Genomic environment of the expression-linked extra copies of genes for surface antigens of Trypanosoma brucei resembles the end of a chromosome

Abstract

The surface coat of trypanosomes consists essentially of a single protein, the variant surface glycoprotein (VSG)^1,2. But more than a hundred different VSGs can be produced³. By switching from the synthesis of one VSG to the next, trypanosomes change the antigenic nature of their surface and thus escape destruction by the host immune system^4–6. Each VSG is encoded by a separate gene and expression of some of these genes involves the duplication and transposition of a silent basic copy (BC) into a new genomic environment^7–9, where the gene is transcribed^10,11. The DNA segment downstream of this transposed expression-linked extra copy (ELC) of the gene has two remarkable properties. It is devoid of restriction endonuclease cutting sites for a stretch of 7 kilobases (kb) which ends abruptly in an apparent cluster of at least 17 restriction endonuclease cutting sites. Here we show that in intact DNA the ‘barren’ region 3′ to two active VSG genes is preferentially attacked by exonuclease Bal31. We conclude that the expressed copies of these genes are located adjacent to a discontinuity in the DNA, presumably the end of a chromosome.