Complementation of a glucose transporter mutant of Schizosaccharomyces pombe by a novel Trypanosoma brucei gene

Trypanosoma brucei: a putative RNA polymerase II promoter

RNA polymerase II (pol II) promoters are rare in the African trypanosome Trypanosoma brucei because gene regulation in the parasite is complex and polycistronic. Here, we describe a putative pol II promoter and its structure-function relationship. The promoter has features of an archetypal eukaryotic pol II promoter including putative canonical CCAAT and TATA boxes, and an initiator element. However, the spatial arrangement of these elements is only similar to yeast pol II promoters. Deletion mapping and transcription assays enabled delineation of a minimal promoter that could drive orientation-independent reporter gene expression suggesting that it may be a bidirectional promoter. In vitro transcription in a heterologous nuclear extract revealed that the promoter can be recognized by the basal eukaryotic transcription complex. This suggests that the transcription machinery in the parasite may be very similar to those of other eukaryotes.

Targeting of toxic compounds to the trypanosome's interior

Drugs can be targeted into African trypanosomes by exploiting carrier proteins at the surface of these parasites. This has been clearly demonstrated in the case of the melamine-based arsenical and the diamidine classes of drug that are already in use in the treatment of human African trypanosomiasis. These drugs can enter via an aminopurine transporter, termed P2, encoded by the TbAT1 gene. Other toxic compounds have also been designed to enter via this transporter. Some of these compounds enter almost exclusively through the P2 transporter, and hence loss of the P2 transporter leads to significant resistance to these particular compounds. It now appears, however, that some diamidines and melaminophenylarsenicals may also be taken up by other routes (of yet unknown function). These too may be exploited to target new drugs into trypanosomes. Additional purine nucleoside and nucleobase transporters have also been subverted to deliver toxic agents to trypanosomes. Glucose and amino acid transporters too have been investigated with a view to manipulating them to carry toxins into Trypanosoma brucei, and recent work has demonstrated that aquaglyceroporins may also have considerable potential for drug-targeting. Transporters, including those that carry lipids and vitamins such as folate and other pterins also deserve more attention in this regard. Some drugs, for example suramin, appear to enter via routes other than plasma-membrane-mediated transport. Receptor-mediated endocytosis has been proposed as a possible way in for suramin. Endocytosis also appears to be crucial in targeting natural trypanocides, such as trypanosome lytic factor (TLF) (apolipoprotein L1), into trypanosomes and this offers an alternative means of selectively targeting toxins to the trypanosome's interior. Other compounds may be induced to enter by increasing their capacity to diffuse over cell membranes; in this case depending exclusively on selective activity within the cell rather than selective uptake to impart selective toxicity. This review outlines studies that have aimed to exploit trypanosome nutrient uptake routes to selectively carry toxins into these parasites.

Gene synteny and evolution of genome architecture in trypanosomatids

The trypanosomatid protozoa Trypanosoma brucei, Trypanosoma cruzi and Leishmania major are related human pathogens that cause markedly distinct diseases. Using information from genome sequencing projects currently underway, we have compared the sequences of large chromosomal fragments from each species. Despite high levels of divergence at the sequence level, these three species exhibit a striking conservation of gene order, suggesting that selection has maintained gene order among the trypanosomatids over hundreds of millions of years of evolution. The few sites of genome rearrangement between these species are marked by the presence of retrotransposon-like elements, suggesting that retrotransposons may have played an important role in shaping trypanosomatid genome organization. A degenerate retroelement was identified in L. major by examining the regions near breakage points of the synteny. This is the first such element found in L. major suggesting that retroelements were found in the common ancestor of all three species.

The Effects of Glucose Concentration on the Reciprocal Regulation of rRNA Promoters in Plasmodium falciparum

The developmental progression of Plasmodium falciparum is remarkably sensitive to glucose concentration. We have investigated the effects of glucose concentration on the parasite development cycle as reflected by changes of ribosomal RNA (rRNA) transcription. We showed that glucose starvation differentially affects transcriptional control of the rRNA genes by sharply repressing transcription from those loci involved with asexual development of the parasite while up-regulating transcription at those loci involved with sexual development of the parasite. Temperature change also effects regulation of transcription. We found that the effects of temperature and glucose were synergistic. We identified and compared the upstream region of the transcription start sites of each gene. These putative promoter structures are considerably different from one another and contain structures remarkably similar to rRNA control elements in other organisms.