Characterization and expression analysis of the tomato telomere-binding protein LeTBP1

We cloned LeTBP1 cDNA that encoded a putative tomato telomere-binding protein. Sequence analysis revealed that LeTBP1 contained an open reading frame of 2067 bp and encoded a protein consisting of 689 amino acids with a predicted molecular mass of 77.0 kDa. The deduced amino sequence of LeTBP1 indicated that a myb-like motif, a structure particular to double-stranded telomere-binding proteins found in various organisms, existed in the C-terminal region. Southern blot analysis revealed that LeTBP1 was a single-copy gene in the tomato genome. Northern blot analyses showed that LeTBP1 was highly expressed in tissues with high cell division capacities as well as in fully differentiated tissues. The high level of expression of LeTBP1 in inflorescences was independent of flower formation, as shown with mutant inflorescences lacking flowers. However, the level of LeTBP1 mRNA was greatly reduced in fruit. Gel shift assay revealed that LeTBP1^509-689, which contained Gly₅₀₉ to Ala₆₈₉ of LeTBP1, including a myb-like motif, bound specifically to the double-stranded telomeric sequence, indicating that LeTBP1 is a double-stranded telomeric DNA-binding protein. We then transformed tobacco BY-2 cells with LeTBP1 to analyze the effect of LeTBP1 expression on telomere lengths in vivo. Telomere lengths decreased from 15-55 to 15-35 kbp with expression of LeTBP1. Therefore it is most likely that LeTBP1 negatively controls the extension of telomere length in tissues with high cell division capacities although its function is unknown in fully differentiated tissues.