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The variant surface glycoprotein (VSG) and procyclin are the major surface proteins of the bloodstream and procyclic stages, respectively, of Trypanosoma brucei. The promoter regions of the VSG and procyclin gene transcription units could be mapped thanks to the specific enrichment of initial transcripts that occurs following UV irradiation. Whereas the VSG gene is 45 kb distant from its promoter, procyclin genes are located immediately downstream. We show, by run‐on assays on isolated nuclei and by cDNA analysis, that transcription occurs from both promoters in bloodstream as well as in procyclic forms. It is inferred that the control of the stage‐specific expression of VSG and procyclin genes is not effected at the level of transcription initiation, but most probably by interfering with the elongation and stability of the specific transcripts.

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In a 7-kilobase (kb) sequence upstream from the 5' barren region, the Trypanosoma brucei AnTat 1.3A expression site carries two putative genes, named ESAG 2 and ESAG 3 for expression site-associated genes, as well as a copy of ESAG 1 (D.F. Cully, H.S. Ip, and G.A.M. Cross, Cell 42:173-182, 1985). At least 3 kb of this expression site exhibits a high degree of homology with the silent telomere carrying the AnTat 1.3A basic copy, whose ESAG 1 is interrupted by stop codons. Like the antigen gene, the region containing the ESAGs is transcribed only in the bloodstream forms, although transcription of 5' barren- and ESAG 2-related sequences also occurs in cultured procyclics. Analysis of steady-state and nascent transcripts suggests a continuous transcription of the whole expression site by an RNA polymerase resistant to alpha-amanitin, possibly initiating at a polymerase I-like promoter located about 17 kb upstream from the antigen gene. This polymerase seems prone to becoming inactivated upon incubation of the trypanosomes at low temperature. The putative protein encoded by ESAG 3 may carry a hydrophobic signal peptide, suggesting interaction with a membrane.

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