed to form a skeleton for the network. Both minicircles and maxicircles were found to attach to these long DNA fibrils. The nature of the long DNA fibrils is not clear, but they may be important in providing a framework for the network structure and a support for the replication of minicircles and maxicircles.

Selection for arsenite resistance causes reversible changes in minicircle composition and kinetoplast organization in Leishmania mexicana

Certain minor minicircle sequence classes in the kinetoplast DNA (kDNA) networks of arsenite- or tunicamycin-resistant Leishmania mexicana amazonensis variants whose nuclear DNA is amplified appear to be preferentially selected to replicate (S. T. Lee, C. Tarn, and K. P. Chang, Mol. Biochem. Parasitol. 58:187-204, 1993). These sequences replace the predominant wild-type minicircle sequences to become dominant species in the kDNA network. The switch from wild-type-specific to variant-specific minicircles takes place rapidly within the same network, the period of minicircle dominance changes being defined as the transition period. To investigate the structural organization of the kDNA networks during this transition period, we analyzed kDNA from whole arsenite-resistant Leishmania parasites by dot hybridization with sequence-specific DNA probes and by electron-microscopic examination of isolated kDNA networks in vitro. Both analyses concluded that during the switch of dominance the predominant wild-type minicircle class was rapidly lost and that selective replication of variant-specific minicircles subsequently filled the network step by step. There was a time during the transition when few wild-type- or variant-specific minicircles were present, leaving the network almost empty and exposing a species of thick, long, fibrous DNA which seemed to form a skeleton for the network. Both minicircles and maxicircles were found to attach to these long DNA fibrils. The nature of the long DNA fibrils is not clear, but they may be important in providing a framework for the network structure and a support for the replication of minicircles and maxicircles.

Intra- and interspecific polymorphisms of Leishmania donovani and L. tropica minicircle DNA

A pair of degenerate polymerase chain reaction (PCR) primers (LEI-1, TCG GAT CC[C,T] [G,C]TG GGT AGG GGC GT; LEI-2, ACG GAT CC[G,C] [G,C][A,C]C TAT [A,T]TT ACA CC) defining a 0.15-kb segment of Leishmania minicircle DNA was constructed. These primers amplified not only inter- but also intraspecifically polymorphic sequences. Individual sequences revealed a higher intraspecific than interspecific divergence. It is concluded that individual sequences are of limited relevance for species determination. In contrast, when a data base of 19 different sequences was analyzed in a dendrographic plot, an accurate species differentiation was feasible.