Telomeres are important protein-DNA structures at the ends of linear
eukaryotic chromosomes that are necessary for genome integrity. Telomeres
are maintained by intermittent action of telomerase. I explored the kinetics of
telomere length homeostasis in the model plant Arabidopsis thaliana by crossing
wild type plants to different generations of telomerase deficient plants, and then
analyzing telomere length in the resulting progeny. Unexpectedly, I found plants
lacking telomerase for seven generations can lengthen telomeres when
telomerase is reintroduced, but one generation is not sufficient to reestablish the
telomere set point.
Est1 is a non-catalytic component of the Saccharomyces cerevisiae
telomerase holoenzyme. To investigate the role of Est1 in higher eukaryotes, I
identified two putative Est1 homologues in Arabidopsis, AtEST1a and AtEST1b.
Plants deficient in AtEST1a displayed no vegetative or reproductive defects.
However, plants deficient for AtEST1b were sterile and had severe vegetative
and reproductive irregularities. Surprisingly, no defects in telomere maintenance
were observed in any single or double mutant line. This suggests that the Est1-
like proteins in plants have evolved new functions outside of telomere length
maintenance and end protection.One consequence of telomere dysfunction is end-to-end chromosome
fusion. In mammals, telomere fusion is mediated through NHEJ and requires
DNA Ligase IV (Lig4). Lig4 is an essential component of the NHEJ pathway
along with the Ku70/Ku80 heterodimer and DNA-PKcs. To address the
mechanism of chromosome fusion in Arabidopsis, we investigated the role of
Lig4 in mutant combinations lacking TERT, the catalytic subunit of telomerase,
and Ku70. Surprisingly, telomere end-to-end fusions were observed in ku70 tert
lig4 triple mutants, suggesting that neither Lig4 nor Ku70 are required for the
fusion of critically shortened telomeres in Arabidopsis. To investigate the origin of
genome instability, terminal restriction fragment analysis was performed on triple
mutants. Strikingly, telomeres diminished five to six-fold faster than in a tert
single mutant. Moreover, in the triple mutants, telomere tracts were extremely
heterogeneous, suggesting that the telomeres were exposed to catastophic
nucleolytic attack. These data provide the first evidence that Lig4 contributes to
telomere maintenance and chromosome end protection.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/4141 |
| Date | 30 October 2006 |
| Creators | Idol, Rachel A |
| Contributors | Shippen, Dorothy |
| Publisher | Texas A&M University |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | Book, Thesis, Electronic Dissertation, text |
| Format | 4034088 bytes, electronic, application/pdf, born digital |
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