Abstract
Telomeres, as physical ends of linear chromosomes, are targets of a number of specific proteins, including primarily telomerase reverse transcriptase. Access of proteins to the telomere may be affected by a number of diverse factors, e.g., protein interaction partners, local DNA or chromatin structures, subcellular localization/trafficking, or simply protein modification. Knowledge of composition of the functional nucleoprotein complex of plant telomeres is only fragmentary. Moreover, the plant telomeric repeat binding proteins that were characterized recently appear to also be involved in non-telomeric processes, e.g., ribosome biogenesis. This interesting finding was not totally unexpected since non-telomeric functions of yeast or animal telomeric proteins, as well as of telomerase subunits, have been reported for almost a decade. Here we summarize known facts about the architecture of plant telomeres and compare them with the well-described composition of telomeres in other organisms.
Highlights
Reviewed by: Takashi Murata, National Institute for Basic Biology, Japan Franziska Katharina Turck, Max Planck Society, Germany
The telomere is mostly composed of Arabidopsis-type TTTAGGG repeats (Richards and Ausubel, 1988; Figure 1A)
The AtTRFL proteins from A. thaliana bind telomeric DNA in vitro and an interaction between AtTRP1 and AtKu70 was observed, suggesting a putative telomere function (Figure 2B; Kuchar and Fajkus, 2004), no functional evidence exists for their role at telomeres
Summary
Telomeres are nucleoprotein structures at the ends of eukaryotic chromosomes that protect linear chromosomes against damage by endogenous nucleases and erroneous recognition as unrepaired chromosomal breaks. Gene duplications and losses in plant phylogeny can be traced in telomere associated protein families. The telomere is mostly composed of Arabidopsis-type TTTAGGG repeats (Richards and Ausubel, 1988; Figure 1A). Known exceptions are species in the order Asparagales, starting from divergence of the Iridaceae family, which shares the human-type telomeric repeat (TTAGGG; probably caused by a mutation that altered the RNA template subunit of telomerase ∼80 Mya; Adams et al, 2001; Weiss and Scherthan, 2002; Sykorova et al, 2003). The length of plant telomeric DNA at a single chromosomal arm can be as small as 500 base pairs (bp) in Physcomitrella patens (Shakirov et al, 2010; Fojtova et al, 2015), as long as 160 kb in Nicotiana tabacum (Fajkus et al, 1995), or 200 kb in Nicotiana sylvestris (Kovarik et al, 1996). Since telomeric DNA serves as a landing pad for a set of proteins, the total length or composition of telomeric tracts could markedly affect the number or selection of telomere-associated proteins and subsequently influence telomere packaging, structural transitions, or launch various biochemical pathways (see below)
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