Characterization of the DNA duplication-transposition that controls the expression of two genes for variant surface glycoproteins in Trypanosoma brucei.

The genome of Trypanosoma brucei carries over a hundred genes coding for different variants of the major surface glycoprotein. Activation of some of these genes is accompanied by a duplication and transposition of the gene (the basic copy) to another region in the genome where it is transcribed. We present here physical maps of the basic and transposition-activated genes for two surface glycoproteins of Trypanosoma brucei, stock 427. In both cases the transposed segment starts 1-2 kb in front of the coding region and ends within the 3'-terminal region of the gene. The DNA segments flanking both transposed genes are indistinguishable and share a 6-kb stretch upstream and a 8-kb stretch downstream of the transposed segment not cut by several restriction endonucleases. The 5' borders of the two transposed segments are homologous and contain sequences present in many copies in the genome. A different repeated sequence has previously been found at the 3' edge of the transposed segment. The replicative transposition may, therefore, involve a unidirectional gene conversion initiated by base pairing between the edges of the transposed sequence and a single expression site elsewhere in the genome.