Early expression of a Trypanosoma brucei VSG gene duplicated from an incomplete basic copy.

Intrachromosomal variant surface glycoprotein (VSG) genes in Trypanosoma brucei are expressed by a mechanism involving gene conversion. The 3' boundary of gene conversion is usually within the last 130 bp of the VSG gene, a region of partially conserved sequences. We report here the loss of the predominant telomeric A VSG gene in the cloned variant antigenic type (VAT) 5A3, leaving only an intrachromosomal A VSG gene (the A-B gene). The nucleotide sequence of the A-B VSG gene reveals that it lacks the normal VSG 3' sequence. Surprisingly, we find cells expressing this A-B VSG gene in relapse populations arising from VAT 5A3. Since the A VSG mRNAs from these cells have a normal 3' sequence, the incomplete A-B VSG gene must be expressed via a partial gene conversion that supplies the functional 3' end. Although the A-B VSG gene is no longer predominant like the telomeric A VSG gene, it is still expressed more frequently than other intrachromosomal VSG genes, suggesting that factors other than a telomeric location determine whether a VSG gene is expressed early in a serodeme.