ANTIGENIC VARIATION IN TRYPANOSOMES

We review here antigenic variation in African trypanosomes with emphas is on genetic mechanisms and on the expression sites in which the gene s for Variant Surface Glycoproteins (VSGs) are expressed. There are mu ltiple expression sites in a trypanosome, but only one of these is act ive at a time. We discuss recent experiments that provide new informat ion on expression site regulation, i.e., how inactive sites are kept i nactive and how the trypanosome switches from expression of one site t o expression of another one. Trypanosomes can also change the gene exp ressed by replacing the gene in an active expression site by another V SG gene. This replacement involves the duplicative transposition of a silent VSG gene into the expression site, We present a model for the m echanism of this transposition that incorporates new features and that explains several unusual characteristics of the transposition process . We also discuss how new knowledge of nutrient uptake, notably uptake of host transferrin by trypanosomes, might be used for vaccine develo pment.