Structure and expression of a Trypanosoma brucei gambiense variant specific antigen gene

The expression-linked copy of the T. b. gambiense variant specific antigen gene LiTat 1.6 is transposed in a 20 kb DNA region devoid of restriction sites, located near a chromosome end. This expression site is very similar to that of T. b. brucei variants 117, 118 (1) and AnTat 1.8. In the basic copy, the transposable element (TE) is flanked by repetitive sequences; it includes the gene copy as well as a sequence of 0.9 to 2.1 kb (probably around 1.1 kb) long, upstream from the gene. Probes derived from the 5' part of the TE specifically reveal three polyadenylated transcripts of 4.2, 1.45 and 0.85 kb, respectively, distinct from the 2.1 kb mRNA. The amount of the 4.2 kb sequence is probably less than 0.01% of total trypanosome RNA. Whereas the mRNAs coding for the three isotypic antigens AnTat 1.8 (T. b. brucei), 12.2 (T. b. rhodesiense) and 3.3 (T. evansi) are recognized by LiTat 1.6 probes extending into the 3' half of the transposed sequence, the 5' genomic probes do not hybridize with any of these RNAs. These observations suggest that the LiTat 1.6 gene could be first transcribed in a large precursor molecule. This precursor would be rapidly processed, loosing a large portion of less conserved sequence from its 5' half. Our data are compatible with a model in which the promoter would be provided by the expression site.