DNA-protein interactions at the telomeric repeats of Schizosaccharomyces pombe

Gel retardation assays using a probe containing the repeat region of a Schizosaccharomyces pombe chromosomal telomere identified four specific DNA- protein complexes in S. pombe total protein extracts (I, I', IIa and IIb). The proteins responsible for these complexes bound to the telomeric repeat region irrespective of whether or not the repeats were in close proximity to the end of a DNA molecule, and none of them bound strongly to single-stranded DNA. The protein responsible for complex I (TeRF I) was separated from the activity responsible for complexes IIa and IIb (TeRF II) using heparin-Sepharose chromatography. Both factors were efficiently cross-competed by an oligonucleotide containing the 18 bp sequence 5'-GGTTACAGGTTACAGGTT-3', which corresponds to two complete telomeric repeat units. Mutation of the T residues at positions 4 and 11 in the oligonucleotide dramatically reduced binding by TeRF II, but had no affect on binding by TeRF I. The protein responsible for complex I' did not bind strongly to either the wild-type or mutant oligonucleotide.