Variant specific transcripts from the co-transposed segments of variant surface glycoprotein genes in Trypanosoma brucei.

We examined steady-state transcripts from the sequences immediately upstream of the 1 and 11 variant surface glycoprotein (VSG) genes in Trypanosoma brucei of the IsTaR 1 serodeme. These sequences, the co-transposed segment, differ in size and sequence, and produce a variety of variant-specific polyadenylated transcripts. Most, if not all, of these transcripts, some of which may be precursors, contain the spliced leader (SL). Variant specific transcription extends from 5' of the co-transposed segment to 100-400 nucleotides downstream of the major VSG mRNA polyadenylation site. The 1 VSG mRNA has at least 2, and possibly 3, SL addition sites; the major site is 120 nucleotides downstream of the polyadenylation site identified by sequencing a cDNA from the co-transposed segment. Two general models for the production of the co-transposed segment transcripts are discussed.