IDENTIFICATION AND CHARACTERIZATION OF A PUTATIVE TELOMERE END-BINDING PROTEIN FROM TETRAHYMENA-THERMOPHILA

Telomeric DNA of Tetrahymena thermophila consists of a long stretch of (TTGGGG)(n) double-stranded repeats with a single-stranded (TTGGGG)(2 ) 3' overhang at the end of the chromosome, We have identified and cha racterized a protein that specifically binds to a synthetic telomeric substrate consisting of duplex DNA and the 3' telomeric repeat overhan g. This protein is called TEP (telomere end-binding protein). A change from G to A in the third position of the TTGGGG overhang repeat conve rts the substrate to a human telomere analog and reduces the binding a ffinity approximately threefold, Changing two G's to C's in the TTGGGG repeats totally abolishes binding. However, permutation of the Tetrah ymena repeat sequence has only a minor effect on binding. A duplex str ucture adjacent to the 3' overhang is required for binding, although t he duplex need not contain telomeric repeats. TEP does not bind to G-q uartet DNA, which is formed by many G-rich sequences. TEP has a greatl y reduced affinity for RNA substrates. The copy number of TEP is at le ast 2 x 10(4) per cell, and it is present under different conditions o f cell growth and development, although its level varies. UV cross-lin king experiments show that TEP has an apparent molecular mass of simil ar to 65 kDa. Unlike other telomere end-binding proteins, TEP is sensi tive to high salt concentrations.