MANIPULATION OF THE VSG CO-TRANSPOSED REGION INCREASES EXPRESSION-SITE SWITCHING IN TRYPANOSOMA-BRUCEI

Disruption of a region of DNA in Trypanosoma brucei immediately upstre am of the expressed telomere-proximal variant surface glycoprotein gen e (vsg), known as the co-transposed region (CTR), can cause a dramatic increase in the rate at which the active expression site (ES) is swit ched off and a new ES is switched on. Deletion of most of the CTR in t wo ESs caused a greater than 100-fold increase in the rate of ES switc hing, to about 1.3 x 10(-4) per generation. A more dramatic effect was observed when the entire CTR and the 5' coding region of the expresse d vsg221 were deleted. In this case a new ES was activated within a fe w cell divisions. This switch also occurred in cell lines where a seco nd vsg had been inserted into the ES, prior to CTR deletion. These cel l lines, which stably co-expressed the inserted and endogenous Vsgs, i n equal amounts, did not differ from the wild type in growth rate or s witching frequency, suggesting that simultaneous expression of two Vsg s has no intrinsic effect. CTR deletion did not disturb the inserted v sg117. We tentatively conclude that it was not the disruption of the v sg221 in itself that destabilized the ES. All of the observed switches occurred without additional detectable DNA rearrangements in the swit ched ES. Deletion of the 70-bp repeats and/or a vsg pseudogene upstrea m of the CTR did not affect ES stability. Several speculative interpre tations of these observation are offered, the most intriguing of which is that the CTR plays some role in modulating chromatin conformation at an ES. (C) 1997 Elsevier Science B.V.