Roles of NtGTBP1 in telomere stability.

Telomeres are protective nucleoprotein structures at the ends of linear eukaryotic chromosomes. In contrast to double-stranded-specific telomere-binding proteins, the cellular roles of single-stranded-specific telomeric proteins are not well understood in higher plants. Three highly conserved tobacco G-strand-specific telomere-binding protein paralogs (NtGTBP1, NtGTBP2, and NtGTBP3) were identified and characterized. All three NtGTBPs were able to bind specifically to the plant single-stranded telomeric repeat elements in vitro with similar affinities. Suppression of NtGTBP1 by means of the RNAi-mediated gene knock-down method resulted in developmental defects in transgenic tobacco plants accompanied by lengthened telomeres, extra-chromosomal telomeric circles, and abnormal anaphase bridges. These results suggest that the down-regulation of NtGTBP1 results in genome instability. NtGTBP1 prevented in vitro strand invasion, a prerequisite process for inter-chromosomal telomeric recombination. Therefore, tobacco NtGTBP1 is one of the essential factors for telomere stability. Because abnormal telomeric elongation and recombination due to the suppression of NtGTBP1 are reminiscent of the recombinational telomere lengthening mechanism that purportedly operates in telomerase negative cancer cells, it is of interest to investigate whether telomeric recombination is associated with cell death in animal systems.