Regulation of vsg expression site transcription and switching in Trypanosoma brucei.

Current understanding of expression-site transcription in Trypanosoma brucei, has been refined by recent results of promoter manipulations at vsg expression sites (ES) and examination of the behavior of ES promoters in ectopic locations both within the ES and at other loci. In summary, ES promoter sequences inserted into non-transcribed rRNA spacers are generally inactive, or have low activity, in bloodstream and procyclic forms. Some mechanism apparently operates to ensure full activation of a single ES in bloodstream-form trypanosomes and the inactivity of all ES promoters in procyclic forms. As previously shown, a rRNA promoter can replace an ES promoter. In bloodstream forms, the replacement rRNA promoter was down-regulated in a 'silent' ES but it was active in procyclic forms. In addition to manipulations of endogenous promoters, we have recently shown that, when an ES promoter is replaced by a T7 promoter, the T7 promoter is unregulated but transcription is attenuated before the vsg, and another ES switches on to maintain cell viability. However, T7 transcription is repressed in the context of core ES-promoter sequences in both stages, particularly in procyclic forms. These observations strongly argue that sequences in the vicinity of the ES core promoter play a role in ES control by nucleating critical events in silencing as well as in activation. Deletions of sequences surrounding the ES core promoter, in situ, did not affect its activity or regulation. In bloodstream forms, rRNA or ES promoters inserted adjacent to silent telomeres or to a non-telomeric 'basic-copy' vsg were > 98% repressed. After transformation to procyclic forms, the sub-telomeric rRNA promoter regained about 10% of its maximal activity but the 'basic-copy' rRNA promoter was fully active. Similarly-positioned ES promoters remained silent in procyclic forms. These results suggest that telomere-proximal or vsg-proximal sequences might mediate suppression of transcription via position-effects that could be sufficient to suppress the expression of chromosome-internal vsgs or telomeric metacyclic vsgs, in bloodstream-form trypanosomes. Recent experiments with T7 promoters indicate that sequences within the ES core promoter might be responsible for silencing ES promoters in procyclic forms. Precedents for regulatory mechanisms that modulate transcription over large chromatin domains are reviewed and possible models for ES regulation are presented.