The 5'-limit of transposition and upstream barren region of a trypanosome VSG gene: tandem 76 base-pair repeats flanking (TAA)90.

We have cloned and sequenced a portion of the region upstream of an expressed VSG gene of Trypanosoma brucei. This "expression-linked copy" arose through the duplication and transposition of a silent, "basic copy" of the gene to an expression site. Comparison of the sequences surrounding the 5'-end of the transposed segment in the two loci indicates the 5'-limit of transposition lies within the first (3'-most) of three repeated segments found at this position in the basic copy locus. These highly conserved repeat segments which average 76 base-pairs in length are also found tandemly repeated upstream of the transposed segment in the expression site. In this latter site, however, they are more numerous (at least 17 repeats) and they are interrupted, within the middle of one repeat, by a 270 base-pair region consisting of (TAA)90. The possible roles of these unusual sequences in transposition and in a model proposing discontinuous transcription of VSG genes are discussed.