2 The Telomerase Ribonucleoprotein Particle

Abstract

All eukaryotes and a few prokaryotes keep their genomes in the form of linear DNA molecules. This requires special mechanisms to fully replicate DNA ends because of the DNA end-replication problem and nucleolytic processing of telomere ends after semiconservative DNA replication (see Chapter 1). The most common solution to the end-replication problem occurs through the ribonucleoprotein (RNP) reverse transcriptase telomerase, discovered in 1985 in the holotrichous ciliate Tetrahymena thermophila by Carol Greider and Elizabeth Blackburn (Greider and Blackburn 1985; Chapter 1). The demonstration that the enzyme contains an RNA moiety, which copurifies with enzymatic activity (Greider and Blackburn 1989) and specifies the sequence of telomeric repeats (Yu et al. 1990), unmasked its nature as a cellular reverse transcriptase. The identification of telomerase protein subunits through genetic screens in yeast (Lendvay et al. 1996) and through the biochemical purification of telomerase from the hypotrichous ciliate Euplotes aediculatus (Lingner and Cech 1996) uncovered the presence of universally conserved reverse transcriptase sequence motifs in the catalytic subunit of telomerase (Lingner et al. 1997). Other less-well-conserved telomerase-associated proteins have crucial roles in the biogenesis of telomerase and the regulation of the interaction of telomerase with chromosome ends. In this chapter, we first describe the structural and functional features of telomerase RNA (TR) and the telomerase reverse transcriptase (TERT). We then give a comprehensive overview of telomerase-associated proteins in ciliates, yeast, and humans. We also summarize biochemical and genetic evidence that telomerase forms higher-order structures in some organisms. For the enzymology and biogenesis of telomerase,...