Specific association of a small protein with the telomeric DNA-protein complex during the onset of leaf senescence in Arabidopsis thaliana

Telomeres and their changes in length throughout the life span of cells hav e been intensively investigated in different organisms. Telomere length is assumed to control replicative senescence in mammalian cells. However, only very few data are available on the developmental dynamics of plant telomer es. Here, changes of telomere length and DNA-protein structure of Arabidops is thaliana telomeres were analysed in different stages of development, wit h the main focus resting on the transition from pre-senescent to senescent leaves. The lengths of the telomeres, ranging from ca. 2.0 to 6.5 kb, do no t significantly change during plant development indicating that telomere le ngth is not involved in differentiation and replicative senescence nor in p ost-mitotic senescence of A. thaliana. In dedifferentiated cultured cells a slight increase in length can be determined. The nucleoprotein structure o f the telomeric DNA was investigated by gel mobility shift assays, with syn thetic oligonucleotides and nuclear protein extracts derived from four defi ned stages of post-mitotic leaf senescence. In all four stages, a highly sa lt-resistant DNA-protein complex was formed with the double-stranded as wel l as with the single-stranded G-rich telomeric DNA. An additional DNA-prote in complex was identified in nuclear protein extracts isolated from plants in the transition stage from pre-senescence to senescence. The protein comp onents of the DNA-protein complexes were analysed on native PAGE and SDS-PA GE gels. A protein of 67 kDa (ATBP1) bound to the telomeric DNA in all deve lopmental stages. An additional protein of merely 22 kDa (ATBP2) was associ ated via protein-protein interaction with ATBP1 to form a higher-order comp lex exclusively during the onset of senescence. DNA interaction of this hig her-order protein complex seems to be restricted to double-stranded telomer ic DNA. The defined period of ATBP1/ATBP2 complex formation with the telome ric DNA probably indicates that ATBP2 is involved in the onset of post-mito tic leaf senescence by either disturbing an established or establishing an additional function exhibited by the telomeres in the interphase nuclei.