Activation of trypanosome surface glycoprotein genes involves a duplication-transposition leading to an altered 3' end

Abstract

Expression of the genes for variant surface glycoproteins 117 and 118 in Trypanosoma brucei is accompanied by the appearance of an extra copy of these genes, the expression-linked copy, which differs in the surrounding restriction enzyme sites from the corresponding basic copy of the genes. We present direct evidence that the expression-linked copy is the one used for messenger RNA synthesis. By S1-nuclease-protection experiments we show that cloned basic-copy genes contain the nucleotide sequence of the corresponding messenger RNA except for the last 100 to 150 nucleotides before the poly(A) tail. Comparison of the 3'-terminal sequence of the 117 basic-copy gene and the 117 complementary DNA shows that this region differs by multiple point mutations, insertions and deletions, the differences starting within the coding sequence. Genomic blots demonstrate that a Bsp I site in the 3'-terminal part of the 118 complementary DNA is present in the expression-linked copy but not in the basic-copy gene. We conclude that expression-linked copies are the active genes, and that the generation of expression-linked copies involves a duplication--transposition in which the 3' end of the gene is replaced.