In Trypanosoma brucei, the active variant surface glycoprotein genes (vsg) are located at telomeric expression sites (ES), whose expression is highly regulated during the life cycle. In the procyclic form, all ESs are repressed. In the bloodstream form, where antigenic variation occurs, only one of approximately 20 ESs is active at a given lime. We have investigated chromatin structure and DNA sequence around the ES promoter to identify cis-acting regulatory regions. A marker gene, inserted 1 kb downstream of the ES promoter, was used as a specific probe to map the position of nuclease hypersensitive sites. A prominent hypersensitive site was detected within the core promoter. This site was present in both active and inactive ES promoters, suggesting that a protein complex is bound to the promoter irrespective of its transcriptional state. However, none of the regions showed differential nuclease sensitivity between active and inactive transcriptional states. A systematic deletion analysis of the sequences surrounding the active ES promoter in situ confirmed the absence of cis-regulatory elements. We find that only 70 bp within the ES promoter are necessary to support ES regulation. Analysis of the reporter activities in an inactive bloodstream-form ES revealed the existence of an intermediate promoter activity in some clones, but we never observed full activation of more than one ES. The vsg mRNA from this intermediate ES was expressed less efficiently. (C) 1998 Elsevier Science B.V. All rights reserved.