Discussion

Trypanosoma brucei bloodstream form cells express a single variant surface glycoprotein (VSG) gene per cell at a given time. VSG Exclusion-1 (VEX1) is a large putative SWIM-type zinc-finger protein that has been recently shown to control this process. VEX1 associates with the active VSG and co-ordinates both positive and negative controls, with both over-expression and knockdown leading to allelic exclusion defects. To dissect VEX1 function, we have expressed VEX1-fragments and analysed their sub-cellular localisation and impact on allelic exclusion. Preliminary results suggest that a region involved in association with the active VSG resides towards the N-terminus, while a nuclear localisation signal appears to reside towards the C-terminus. VEX1 has also been shown to drive its own expression when under the control of a pol-I promoter so we are setting up strains that allow for VEX1 expression from a T7-promoter to determine whether this positive-feedback control is promoter-specific.