| Abstract
| - The fission yeast Pot1 (protection of telomeres) protein is a single-stranded telomeric DNA-binding protein and is required to protect the ends of chromosomes. Its N-terminal DNA-binding domain,Pot1pN, shows sequence similarity to the first OB fold of the telomere-binding protein α subunit ofOxytricha nova. The minimal-length telomeric ssDNA required to bind Pot1pN was determined to consistof six nucleotides, GGTTAC, by gel filtration chromatography and filter-binding assay (KD = 83 nM).Pot1pN is a monomer, and each monomer binds one hexanucleotide. Experiments with nucleotidesubstitutions demonstrated that the central four nucleotides are crucial for binding. The dependence ofPot1pN−ssDNA binding on salt concentration was consistent with a single ionic contact between theprotein and the ssDNA phosphate backbone, such that at physiological salt condition 83% of the freeenergy of binding is nonelectrostatic. Subsequent binding experiments with longer ssDNAs indicated thatPot1pN binds to telomeric ssDNA with 3‘ end preference and in a highly cooperative manner that mainlyresults from DNA-induced protein−protein interactions. Together, the binding properties of Pot1pN suggestthat the protein anchors itself at the very 3‘ end of a chromosome and then fills in very efficiently, coatingthe entire single-stranded overhang of the telomere.
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