15 Ciliate Telomeres

Abstract

Ciliated protozoa hold a unique place in telomere biology because so many key discoveries have been made with these organisms. The first telomeres to be sequenced were from Tetrahymena (Blackburn and Gall 1978) and the first telomere-binding protein was isolated from Oxytricha ( Sterkiella 1) (Gottschling and Zakian 1986; Price and Cech 1987). Telomerase was discovered in Tetrahymena (Greider and Blackburn 1985) and, after a decade-long search, the catalytic subunit was finally isolated from Euplotes and shown to be a reverse transcriptase (Lingner and Cech 1996; Lingner et al. 1997). In recent years, Tetrahymena and Euplotes have continued to provide important models for understanding telomerase biochemistry, telomere replication, and G-overhang processing (Collins 1999; Ray et al. 2002; Jacob et al. 2003), whereas the crystal structure of the Oxytricha telomere-binding protein has provided insight into G-overhang protection and telomere capping (Larson et al. 1987; Horvath et al. 1998; Lei et al. 2003). The reason ciliates have played such a central role in telomere biology lies in their unusual genomic organization, because it results in each cell containing literally thousands or millions of telomeres. Ciliates have two functionally distinct nuclei: the germ-line micronucleus, which is transcriptionally silent and serves as a genetic repository during sexual development, and the somatic macronucleus, which is transcriptionally active and is used to support vegetative growth (Prescott 1994; Jahn and Klobutcher 2002). The micronucleus is diploid and it divides by mitosis. It contains relatively few large chromosomes: 5 in Tetrahymena and ~ 120 in Oxytricha . In contrast, the macronucleus...