A structural and transcription pattern for variant surface glycoprotein gene expression sites used in metacyclic stage Trypanosoma brucei

African trypanosomes first express the variant surface glycoprotein (VSG) a t the metacyclic stage in the tsetse fly vector, in preparation for transfe r into the mammal. Metacyclic (M)VSGs comprise a specific VSG repertoire su bset and their expression is regulated differently from that of bloodstream VSGs, involving exclusively transcriptional regulation during the life cyc le. To identify basic structural and functional features that may be common to MVSG telomeric transcription units, we have characterized the anatomy a nd transcription of the telomere containing the ILTat 1.61 MVSG gene. This telomere contains pseudogenes of the ESAG1 and ESAG9 families found in bloo dstream VSG transcription units. The 1.61 MVSG occupies a monocistronic tra nscription unit and is transcriptionally controlled through the life cycle. The 1.61, and also the 1.22, MVSG transcription initiation site sequences resemble eukaryotic initiator elements. Sequence comparison reveals that fo ur out of five characterized MVSC expression sites have a conserved region 2.0-4.7 kb long upstream of the MV,SG. In some cases, this region contains not only the transcription initiation site that we have observed to be acti ve in fly-transmitted trypanosomes but also, upstream, another sequence, de scribed elsewhere as a 'putative promoter' for the MVAT set of MVSGs (Nagos hi YL, Alarcon CM, Donelson JE. A monocistronic transcript for a trypanosom e variant surface glycoprotein, Mol Biochem Parasitol 1995,72:33-45). In fl y-transmitted trypanosomes, the latter element is transcriptionally silent. Our analysis of the structure of MVSG telomeres suggests that metacyclic e xpression sites arose from bloodstream expression sites. (C) 1999 Elsevier Science B.V. All rights reserved.