Modifications of a Trypanosoma b. brucei antigen gene repertoire by different DNA recombinational mechanisms.

In the Trypanosoma b. brucei AnTat 1.1C clone, the gene coding for the variant-specific surface antigen is telomeric and appears as a hybrid sequence, partially modified by gene conversion. This conversion is very similar to that observed in another AnTat 1.1-expressor clone (AnTat 1.1B). This sequence is not activated by duplicative transposition, although it could be activated by duplication in another clone (AnTat 1.10). Instead activation of the AnTat 1.1C gene seems operated by reciprocal recombination between its own telomere and the telomere carrying the previous (AnTat 1.16) ELC. Indeed, from the switch to AnTat 1.1C onward, the AnTat 1.16 ELC becomes a new silent member of its gene family, whereas in the variant directly derived from AnTat 1.1C (AnTat 1.3B), the AnTat 1.1C-containing telomere is lost, probably replaced by a large duplicate, at least 40 kb long, of the AnTat 1.3 gene-containing telomere. Different DNA rearrangement mechanisms used by the trypanosome to change its antigenic type thus contribute, by gain and loss of genes, to the evolution of the repertoire for surface antigens.