3 Telomerase Biochemistry and Regulation

Abstract

TELOMERE REPLICATION
Telomere replication poses a special problem for eukaryotic cells. The mechanism of leading- and lagging-strand replication, initiated at internal origins, allows copying of the majority of the genome. However, the molecular ends of chromosomes are not completely copied by this mechanism (Watson 1972; Olovnikov 1973). Before the molecular structure of telomeres was known, a number of models were proposed for the way the replication of ends might be accomplished (Blackburn and Szostak 1984; Zakian 1989). Many of these models were based on viral modes of end replication. Linear phage and viral genomes face the same problems as linear eukaryotic chromosomes in replicating DNA ends (see Gall, this volume). However, with the initial cloning of telomeres in ciliates (Blackburn and Gall 1978; Klobutcher et al. 1981), it became apparent that many viral models did not fit with the DNA structures at chromosome ends. The number of possible replication mechanisms narrowed to just a few.

The telomere DNA identified in ciliates comprises simple tandem repeats of GT-rich sequences, d(TTGGGG) in Tetrahymena and d(TTTTGGGG) in Oxytricha and Stylonychia. Tandem short repeats were subsequently identified on chromosome ends in many other organisms (see Henderson, this volume). These sequences are present on chromosome ends in variable numbers. Any given Tetrahymena telomere might have 50–70 repeats; the number is not fixed. In hypotrichous ciliates such as Oxytricha and Euplotes, there are only 3–4 repeats on the ends of mature chromosomes. The variable number of repeats results in telomere restriction fragments having a...