Transcription of a transposed trypanosome surface antigen gene starts upstream of the transposed segment

The non-telomeric variant surface glycoprotein (VSG) genes in Trypanosoma brucei are activated by a duplicative transposition to a telomeric expression site. We have determined the 5' end of the transposed segment of the gene for VSG 117 and infer from comparison with similar data obtained by others that the crossover can occur at variable positions within short repeats present upstream of this gene and in the expression site. We have analysed nascent and steady state transcripts of the transposed gene and its neighbouring expression site DNA. The results indicate that transcription starts upstream of the transposed gene segment in the expression site and that transcripts are rapidly processed at specific points identified by protection of DNA-RNA hybrids against digestion by nuclease S1 or Exo VII. Hence, this gene appears to be activated by a process akin to promoter addition.