GENE CONVERSIONS MEDIATING ANTIGENIC VARIATION IN TRYPANOSOMA-BRUCEI CAN OCCUR IN VARIANT SURFACE GLYCOPROTEIN EXPRESSION SITES LACKING 70-BASE-PAIR REPEAT SEQUENCES

African trypanosomes undergo antigenic variation of their variant surf ace glycoprotein (VSG) coat to avoid immune system-mediated killing by their mammalian host. An important mechanism for switching the expres sed VSG gene is the duplicative transposition of a silent VSG gene int o one of the telomeric VSG expression sites of the trypanosome, result ing in the replacement of the previously expressed VSG gene. This proc ess appears to be a gene conversion reaction, and it has been postulat ed that sequences within the expression site may act to initiate and d irect the reaction. All bloodstream form expression sites contain huge arrays (many kilobase pairs) of 70-bp repeat sequences that act as th e 5' boundary of gene conversion reactions involving most silent VSG g enes. For this reason, the 70-bp repeats seemed a likely candidate to be involved in the initiation of switching. Here, we show that deletio n of the 70-bp repeats from the active expression site does not affect duplicative transposition of VSG genes from silent expression sites. We conclude that the 70-bp repeats do not appear to function as indisp ensable initiation sites for duplicative transposition and are unlikel y to be the recognition sequence for a sequence-specific enzyme which initiates recombination-based VSG switching.