A XENOPUS EGG FACTOR WITH DNA-BINDING PROPERTIES CHARACTERISTIC OF TERMINUS-SPECIFIC TELOMERIC PROTEINS

We have identified a Xenopus laevis protein factor that specifically r ecognizes vertebrate telomeric repeats at DNA ends. This factor, calle d Xenopus telomere end factor (XTEF), is detected predominantly in ext racts of Xenopus eggs and ovaries, which are estimated to contain suff icient XTEF to bind approximately 3 x 10(7) DNA ends. In contrast, XTE F is much less abundant (approximately 90 per cell) in extracts of som atic cell nuclei. Mobility retardation analysis of the XTEF activity i n egg extracts indicates that this factor binds the vertebrate telomer ic repeat sequence (TTAGGG)2 when present in a single-stranded 3' over hang. Single-stranded 3' extensions of (TTTGGG)2, (AAAGGG)2, (TTACCC)2 , or a nonrepetitive sequence fail to bind XTEF efficiently, whereas c hanges in the double-stranded sequence 5' to the TTAGGG repeat tail ar e tolerated. TTAGGG repeats are not recognized at internal positions, at a 5' protruding end, or in double-stranded DNA. In addition, the fa ctor does not bind RNA with single-stranded UUAGGG repeats at a 3' end . XTEF-DNA complexes form and are stable in high salt. The DNA-binding properties of XTEF resemble the characteristics of a class of terminu s-specific telomere proteins identified previously in hypotrichous cil iates.