Telomere Maintenance in Organisms without Telomerase

Abstract

Telomeres serve two vital functions to eukaryotes. They act as a protective chromosome cap to distinguish natural chromosome ends from double stranded DNA breaks and to avoid inappropriate fusions of telomeric sequences, and they maintain chromosome length by adding DNA to the ends of chromosomes. Telomeres thus balance the loss of terminal DNA due to the inability of the replication machinery to completely replicate linear DNA molecules (Olovnikov, 1973; Watson, 1972). In many cases the newly replicated chromosome ends are resected to allow for the formation of a t-loop that helps to hide the tip (Griffith et al., 1999; Wellinger et al., 1996). Most eukaryotes elongate chromosome ends with a special reverse transcriptase, telomerase, that carries a specific RNA template with telomeric sequence (Greider, 1996). The telomerase enzyme repeatedly adds copies of the short telomeric DNA sequence to the chromosome end. While there is strict conservation of telomeric sequence repeat in most species, the repeat unit has changed over evolutionary time. Holotrichous ciliates, e. g. Tetrahymena, use the sequence (TTGGGG)n (Blackburn & Gall, 1978), while hypotrichous ciliates, e. g. Oxytricha, use (TTTTGGGG)n (Klobutcher et al., 1981; Oka et al., 1980). The primary telomeric sequence in plants is (TTTAGGG)n (Richards & Ausubel, 1988; Zellinger & Riha, 2007), although the alga Chlamydomonas uses (TTTTAGGG)n. In the yeasts the telomeric sequence has the same general motif, but is not as tightly controlled. Saccharomyces for example uses (TG1-3)n (Shampay et al., 1984; Wang & Zakian, 1990), while Schizosaccharomyces has (TTACAG1-8)n (Matsumoto et al., 1987). The sequence found at the telomeres of most metazoans is (TTAGGG)n (Meyne et al., 1989; Traut et al., 2007), although arthropods use (TTAGG)n (Okazaki et al., 1993). Lack of the predominant telomeric sequence in a species does not, however, signify that telomerase-generated terminal sequences are missing. For example, the metazoan-type telomeric sequence is found in place of the plant sequence in Aloe species (Weiss & Scherthan, 2002). In order to establish that a telomeraseindependent, chromosome maintenance system exists it is also necessary to show a lack of a telomerase gene and telomerase activity, and to identify the nature of the DNA sequence at the chromosome termini. Establishing the negative is always difficult, and confirming that a specific sequence is at, not merely near, the chromosome tip is not trivial. Conversely, the presence of a