186 Update TRENDS in Parasitology Vol.23 No.5

22 Ralph, S.A. et al. (2005) Antigenic variation in Plasmodium falciparumis associated with movement of var loci between subnuclear locations.Proc. Natl. Acad. Sci. U. S. A. 102, 5414–5419

23 Marty, A.J. et al. (2006) Evidence that Plasmodium falciparumchromosome end clusters are cross-linked by protein and are thesites of both virulence gene silencing and activation. Mol. Microbiol.62, 72–83

24 Freitas-Junior, L.H. et al. (2005) Telomeric heterochromatinpropagation and histone acetylation control mutually exclusive expre-ssion of antigenic variation genes in malaria parasites. Cell 121, 25–36

25 Gasser, S.M. and Cockell, M.M. (2001) Themolecular biology of the SIRproteins. Gene 279, 1–16

Corresponding author: Sutherland, C. (colin.sutherland@lshtm.ac.uk).Available online 7 March 2007.

www.sciencedirect.com

26 Wyrick, J.J. et al. (1999) Chromosomal landscape of nucleosome-dependent gene expression and silencing in yeast. Nature 402, 418–421

27 Duffy, M.F. et al. (2002) Transcription of multiple var genes byindividual, trophozoite-stage Plasmodium falciparum cells expressinga chondroitin sulphate A binding phenotype. Mol. Microbiol. 43, 1285–1293

28 Chen, Q. et al. (1998) Developmental selection of var gene expression inPlasmodium falciparum. Nature 394, 392–395

1471-4922/$ – see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.pt.2007.02.010

Research Focus Response

Response to Duffy and Tham: Transcription andcoregulation of multigene families in Plasmodiumfalciparum

Sarah Sharp1, Louisa McRobert1, Colin Sutherland1 and Thomas Lavstsen2

1 London School of Hygiene & Tropical Medicine, Keppel Street, London WC1E 7HT, UK2 Centre for Medical Parasitology, Institute for Medical Microbiology and Immunology, University of Copenhagen, DK-1017

Copenhagen, Denmark

In this issue, Duffy and Tham [1] present a welcomecommentary on our recent article [2] that describes theabundance of transcripts that encode Plasmodium falci-parum erythrocyte membrane protein (PfEMP)1 andSTEVOR (subtelomeric variable open reading frame)variants in P. falciparum asexual parasites and gameto-cytes. Understanding the dynamic expression patterns ofthese two antigen families is hampered by the lack ofvariant-specific reagents at the protein level, althoughgroups at the University of Copenhagen (http://www.cmp.dk), working on PfEMP1, and Nanyang Tech-nology University, Singapore (http://www.ntu.edu.sg),working on STEVOR [3], are currently addressing this.We have been able to develop variant-specific reagents atthe mRNA level and used these to identify a gametocyte-specific transcriptional programme of the var gene family(which encodes PfEMP1) that is independent of var tran-scription phenotype among asexual progenitors. We didnot find evidence of gametocyte-specific transcription ofstevor loci.

Another key finding is that stevor transcript profilesin adhesion-selected asexual cultures that expresspredominantly type A var transcripts were essentiallyindistinguishable from those of unselected P. falciparum3D7 strain progenitor cultures. We concluded thatvar and stevor phenotypes are not coregulated. Theseresults do not preclude the possibility that subsets ofvar loci other than type A might be coregulated withparticular stevor profiles. We do have unpublished datathat indicate that stevor transcript profiles in parasite

populations that express the type E var2csa phenotype,which is strongly implicated in placental sequestrationof P. falciparum, do not differ from the profiles seen inunselected 3D7 cultures. Further studies of stevor tran-scription in other adhesion-selected parasite lines, andin lines of non-3D7 lineages, are still required to addressthis issue.

Ascribing functional importance to patterns of trans-cript abundance needs to be done with caution. We agreewith Duffy and Tham that var transcripts that aredetected in gametocytes might encode PfEMP1 adhesionmolecules that mediate gametocyte sequestration duringtheir long developmental phase. However, there are noprotein-level data to support the hypothesis that PfEMP1is associated with the gametocyte-infected erythrocytemembrane after stage IIA. Although in the case of STE-VOR there is such evidence, it is not certain whetherthese STEVOR molecules are surface exposed and able tocontribute directly to gametocyte adhesion [4]. Further-more, we found that stevor transcription ceased severaldays before STEVOR proteins were trafficked from thegametocyte to the host-erythrocyte membrane. Thus, thetiming of transcription during gametocyte developmentcannot be used to infer any function of the encodedvariant proteins. We, therefore, consider it plausible thatsome or all PfEMP1 molecules that are produced inthe developing intra-erythrocytic gametocyte could bedestined for processes that occur after ingestion by anAnopheles mosquito. It is not known whether PfEMP1production continues in the mosquito stages of theP. falciparum life cycle. These questions also apply toSTEVOR because we and others have demonstratedthe presence of STEVOR proteins in sporozoites [4,5].

