Genetic controls for the expression of surface antigens in African trypanosomes.

The major surface antigens of African trypanosomes, variant surface glycoprotein (VSG) and procyclin, are typical markers of their respective developmental stages, the bloodstream form and the insect-specific procyclic form. Although the role of procyclin is still unclear, variation of the VSG in the blood allows the parasite to escape the immune response of the host and develop a chronic infection. In this review, we 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 appear to primarily control the expression of their genes through a specific modulation of promoter activity. Antigenic variation in the bloodstream results either from DNA rearrangements or from a change in telomeric chromatin structure, and stage-specific regulation of antigen synthesis is linked to differential control of RNA elongation, processing, stability, and/or translation. Trypanosomes' apparent lack of transcription-initiation control probably relates to the general organization of genes in long polycistronic transcription units. Only two promoters for protein-encoding genes, those of VSG and procyclin, are known in trypanosomes, and these share properties with the ribosomal gene promoter.