Variant surface glycoprotein (VSG)genes ofAfrican trypanosomes areexpressed whentheyareinserted into oneofseveral telomere-linked expression sites. Wecloned andcharacterized an11-kilobase (kb) DNAfragment located upstream ofanexpressed VSGgene. A DNAsequence of1.8kbthatislocated immediately upstream oftheinserted VSG genecontains sequences homologous tothe76-base-pair repeats described asbeing upstream ofVSG genesinTrypanosoma brucei (D.A.Campbell, M.P.VanBree,andJ.C.Boothroyd, Nucleic Acids Res.12:2759-2774). Therearenosuchsequences elsewhere inthe11-kb cloned region. Southern blot analysis using probes fromthecloned region revealed multiple unlinked copies ofthesameorverysimilar regions. Atleast three ofthese arelocated neartelomeres, andtwohavebeenshowntobeusedforthe expression ofknownTrypanosoma equiperdum VSGgenes. LikeVSGgenes, theupstream sequences themselves canbeduplicated anddeleted. Thechoice ofexpression site tobeusedbyaduplicated VSGgeneisnonrandom; thesite usedforexpression oftheparental VSGgeneisstrongly favored foruseinthedaughter variant. Furthermore, evenwhentheparental expression site isnotused, theVSGgeneoccupying itisreplaced. Thus, anactive expression site isapreferential target forgeneconversion inthenextvariation event. African trypanosomes alternately present different VSGs (seeMaterials andMethods fordefinitions ofabbreviations andatrypanosome glossary) inthemammalian host. This process ofantigenic variation ispresumed toaidthepara
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