Expression of a retroposon-like sequence upstream of the putative Trypanosoma brucei variant surface glycoprotein gene expression site promoter.

We have cloned the region spanning the putative promoter from two variant surface glycoprotein gene expression sites that are at each end of chromosome M4 of Trypanosoma brucei IsTat 7. Both expression sites contain a retroposon-like sequence (ESR) pseudogene whose 3' end is approximately 30 bp upstream of the putative expression site promoter. The ESRs from both expression sites share considerable sequence homology and are related to LINE-like elements, especially the T. brucei ingi retroposon. Other ESRs are located on large, but not intermediate or mini-, chromosomes in the IsTaR 1 serodeme, and the total copy number is 10 to 20, similar to that estimated for variant surface glycoprotein expression sites. No DNA rearrangements in the vicinity of the ESR and putative expression site promoter were detected following antigenic switches in the IsTaR 1 serodeme. ESR transcripts are present in bloodstream, but not procyclic, forms. Variation in transcript size and sequence between bloodstream variant antigenic types implies that only the ESR from the active expression site is transcribed. This pattern of expression reflects that of sequences downstream of the putative expression site promoter, suggesting that the region of coordinately controlled expression extends upstream of this promoter.