Trypanosoma brucei: conserved sequence organization 3' to telomeric variant surface glycoprotein genes

We have previously postulated that telomeric variant surface glycoprotein (VSG) genes in Trypanosoma brucei serve more frequently than intrachromosomal VSG genes as basic copies for gene conversion. To examine this further we determined the sequence for approximately 1200 nucleotides 3' to the telomeric IsTat 1 VSG gene, expressed in early variant antigenic types, and compared this sequence with those 3' to other VSG genes. We found that about 200 nucleotides immediately 3' to the 1 VSG gene are homologous to sequences immediately 3' to other telomeric VSG genes. These sequences may function in extended duplex formation 3' to telomeric VSG genes and partially explain their more frequent gene conversion. In addition, further 3' is a highly conserved 49 bp direct repeat, which is not transcribed into stable RNA. These sequences appear to be conserved in various T. brucei stocks, and we have therefore proposed a model which is a modification of one previously proposed (E. H. Blackburn and P. B. Challoner, 1984, Cell, 36, 447-457; L. H. T. Van der Ploeg, A. Y. C. Liu, and P. Borst, 1984, Cell, 36, 459-468) for the sequence organization of a trypanosome telomeric region.