Novel expression-linked copies of the genes for variant surface antigens in trypanosomes

Pathogenic African trypanosomes evade the immune system of their mammalian hosts by the sequential expression of alternative cell-surface glycoproteins (reviewed in refs 1, 2). Variant surface glycoproteins (VSGs) purified ³ from cloned variants of Trypanosoma brucei have similar molecular weights (about 60,000), but differ in amino acid composition³, N-terminal amino acid sequence⁴ and C-terminal structure⁵. We have cloned DNA complementary to the messenger RNAs for four immunologically distinct VSGs⁶ and hybridised these complementary DNAs (cDNAs) with restriction digests of T. brucei nuclear DNA, fractionated by gel electrophoresis and transferred to nitrocellulose strips. Each cDNA recognises a unique set of fragments and this basic set is present unaltered in the nuclear DNAs from the four variants. In addition, each probe recognises an extra fragment only in nuclear DNA isolated from cells expressing the VSG corresponding to the cDNA probe. We infer that activation of a VSG gene involves the production of an expression-linked copy of that gene.