African trypanosomes are a major plague in sub-Saharan Africa. They cause sleeping sickness in humans and nagana in cattle. These parasites are transmitted between their mammalian hosts by tsetse flies. They are adapting to their different environments through differentiation processes. These processes involve, amongst other things, the expression of different surface coats. These coats are made of procyclin protein at the insect midgut procyclic stage and of variant surface glycoprotein (VSG) at the mammalian bloodstream stage. At a given time, one VSG is expressed from a single VSG gene out of a repertoire of more than 1500 VSG genes present in the trypanosomes genome. The expressed VSG gene is always located at one of fifteen telomeric polycistronic transcription units called expression sites (ES). The VSG coat is changed regularly in a process called antigenic variation allowing trypanosomes to escape the immune response. The exact mechanism controlling the selection of the active ES is not yet known and controversies have been raised concerning the ES transcription control. Although several molecular factors involved in the ES monoallelic-expression have been identified, none of them seems to be a critical regulator.<p><p>Thus during my thesis we decided to explore two aspects of ES expression: (A) deciphering the level at which this expression is controlled and (B) fishing for new protein factors controlling this expression.<p>A) It is not even clear at which level the ES transcription control takes place. In particular, there has been debate on whether it is taking place at the transcription initiation or elongation level. Previous experiments generated contradictory conclusions and gave rise to two different models. The first model suggested that transcription initiation takes place in all ESs simultaneously. The second model suggested that transcription is initiated in only two ESs, one being fully active and a second being pre-active. These two models were equally able to account for the finding of transcripts from different ES within a trypanosome population provided the pre-active ES differs between individual cells. In order to decide if a single or multiple ES promoters can initiate transcription in a given cell, single cell RT-PCR targeting the beginning of the ES was required. Thus single cell RT-PCR was performed and an analysis of the obtained transcripts showed that transcription initiation is taking place on many ES while only one VSG is transcribed. This permitted the unambiguous conclusion that the monoallelic expression of VSG is exerted by controls operating downstream from transcription initiation, suggesting transcription elongation or RNA processing as critical control steps. <p>B) We have characterized a new nuclear protein, Tb alba3, involved in the repression of silent VSGs. Its invalidation lead to chromatin opening in the silent expression sites and to a raise in their expression. As this protein is cytoplasmic and binding procyclin mRNAs at the procyclic stage, it could be a new versatile factor, shuttling between the cytoplasm and the nucleus and involved both in the inverse regulation of major surface antigens at different differentiation stages and the control of antigenic variation.<p><p>These results enhance our understanding of ES transcription control and of ES monoallelic expression. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished