Tandem repetition of the 5' mini-exon of variant surface glycoprotein genes: a multiple promoter for VSG gene transcription?

Activation of some variant surface glycoprotein (VSG) genes involves a duplicative transposition to an expression site, which completes the gene by addition of a mini-exon coding for the 5' 35 nucleotides of VSG mRNAs. Using a 22 nucleotide probe we have found some 200 copies of the mini-exon on a tandemly arranged 1.35 kb repetitive element. This repeat is highly conserved in three trypanosome species. The mini-exon on the repeat is flanked by a 5' splice site that resembles the consensus sequence. We have not found a single mini-exon within 10 kb of the transposed VSG gene exon in the expression site. We propose a model in which the arrays of mini-exon repeats function as a repetitive promoter for efficient transcription of VSG genes.