The telomere binding protein of Euplotes crassus prevents non-specific transcription initiation but has no role in positioning transcription initiation complexes

Model substrates mimicking the telomeric as well as the 5'-untranslated regions in front of a 5'-terminal part of a macronuclear gene of Euplotes crassus were transcribed in vitro using cellular extracts. The obtained transcripts were characterized by primer extension and shown to start at the natural initiation points. The situation changed in the absence of telomere binding protein or with substrates lacking functional telomeric sequences. In these cases non-specific transcription was observed. Since it had been previously found that transcription starts are frequently located at an apparently fixed distance from the telomere, a model substrate was constructed which carried a duplication of the non-transcribed region. This resulted in only one transcription start site, the position of which was conserved relative to the start of the open reading frame but moved away from the telomere by the distance of the duplication.