Role of STN1 and DNA polymerase α in telomere stability and genome-wide replication in Arabidopsis.

The CST (Cdc13/CTC1-STN1-TEN1) complex was proposed to have evolved kingdom specific roles in telomere capping and replication. To shed light on its evolutionary conserved function, we examined the effect of STN1 dysfunction on telomere structure in plants. STN1 inactivation in Arabidopsis leads to a progressive loss of telomeric DNA and the onset of telomeric defects depends on the initial telomere size. While EXO1 aggravates defects associated with STN1 dysfunction, it does not contribute to the formation of long G-overhangs. Instead, these G-overhangs arise, at least partially, from telomerase-mediated telomere extension indicating a deficiency in C-strand fill-in synthesis. Analysis of hypomorphic DNA polymerase α mutants revealed that the impaired function of a general replication factor mimics the telomeric defects associated with CST dysfunction. Furthermore, we show that STN1-deficiency hinders re-replication of heterochromatic regions to a similar extent as polymerase α mutations. This comparative analysis of stn1 and pol α mutants suggests that STN1 plays a genome-wide role in DNA replication and that chromosome-end deprotection in stn1 mutants may represent a manifestation of aberrant replication through telomeres.

If the cap fits, wear it: an overview of telomeric structures over evolution.

Genome organization into linear chromosomes likely represents an important evolutionary innovation that has permitted the development of the sexual life cycle; this process has consequently advanced nuclear expansion and increased complexity of eukaryotic genomes. Chromosome linearity, however, poses a major challenge to the internal cellular machinery. The need to efficiently recognize and repair DNA double-strand breaks that occur as a consequence of DNA damage presents a constant threat to native chromosome ends known as telomeres. In this review, we present a comparative survey of various solutions to the end protection problem, maintaining an emphasis on DNA structure. This begins with telomeric structures derived from a subset of prokaryotes, mitochondria, and viruses, and will progress into the typical telomere structure exhibited by higher organisms containing TTAGG-like tandem sequences. We next examine non-canonical telomeres from Drosophila melanogaster, which comprise arrays of retrotransposons. Finally, we discuss telomeric structures in evolution and possible switches between canonical and non-canonical solutions to chromosome end protection.

Chromosome end protection by blunt-ended telomeres.

Single-stranded telomeric DNA protrusions are considered to be evolutionarily conserved structural elements essential for chromosome end protection. Their formation at telomeres replicated by the leading strand mechanism is thought to involve poorly understood post-replicative processing of blunt ends. Unexpectedly, we found that angiosperm plants contain blunt-ended and short (1- to 3-nucleotide) G-overhang-containing telomeres that are stably retained in post-mitotic tissues, revealing a novel mechanism of chromosome end protection. The integrity of blunt-ended telomeres depends on the Ku70/80 heterodimer but not on another telomere capping protein, STN1. Curiously, Ku-depleted telomeres are fully functional. They are resected by exonuclease 1, promoting intrachromatid recombination, which may facilitate formation of an alternative capping structure. These data challenge the view that telomeres require ssDNA protrusions for forming a functional capping structure and demonstrate flexibility in solutions to the chromosome end protection problem.