VSG switching in Trypanosoma brucei: antigenic variation analysed using RNAi in the absence of immune selection.

Trypanosoma brucei relies on antigenic variation of its variant surface glycoprotein (VSG) coat for survival. We show that VSG switching can be efficiently studied in vitro using VSG RNAi in place of an immune system to select for switch variants. Contrary to models predicting an instant switch after inhibition of VSG synthesis, switching was not induced by VSG RNAi and occurred at a rate of 10(-4) per division. We find a highly reproducible hierarchy of VSG activation, which appears to be capable of resetting, whereby more than half of the switch events over 12 experiments were to one of two VSGs. We characterized switched clones according to switch mechanism using marker genes in the active VSG expression site (ES). Transcriptional switches between ESs were the preferred switching mechanism, whereby at least 10 of the 17 ESs identified in T. brucei 427 can be functionally active in vitro. We could specifically select for switches mediated by DNA rearrangements by inducing VSG RNAi in the presence of drug selection for the active ES. Most of the preferentially activated VSGs could be activated by multiple mechanisms. This VSG RNAi-based procedure provides a rapid and powerful means for analysing VSG switching in African trypanosomes entirely in vitro.

Isolation of the repertoire of VSG expression site containing telomeres of Trypanosoma brucei 427 using transformation-associated recombination in yeast.

Trypanosoma brucei switches between variant surface glycoproteins (VSGs) allowing immune escape. The active VSG is in one of many telomeric bloodstream form VSG expression sites (BESs), also containing expression site-associated genes (ESAGs) involved in host adaptation. The role of BES sequence diversity in parasite virulence can best be understood through analysis of the full repertoire of BESs from a given T. brucei strain. However, few BESs have been cloned, as telomeres are highly underrepresented in standard libraries. We devised a strategy for isolating the repertoire of T. brucei 427 BES-containing telomeres in Saccaromyces cerevisiae by using transformation-associated recombination (TAR). We isolated 182 T. brucei 427 BES TAR clones, 167 of which could be subdivided into minimally 17 BES groups. This set gives us the first view of the breadth and diversity of BESs from one T. brucei strain. Most BESs ranged between 40 and 70 kb (average, 57 +/- 17 kb) and contained most identified ESAGs. Phylogenetic comparison of the cohort of BES promoter and ESAG6 sequences did not show similar trees, indicating rapid evolution most likely mediated by sequence exchange between BESs. This cloning strategy could be used for any T. brucei strain, facilitating research on the biodiversity of telomeric gene families and host-pathogen interactions.

Properties of trypanothione synthetase from Trypanosoma brucei.

Trypanothione [N(1),N(8)-bis(glutathionyl)spermidine] plays a central role in defence against oxidant damage, ribonucleotide metabolism and in resistance to certain drugs in trypanosomatids. In Crithidia fasciculata, synthesis of trypanothione involves sequential conjugation of two molecules of glutathione (GSH) to spermidine by two enzymes: glutathionylspermidine synthetase (GspS; EC 6.3.1.8) and trypanothione synthetase (TryS; EC 6.3.1.9), whereas in Trypanosoma cruzi both steps are catalysed by an unusual TryS with broad substrate specificity. To determine which route operates in T. brucei, we have cloned and expressed a single copy gene with similarity to C. fasciculata and T. cruzi TRYS. The purified recombinant protein catalyses formation of trypanothione from either spermidine and GSH, or glutathionylspermidine and GSH. The enzyme displays high substrate inhibition with GSH as variable substrate (apparent K(m)=56 microM, K(i)(s)=37 microM, k(cat)=2.9s(-1)). At a fixed subsaturating GSH concentration (100 microM), the enzyme obeys simple hyperbolic kinetics yielding apparent K(m) values for spermidine, glutathionylspermidine and MgATP of 38, 2.4, and 7.1 microM, respectively. Recombinant TryS can also catalyse conversion of spermine to glutathionylspermine and bis(glutathionyl)spermine, as recently reported for T. cruzi. The enzyme has amidase activity that can be inhibited by iodoacetamide. Studies using GSH and polyamine analogues identified GSH as the critical determinant for recognition by the amidase domain. Thus, the biosynthesis and degradation of trypanothione are similar in African and American trypanosomes, and different from the insect trypanosomatid, C. fasciculata.

Downregulation of Trypanosoma brucei VSG expression site promoters on circular bacterial artificial chromosomes.

Trypanosoma brucei has about 20 telomeric variant surface glycoprotein (VSG) gene expression sites (ESs), which are downregulated in the insect form. We investigated the transcriptional behaviour of ES promoters on bacterial artificial chromosomes (BACs) containing two different ESs and their flanking regions on fragments of about 140kb. Four different BACs containing either the 221 or the VO2 ES were introduced into insect form T. brucei. The BACs replicated as circular episomes as shown using pulsed field gel (PFG) analysis of DNA exposed to increasing doses of gamma radiation, and digestion with Dam methylation-sensitive restriction enzymes. BAC copy number per cell varied from about 3 for the 221 ES BACs to about 15 for the VO2 ES BACs. Increasing drug selection pressure on the VO2 BAC T. brucei transformants resulted in amplification to about 80 BACs per cell. Although BACs were maintained in the absence of drug selection for at least 56 days, copy number fell and there was no evidence for centromere activity. ES promoters on small plasmid episomes introduced into insect form T. brucei in transient transfections are derepressed. In contrast, ES promoters on large BAC episomes are downregulated both on the original ES BACs, and on ES BACs selected for a drug marker driven by a rDNA promoter fused to the BAC vector. This indicates that downregulation of ES promoters in insect form T. brucei is influenced by genomic context, but does not necessitate proximity to a chromosome end.