Update TRENDS in Parasitology Vol.23 No.5 187

Therefore, the limited repertoire of stevor transcriptsthat is observed in the asexual and sexual blood stages,which is cited by Duffy and Tham as evidence thatexclusive transcription of a subset of stevor might occur,could also reflect some specialization of STEVOR var-iants for particular life-cycle compartments.

References1 Duffy, M. and Tham, W-H. (2007) Transcription and coregulation of

multigene families in Plasmodium falciparum. Trends Parasitol. 23,183–186

Corresponding author: Bellofatto, V. (bellofat@umdnj.edu).Available online 19 March 2007.

www.sciencedirect.com

2 Sharp, S. et al. (2006) Programmed transcription of the var gene family,but not of stevor, inPlasmodium falciparum gametocytes.Eukaryot. Cell5, 1206–1214

3 Kaviratne, M. et al. (2002) Small variant STEVOR antigen is uniquelylocated within Maurer’s clefts in Plasmodium falciparum-infected redblood cells. Eukaryot. Cell 1, 926–935

4 McRobert, L. et al. (2004) Distinct trafficking and localization ofSTEVOR proteins in three stages of the Plasmodium falciparum lifecycle. Infect. Immun. 72, 6597–6602

5 Florens, L. et al. (2002) A proteomic view of the Plasmodium falciparumlife cycle. Nature 419, 520–526

1471-4922/$ – see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.pt.2007.02.008

Research Focus

Pyrimidine transport activities in trypanosomes

Vivian Bellofatto

Department of Microbiology and Molecular Genetics, University of Medicine and Dentistry of New Jersey, Newark, NJ 07101, USA

Parasites of the Trypanosomatidae family are unable tosynthesize purines. Instead, they rely on their hosts tosupply these necessary compounds. The article by Gudinet al. identifies three transport mechanisms of the equi-librative nucleoside transporter family by which nucleo-sides and nucleobases are transported in this medicallyimportant family of organisms. The work by Gudin et al.characterizes the dynamics of these transporters andpoints to further areas for future genetic and therapeuticexperiments.

Purine and pyrimidine biosynthesisParasites derive the nutrients they need from their hosts[1]. The Trypanosomatidae family of protozoan parasitesdepends upon purine acquisition because they cannot syn-thesize this class of heterocyclic nitrogenous compounds[2]. Purine synthesis requires more energy and is metabo-lically more complex than pyrimidine synthesis. Trypano-somes can synthesize pyrimidines de novo but scavenge arange of purine molecules.

Many trypanosomatids, including Trypanosoma andLeishmania, transition from mammalian host to arthropodvector during their complex life cycle [3]. Inhumans,Africantrypanosomes spend weeks or months in the bloodstream,depending on the specific subspecies ofTrypanosome brucei,and eventually lodge in the brain [4]. In either locale,trypanosomes avail themselves of various purines.Although it is extremely difficult to measure serum purineand pyrimidine levels in the bloodstreamaccurately, hypox-anthine and xanthine [both nucleobases (Table 1)] are pre-sent in serum and help to satisfy the purine needs of thetrypanosomes. During parasite invasion, immunostimula-

tion causes an increase in blood levels of extracellularadenosine [5]. In the final stages of human African trypa-nosomiasis (HAT), parasites migrate and proliferate in thehost’s central nervous system (CNS) and eventually cause acoma [6]. The CNS is replete with extracellular purinenucleosides, specifically adenosine, that function as neuro-modulators [7]. Arthropod vectors produce saliva that is richin nucleotide-metabolizing enzymes that function as ananticoagulatory and anti-inflammatory defensemechanism[8]. The resultant purine nucleosides and bases satisfy thenutritional needs of themetacylic parasites that are primedto enter the mammalian host from the tsetse vector.

Membrane transportersMembrane transporters that salvage nucleosides andnucleobases have been intensely studied [9,10]. The classof transporters that is designated as equilibrativenucleoside transporters (ENTs) is common in eukarya.ENTs have 11 transmembrane helices and are situatedin the cellular membrane [11]. The ENT family includesproteins that bind to and deliver both purine and pyrimi-dine nucleosides from the extracellular milieu across thecell membrane (Table 1). ENT transporters are equilibra-tive or facilitative permeases in metazoa but several havebeen shown to be proton-dependent concentrative trans-porters in protozoa [12].

Fungal systems lack ENT-type permeases. Yeast andpathogenic fungi salvage nucleobases and nucleosidesthrough a set of transmembrane proteins in the structu-rally distinct family of Fur permeases, which includes theproton symporters Fui1p and Fur4p [13]. Fur orthologshave not been found in trypanosome database searches.

In trypanosomatids, all purine and pyrimidine trans-porters that have been studied to date are members of theENT family. Although these ENT-type transporters takeup a range of nucleosides and nucleobases, some are purine