Structure and arrangement of the beta-tubulin genes of Leishmania tropica

A negative regulatory element controls mRNA abundance of the Leishmania mexicana Paraflagellar rod gene PFR2

The Leishmania mexicana PFR2 locus encodes a component of the paraflagellar rod (PFR), a flagellar structure found only in the insect stage of the life cycle. PFR2 mRNA levels are 10-fold lower in the mammalian stage than in the insect stage. Nuclear run-on experiments indicate that the change in PFR2 mRNA abundance is achieved posttranscriptionally. Deletion and block substitution analysis of the entire 1,400-nucleotide 3' untranslated region (UTR) of PFR2C led to the identification of a regulatory element contained within 10 nucleotides of the 3' UTR, termed the PFR regulatory element (PRE), that is necessary for the 10-fold regulation of PFR2 mRNA levels. Comparison of the half-lives of PFR2 transcripts, identical except for the presence or absence of the PRE, revealed that the PRE acts by destabilizing the PFR2 mRNA in amastigotes. The PRE was inserted into a construct which directs the constitutive expression of a chimeric PFR2 transcript. Insertion of the PRE resulted in regulated expression of this transcript, demonstrating that the regulatory element is sufficient for promastigote-specific expression. Since the PRE is present in the 3' UTR of all L. mexicana PFR genes examined so far, we propose that it serves a means of coordinating expression of PFR genes.

The Leishmania genome comprises 36 chromosomes conserved across widely divergent human pathogenic species

All the physical linkage groups constituting the genome of Leishmania infantum have been identified for the first time by hybridization of specific DNA probes to pulsed field gradient-separated chromosomes. The numerous co-migrating chromosomes were individualised using the distinctive size polymorphisms which occur among strains of the L. infantum/L. donovani complex as a tool. A total of 244 probes, consisting of 41 known genes, 66 expressed sequence tags (ESTs) and 137 anonymous DNA sequences, were assigned to a specific linkage group. We show that this genome comprises 36 chromosomes ranging in size from 0.35 to -3 Mb. This information enabled us to compare the genome structure of L. infantum with those of the three other main Leishmania species that infect man in the Old World, L. major, L. tropica and L. aethiopica. The linkage groups were consistently conserved in all species examined. This result is in striking contrast to the large genetic distances that separate these species and suggests that conservation of the chromosome structure may be critical for this human pathogen. Finally, the high density of markers obtained during the present study (with a mean of 1 marker/130 kb) will speed up the construction of a detailed physical map that would facilitate the genetic analysis of this parasite, for which no classical genetics is available.

Leishmaniases of the New World: current concepts and implications for future research

Recent epidemiologic studies indicate that leishmaniasis in the Americas is far more abundant and of greater public health importance than was previously recognized. The disease in the New World is caused by a number of different parasite species that are capable of producing a wide variety of clinical manifestations. The outcome of leishmanial infection in humans is largely dependent on the immune responsiveness of the host and the virulence of the infecting parasite strain. This article reviews current concepts of the clinical forms, immunology, pathology, laboratory diagnosis, and treatment of the disease as well as aspects of its epidemiology and control. Recommendations for future research on the disease and its control are made.

Development of a stable Leishmania expression vector and application to the study of parasite surface antigen genes

Trypanosomatid protozoan parasites cause several important tropical diseases and have been a fertile ground for the discovery of molecular paradigms such as trans-splicing and RNA editing. Transfection-based methods for the study of these organisms have recently been developed, and we have now designed an expression vector, pX, which contains only 2.3 kilobases of Leishmania DNA and can be stably transfected with high efficiency. Genes encoding Escherichia coli beta-galactosidase or a Leishmania amazonensis protective membrane glycoprotein (GP46A/M-2) were inserted into the pX expression site and transfected into Leishmania major, where they directed the synthesis of high levels of mRNAs formed by 5' and 3' processing events occurring predominantly at the sites used by the normal transcripts. Colony assays and immunoblot analysis showed that both proteins were produced; enzymatically active beta-galactosidase comprised approximately 1% of total protein. Sizes of the GP46A protein synthesized in transfected L. major or L. amazonensis were similar and differed from the predominant L. amazonensis GP46, suggesting that the GP46A gene may encode a variant GP46 family member. Because these vectors function efficiently in pathogenic species of Leishmania, pX will facilitate the genetic analyses of parasite proteins crucial for infectivity as well as the identification of cis-acting elements mediating transcription and replication.

Evolution of the genus Leishmania as revealed by comparisons of nuclear DNA restriction fragment patterns

Restriction endonuclease DNA fragment patterns have been used to examine the relationships among 28 isolates of Leishmania as well as Crithidia, Endotrypanum, and Trypanosoma cruzi. Fragments of nuclear DNA were generated with six restriction enzymes, and blots were hybridized with probes from three loci. Among the major lineages the fragment patterns are essentially completely different, while within the major lineages various degrees of divergence are found. Molecular evolutionary trees were constructed using the method of Nei and Li to estimate the percent nucleotide sequence divergence among strains from the fraction of fragments shared. Defined groups, such as species or subspecies within the major lineages, are also grouped by nuclear DNA comparisons. Within the donovani complex, we find Leishmania donovani chagasi and Leishmania donovani infantum to be as similar as strains within Leishmania donovani donovani, consistent with the proposal by other workers that New World visceral leishmaniasis originated quite recently.

Apparent generation of a segmented mRNA from two separate tandem gene families in Trypanosoma cruzi

Using a cDNA for an abundant Trypanosoma cruzi mRNA as probe, we have cloned and sequenced a gene which is organized in at least 20 nearly perfect tandem repeats of 940 base pairs. The 5' end of the mRNA has been sequenced by primer extension and found to contain a 35 nucleotide mini-exon (or spliced-leader) sequence that is ubiquitous in trypanosome mRNAs. This sequence, however, is not present in the tandem genomic repeats which encode the exon containing the major portion of the mRNA. Previous studies have shown that the 35-nucleotide sequence is encoded by a separate tandem gene family. One model to explain the formation of a segmented mRNA invokes priming of transcription by a small RNA which contains the leader sequence at its 5' end. However, northern blot analysis of total trypanosome RNA reveals a ladder of molecules larger than the mature mRNA, which appear to be faithful multimeric copies of the tandem gene. The discrete sizes of these RNAs correspond to those expected for partially processed precursors. These observations lend credence to the possibility of an alternative model where segmented mRNAs are generated by inter-molecular splicing.

The molecular karyotype of Leishmania major and mapping of alpha and beta tubulin gene families to multiple unlinked chromosomal loci

The arrangement of tubulin genes in the genome of the protozoan parasite Leishmania major was studied by genomic Southern blot analysis and mapping of genes to chromosomes fractionated by pulsed field gradient gel (PFG) electrophoresis. alpha-tubulin genes exist as a tandem array of 2.4 kb PstI fragments. beta-tubulin genes are found as a tandem array of 3.9 kb AvaI or PvuI fragments, but additional genes are also found on other genomic DNA fragments. Chromosome-sized DNA molecules released from promastigotes of L. major were fractionated into at least 17 chromosome bands of approximate size 400-4000 kb by PFG gel electrophoresis. Some bands may be present in non-equimolar amounts suggesting that there may be more than 17 chromosomes. All alpha-tubulin genes were localized to a single band (chromosome 7). beta-tubulin genes were localized to four bands (chromosomes 6, 10, 16 and 17). This shows that the alpha- and beta- tubulin gene families are unlinked in L. major. There is a single chromosomal locus for the alpha-tubulin tandem array whereas beta-tubulin genes exist both as a tandem array and as dispersed genes at four chromosomal loci.