Telomerase-independent mechanisms of telomere elongation

Abstract

The ends of linear chromosomes must be elongated in a DNA-replication-independent fashion. For chromosome end elongation the majority of eukaryotes use a specialized reverse transcriptase, telomerase, which adds a short, tandemly repeated DNA sequence motif to chromosome ends. Chromosome elongation can also be achieved, however, by mechanisms other than telomerase. Such elongation events have been detected under conditions where telomerase has been inactivated experimentally and in the few organisms that naturally lack telomerase. We will summarize current knowledge on these telomerase-independent elongation mechanisms in yeast and mammalian cells and will discuss in more detail the telomere elongation mechanism by retrotransposons in Drosophila melanogaster.

Drosophila telomeres: an exception providing new insights

“…Recombination between these complex repeat sequences has been proposed as a mechanism for chromosome length maintenance in these species. It should be noted that 'lower' dipterans, such as the midge Chironomus and the mosquito Anopheles, which may resemble a Drosophila ancestor, fall into this category (55,98,99).…”

HP1 Is Distributed Within Distinct Chromatin Domains at Drosophila Telomeres

“…T ELOMERES are nucleoprotein structures at the ends of eukaryotic chromosomes with important roles in chromosome replication, stability, segregation, and position within the nucleus (Hochstrasser et al 1986;Blackburn 1991;Hari et al 2001;Chan and Blackburn 2002;Biessmann and Mason 2003;Abad et al 2004). In most eukaryotes, chromosomes terminate in an array of simple repeats that is synthesized by telomerase (Blackburn 1991).…”

“…The terminal arrays at Drosophila telomeres, however, are composites of three telomere-specific nonlong terminal repeat (non-LTR) retrotransposons, HeT-A, TAHRE, and TART (Mason and Biessmann 1995;Mason et al 2008), whose stochastic transposition creates an array (HTT) that differs in length at different chromosomal ends in a range of 147-26 kb in one stock (Abad et al 2004). Telomeric retrotransposons maintain chromosome length by targeted transposition to chromosome tips and by terminal recombination/gene conversion (Kahn et al 2000;Biessmann and Mason 2003). The attachment of the elements by their 39 oligo (A) tails to the chromosome end probably occurs via target-primed reverse transcription (Luan et al 1993) and does not depend on the DNA sequence at the terminus (Biessmann et al 1992;Biessmann and Mason 2003).…”

“…Telomeric retrotransposons maintain chromosome length by targeted transposition to chromosome tips and by terminal recombination/gene conversion (Kahn et al 2000;Biessmann and Mason 2003). The attachment of the elements by their 39 oligo (A) tails to the chromosome end probably occurs via target-primed reverse transcription (Luan et al 1993) and does not depend on the DNA sequence at the terminus (Biessmann et al 1992;Biessmann and Mason 2003). The HeT-A element is the most abundant telomeric retroelement; it has a promoter located at its 39 end that directs transcription of a downstream sequence (Danilevskaya et al 1997;Capkova Frydrychova et al 2007).…”

Three distinct chromatin domains in telomere ends of polytene chromosomes inDrosophila melanogaster Telmutants

“…They compensate for incomplete DNA replication and contribute to the stability of chromosomes and karyotype (Muller, 1932;Pardue and Debaryshe, 1999;Biessmann and Mason, 2003). Telomeres also participate in nuclear architecture maintenance, and are known to associate with nuclear lamina (Hochstrasser et al, 1986;Marshall et al, 1996;Hari et al, 2001) or with nuclear matrix (de Lange, 1992;Luderus et al, 1996).…”