GENETIC-CONTROLS FOR THE EXPRESSION OF SURFACE-ANTIGENS IN AFRICAN TRYPANOSOMES

The major surface antigens of African trypanosomes, variant surface gl ycoprotein (VSG) and procyclin, are typical markers of their respectiv e developmental stages, the bloodstream form and the insect-specific p rocyclic form. Although the role of procyclin is still unclear, variat ion of the VSG in the blood allows the parasite to escape the immune r esponse of the host and develop a chronic infection. In this review, w e discuss the available information concerning the genetic mechanisms that control the expression of VSG and procyclin during the life-cycle of the trypanosome. Unlike other eukaryotes, trypanosomes do not appe ar to primarily control the expression of their genes through a specif ic modulation of promoter activity. Antigenic variation in the bloodst ream results either from DNA rearrangements or from a change in telome ric chromatin structure, and stage-specific regulation of antigen synt hesis is linked to differential control of RNA elongation, processing, stability, and/or translation. Trypanosomes' apparent lack of transcr iption-initiation control probably relates to the general organization of genes in long polycistronic transcription units. Only two promoter s for protein-encoding genes, those of VSG and procyclin, are known in trypanosomes, and these share properties with the ribosomal gene prom oter.