Chlamydomonas telomere sequences are A+T-rich but contain three consecutive G·C base pairs

We have isolated telomeric DNA and telomere-associated sequences from Chlamydomonas reinhardtii. The terminal telomere sequences of the green alga Chlamydomonas are composed of (TTTTAGGG)_n repeats that are similar, but not identical, to those of the higher plant Arabidopsis thaliana. We demonstrate that these repeats are telomeric by their preferential sensitivity to nuclease Bal-31 digestion, their similarity to A. thaliana telomeres, their orientation relative to the end of the chromosome, and the methods used for their isolation. Five independent telomere clones were isolated, and three of these clones include closely related telomere-associated sequences. One of these telomere-associated sequences hybridizes to a number of genomic fragments sensitive to digestion with the exonuclease Bal-31. Like telomere sequences from other organisms, the C. reinhardtii telomeres display a bias for guanine and thymine nucleotides on the 3′-end strand. However, the sequence of Chlamydomonas telomeres is more A+T-rich than any other known telomere sequence. We propose that the common feature of all known telomeres is the frequent occurrence of tracts of three or more adjacent guanine residues.