Telomerase extends the 3′ end by synthesizing multiple copies of the telomere repeat using its specialized telomerase reverse transcriptase (TERT) and its telomerase RNA (TER), which contains the template that directs repeat synthesis ( Blackburn et al. The DNA at the ends of eukaryotic chromosomes has a repetitive sequence, the telomeric repeat, that is generally a 6–8-nt G-rich sequence (e.g., TTGGGG in ciliates, TTAGGG in vertebrates) on the 3′-end strand (G-strand) and ends with a single-stranded overhang of variable length ( Blackburn and Collins 2011). This “end replication problem” led to the search for a specialized enzyme that might maintain the DNA at the ends of chromosomes and the discovery of telomerase ( Greider and Blackburn 1989). Eventually, this loss of DNA leads to cellular senescence ( Armanios and Blackburn 2012). At the 3′ ends of a replicating chromosome, the removal of the RNA primer would result in loss of ∼20 nucleotides (nt) for each round of DNA replication. The RNA primers are subsequently removed and replaced by DNA.
#FREETER XIE FREE#
DNA replication normally starts with an RNA primer that pairs to the opposite strand and provides the free 3′-OH required for DNA polymerases to initiate synthesis. The linear chromosomes of higher organisms present a special problem for DNA replication.
As the resolution has improved from ∼30 Å to ∼5 Å, these studies have provided increasingly detailed information on telomerase architecture and mechanism.
Here we describe how nuclear magnetic resonance (NMR) spectroscopy and X-ray crystallography of TER and protein domains helped define the structure and function of the core RNP, laying the groundwork for interpreting negative-stain and cryo electron microscopy (cryo-EM) density maps of Tetrahymena thermophila and human telomerase holoenzymes. It is a unique ribonucleoprotein (RNP) containing a specialized telomerase reverse transcriptase (TERT) and telomerase RNA (TER) with its own template and other elements required with TERT for activity (catalytic core), as well as species-specific TER-binding proteins important for biogenesis and assembly (core RNP) other proteins bind telomerase transiently or constitutively to allow association of telomerase and other proteins with telomere ends for regulation of DNA synthesis. Telomerase is a DNA polymerase that extends the 3′ ends of chromosomes by processively synthesizing multiple telomeric repeats.
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