Trypanosoma brucei survives immune attack in its mammalian hosts by periodically changing the variant surface glycoprotein (VSG). From the -1000 VSG genes (VSGs) scattered throughout the genome, a few are located at telomere loci called expression sites (ESs) one is expressed at any time, suggesting a stringent regulatory mechanism. Here, it is demonstrated that trypanosomes engineered to express two VSGs simultaneously from the same ES can multiply in cultures and infect animals normally. Mechanisms involved in the control ES-associated gene expression are thought to involve some kind of telomere position effect. Telomeres typically contain simple DNA repeats (TTAGGG in animals and trypanosomatids) that are the binding sites for telomeric proteins, and end in 3\u27 protrusions known as overhangs. In mammals, telomeres fold back in structures known as t-loops, in which the overhang is intercalated in the duplex DNA. The length of the TTAGGG tracts was measured (-15 kb) and the presence of 3\u27 overhangs was established in trypanosome telomeric DNA. EM studies revealed the presence of t-loops in trypanosome telomeric DNA and at both ends of minichromosomes. Therefore, trypanosome telomeres have conserved features and probably contain proteins with conserved functions. Binding of the human protein TRFI to telomeres procyclic trypanosomes caused transient cell cycle arrest and abrupt telomere shortening, indicating that TRFI is interfering with a presumed trypanosome telomere complex. Trypanosome protein extracts did not exhibit a detectable TRFI-like DNA-binding activity. A yeast one-hybrid screen was developed to detect trypanosome proteins with the ability to TTAGGG. However, this screen was interfered by the presence of an endogenous TTAGGG binding activity and by the trypanosome nucleosome assembly protein NAP1
