Telomeres comprise the physical ends of chromosomes. Essential functions of
telomeres include protecting the terminus from being recognized as a DNA doublestrand
break and facilitating the complete replication of the physical end of the DNA.
Telomere functions are mediated by a large array of telomere-associated proteins.
Mutations in telomere-related genes cause immediate telomere dysfunction, activation
of DNA damage response, and accumulation of end-to-end chromosome fusions. In
addition, changes in telomere complex composition may affect the ability of the
telomerase enzyme to maintain telomeres in vivo.
Here, we describe the characterization of telomere-associated proteins in the
flowering plant, Arabidopsis thaliana. Using a bioinformatics approach, we identified
twelve proteins with sequence similarity to vertebrate duplex telomere DNA binding
proteins TRF1 and TRF2. We showed that, like their vertebrate counterparts, some of
the Arabidopsis TRFL (TRF-LIKE) proteins can homodimerize and bind telomeric DNA
in vitro, indicating that Arabidopsis encodes a large family of double-strand telomeric
DNA binding proteins. We have also characterized three Arabidopsis POT1 proteins
whose homologs in yeast and vertebrates associate with the single-stranded portion of
telomeric DNA. Unexpectedly, we found that unlike POT1 protein in other organisms,
Arabidopsis AtPOT1a protein associates with telomeres only in the S phase of the cell cycle and is a physical component of the active telomerase RNP complex, providing
positive telomere length regulation. Our data implicated AtPOT1b, another Arabidopsis
POT1 protein, in chromosome end protection. Finally, we showed that Arabidopsis
thaliana has evolved a third POT1 protein, AtPOT1c, which contributes to both telomere
length regulation and telomerase activity, and maintenance of the structure of the
chromosome terminus. Thus, Arabidopsis has evolved a set of POT1 proteins that
make distinct and novel contributions to telomere biology.
Finally, we describe the identification and characterization of a novel
Arabidopsis protein CIT1 (Critical for Integrity of Telomeres 1), and show that CIT1
deficiency leads to an immediate and profound telomere dysfunction and chromosome
end deprotection. Altogether, these data provide new insight into plant telomereassociated
factors and significantly improve our understanding of the overall
architecture and evolution of telomeric complex in Arabidopsis
