An evolutionary change in telomere sequence motif within the plant section Asparagales had significance for telomere nucleoprotein complexes

Abstract

In association with a phylogenetic tree of Asparagales, our previous results showed that a distinct clade included plant species where the ancestral, Arabidopsis-type of telomeric repeats (TTTAGGG)n had been partially, or fully, replaced by the human-type telomeric sequence (TTAGGG)n. Telomerases of these species synthesize human repeats with a high error rate in vitro. Here we further characterize the structure of telomeres in these plants by analyzing the overall arrangement of major and minor variants of telomeric repeats using fluorescence in situ hybridization on extended DNA strand(s). Whilst the telomeric array is predominantly composed of the human variant of the repeat, the ancestral, Arabidopsis-type of telomeric repeats was ubiquitously observed at one of the ends and/or at intercalary positions of extended telomeric DNAs. Another variant of the repeat typical of Tetrahymena was observed interspersed in about 20% of telomerics. Micrococcal nuclease digestions indicated that Asparagales plants with a human-type of telomere have telomeric DNA organised into nucleosomes. However, unexpectedly, the periodicity of the nucleosomes is not significantly shorter than bulk chromatin as is typical of telomeric chromatin. Using electrophoretic mobility shift assays we detected in Asparagales plants with a human type of telomere a 40-kDa protein that forms complexes with both Arabidopsis- and human-type G-rich telomeric strands. However, the protein shows a higher affinity to the ancestral Arabidopsis-type sequence. Two further proteins were found, a 25-kDa protein that binds specifically to the ancestral sequence and a 15-kDa protein that binds to the human-type telomeric repeat. We discuss how the organisation of the telomere repeats in Asparagales may have arisen and stabilised the new telomere at the point of mutation.

Renaturation of telomere-binding proteins after the fractionation by SDS-polyacrylamide gel electrophoresis

“…Sýkorová et al (2003) found that in several families of this order, the "typical" (A. thaliana-like) telomeres are partially or fully replaced by alternative telomere sequences. Consistent with this change, our previous results show changes in telomeric chromatin structure of these plants (Rotková et al 2004). The present method of rapid renaturation of DNA-binding proteins after their separation by SDS-polyacrylamide gel electrophoresis can further elucidate changes to cope with the telomere DNA sequence change, which happens at the proteomic level.…”

Telomeres in evolution and evolution of telomeres

“…Many of the plants identified by Sykorova et al (2003c) with substantial (TTAGGG)n sequences also had significant (although less) Arabidopsis and Tetrahymena-type repeats which co-localised with the telomeric array of TTAGGG repeats (Sykorova et al 2003c, Rotkova et al 2004, Fajkus et al 2005. Nevertheless the terminal positions of the vertebratetype repeat at the telomeres was confirmed by Bal31 exonuclease digestion and terminal restriction fragment analysis of Othocallis siberica genomic DNA (Weiss-Schneeweiss et al 2004).…”

“…The telomerase Bmistakes^could have had a role in the adaptation of telomere-binding proteins. Analysis of putative telomere-binding proteins in Muscari armeniacum and Scilla peruviana (both with vertebrate-type telomere motifs) has shown proteins with affinity to both Arabidopsistype and vertebrate-type telomere motifs (Rotkova et al 2004). It can be envisaged that at the potentially catastrophic event in the ancestor to Asparagales when telomerase mutated to generate an alternative telomere sequence, survival depended on either the rapid co-evolution of telomeric proteins, or sufficient similarity between the old and the new type of telomere for telomeric function to be maintained.…”

Analysis of telomere length and telomerase activity in tree species of various life-spans, and with age in the bristlecone pine Pinus longaeva

“…Telomeres in nearly all plants and trees consist of the heptanucleotide repeat (TTTAGGG) n (Ganal et al 1991;McKnight et al 1997), although recent research has suggested that the plant order Asparagales have nearly all TTAGGG telomeric repeats (Sykorova et al 2003;Rotkova et al 2004). Telomere length can be used as a predictor of the future replicative capacity of cells , and depends on both the age of the cell and the number of times the cell has already divided (Harley et al 1990).…”