TRF1 PROMOTES PARALLEL PAIRING OF TELOMERIC TRACTS IN-VITRO

Human telomeres consist of long arrays of TTAGGG repeats bound to the telomere-specific proteins, TRF1 and TRF2. Here we describe the struct ure of in vitro complexes formed between telomeric DNA and TRF1 as ded uced by electron microscopy. Visualization of TRF1 bound to DNA contai ning six or 12 tandem TTAGGG repeats revealed a population of DNAs con taining a spherical protein complex localized just to the repeats. Mas s analysis of the protein complexes suggested binding of TRF1 dimers a nd tetramers to the TTAGGG repeats. The DNA was not significantly comp acted or extended by protein binding. TRF1 formed filamentous structur es on longer telomeric repeat arrays (greater than or equal to 27 repe ats) consistent with the presence of an array of bound TRF1 dimers. Un expectedly, there was a strong propensity for two telomeric tracts to form paired synapses over the TRF1 covered segment. Up to 30% of the T RF1-bound DNAs could be found in a paired configuration with a strong bias for a parallel as contrasted to an antiparallel arrangement. TRF1 -induced pairing was confirmed using a ligation assay which detected t he formation of DNA multimers dependent on the presence of TRF1 and a 27mer repeat array in the DNA. These findings suggests that this prote in may have an architectural role at telomeres. We discuss the possibi lity that TRF1-dependent changes in the conformation of telomeres are involved in the regulation of telomere length. (C) 1998 Academic Press Limited.