Molecular characterization of Chlamydomonas reinhardtii telomeres and telomerase mutants

Telomeres are repeated sequences found at the end of the linear chromosomes of most eukaryotes and are required for chromosome integrity. They shorten with each cell division because of the end-replication problem. Expression of the reverse transcriptase telomerase allows for extension of telomeric repeats to counteract 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 non-degenerate and that the telomeres form a protective structure, ending with a 3' overhang. While 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 mutants of this gene display an \"ever shortening telomere\" phenotype and eventually enter replicative senescence, demonstrating that telomerase is required for long-term maintenance of telomeres in C. reinhardtii.