Trypanosomatid shuttle vectors: new tools for the functional dissection of parasite genomes

The Trypanosoma cruzi Diamine Transporter Is Essential for Robust Infection of Mammalian Cells

Trypanosoma cruzi is incapable of synthesizing putrescine or cadaverine de novo, and, therefore, salvage of polyamines from the host milieu is an obligatory nutritional function for the parasite. A high-affinity diamine transporter (TcPOT1) from T. cruzi has been identified previously that recognizes both putrescine and cadaverine as ligands. In order to assess the functional role of TcPOT1 in intact parasites, a Δtcpot1 null mutant was constructed by targeted gene replacement and characterized. The Δtcpot1 mutant lacked high-affinity putrescine-cadaverine transport capability but retained the capacity to transport diamines via a non-saturable, low-affinity mechanism. Transport of spermidine and arginine was not impacted by the Δtcpot1 lesion. The Δtcpot1 cell line exhibited a significant but not total defect in its ability to subsist in Vero cells, although initial infection rates were not affected by the lesion. These findings reveal that TcPOT1 is the sole high-affinity diamine permease in T. cruzi, that genetic obliteration of TcPOT1 impairs the ability of the parasite to maintain a robust infection in mammalian cells, and that a secondary low-affinity uptake mechanism for this key parasite nutrient is operative but insufficient for optimal infection.

RNA interference in Trypanosoma brucei: a high-throughput engine for functional genomics in trypanosomatids?

RNA interference (RNAi) is the technique of choice for down-regulating the gene function of suitable genes in African trypanosomes. A recent report by Subramanian and co-workers describes a high-throughput method for gene function discovery using RNAi in Trypanosoma brucei. The phenotype of most of the Open Reading Frames from chromosome 1 of T. brucei was analysed using a battery test of standard protocols. The authors propose that this technique could be used to mine the full genome of T. brucei and to reveal the core proteomic map of the other two major trypanosomatids, Trypanosoma cruzi and Leishmania major, despite the lack of a homologous mechanism of genetic silencing.

A new expression vector for Crithidia fasciculata and Leishmania

Crithidia fasciculata is a monogenetic parasite of insects. It grows in fully defined media without requiring serum, which facilitates biochemical analysis. We have constructed a series of expression systems that allows expression of transfected genes in the kinetoplastid protozoa Crithidia and Leishmania. These cells can be readily transfected with plasmid DNA by electroporation and transformants selected with various antibiotic resistance markers. 5'-Trans-splicing signals and poorly defined regions within the 3'-untranslated regions of genes are required for optimal expression of genes in trypanosomatids. We, therefore, inserted the intergenic region of the C. fasciculata phosphoglycerate kinase (PGK) genes A and B, which allows polyadenylation of the target gene and spliced leader addition to the selectable marker gene. Part of the intergenic region of the PGK locus was added upstream of the target gene to permit its trans-splicing. A 3'-untranslated sequence from the Crithidia glutathionylspermidine synthetase (GSPS) was also added to allow the polyadenylation of the selectable marker gene. Genes can be readily inserted using a multiple cloning site and can be expressed as a fusion protein with a poly-histidine sequence at either the N or C-terminus or fused with green fluorescent protein. Biologically active proteins can be expressed in C. fasciculata or L. amazonensis promastigotes and purified by affinity chromatography using a metal chelating column.

Genetic manipulation of kinetoplastida

During the 1980s, many kinetoplastid genes were cloned and their function inferred from homology with genes from other organisms, location of the corresponding proteins or expression in heterologous systems. Up until 1990, before the availability of DNA transfection methodology, we could not analyze the function of kinetoplastid genes within the organisms themselves. Since then, it has become possible to create and complement mutants, to overexpress foreign proteins in the parasites, to knock out genes and even to switch off essential functions. However, these methods are not equally applicable in all parasites. Here, Christine Clayton highlights the differences and similarities between the most commonly used model organisms, and assesses the relative advantages of different approaches and parasites for different types of investigation.

Studies with artificial extrachromosomal elements in trypanosomatids: Could specificity in the initiation of DNA replication be linked to that in transcription?

Historically, artificial replicons have served as useful models for the definition of regulatory elements involved in chromosomal replication and transmission in yeast and DNA replication in bacteria. Here, Pradeep Patnaik examines what we have learnt so far from the replicative behaviour of various artificial extrachromosomal elements available for trypanosomatids. He highlights the involvement of transcription regulatory elements in virtually every eukaryotic origin of replication analysed in detail and, by drawing upon the extensive literature supporting a close association between DNA replication and transcription, he speculates that the nature and organization of origins of replication on a chromosome also may hold clues to the manner by which an organism regulates gene expression.