Telomeres are repeated sequences found at the end of the linear chromosomes of most eukaryotes and are required for chromosome integrity. Expression of the reverse-transcriptase telomerase allows for extension of telomeric repeats to counteract natural telomere shortening. Although Chlamydomonas reinhardtii, a photosynthetic unicellular green alga, is widely used as a model organism in photosynthesis and flagella research, and for biotechnological applications, the biology of its telomeres has not been investigated in depth. Here, we show that the C. reinhardtii (TTTTAGGG)n telomeric repeats are mostly nondegenerate and that the telomeres form a protective structure, with a subset ending with a 3' overhang and another subset presenting a blunt end. Although telomere size and length distributions are stable under various standard growth conditions, they vary substantially between 12 genetically close reference strains. Finally, we identify CrTERT, the gene encoding the catalytic subunit of telomerase and show that telomeres shorten progressively in mutants of this gene. Telomerase mutants eventually enter replicative senescence, demonstrating that telomerase is required for long-term maintenance of telomeres in C. reinhardtii.

Central European Institute of Technology Masaryk University Brno Czech Republic

Sorbonne Université CNRS UMR 7141 Institut de Biologie Physico Chimique Biologie du Chloroplaste et Perception de la Lumière chez les Micro algues Paris France

Sorbonne Université CNRS UMR 7238 Institut de Biologie Paris Seine Laboratory of Computational and Quantitative Biology Paris France

Sorbonne Université PSL Research University CNRS UMR 8226 Institut de Biologie Physico Chimique Laboratoire de Biologie Moléculaire et Cellulaire des Eucaryotes Paris France

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Telomeres and Their Neighbors