Analysis of the G-overhang structures on plant telomeres: evidence for twodistinct telomere architectures

Telomeres are highly conserved structures essential for maintaining the int egrity of eukaryotic genomes. In yeast, ciliates and mammals, the G-rich st rand of the telomere forms a 3' overhang on the chromosome terminus. Here w e investigate the architecture of telomeres in the dicot plants Silene lati folia and Arabidopsis thaliana using the PENT (primer extension/nick transl ation) assay. We show that both Arabidopsis and Silene telomeres carry G-ov erhangs longer than 20-30 nucleotides. However, in contrast to yeast and ci liate telomeres, only half of the telomeres in Silene seedlings possess det ectable G-overhangs. PENT reactions using a variety of primers and reaction conditions revealed that the remaining fraction of Silene telomeres carrie s either no overhangs or overhangs less than 12 nucleotides in length. G-ov erhangs were observed in Silene seeds and leaves, tissues that lack telomer ase activity. These findings suggest that incomplete DNA replication of the lagging strand, rather than synthesis by telomerase, is the primary mechan ism for G-overhang synthesis in plants. Unexpectedly, we found that the fra ction of telomeres with detectable G-overhangs decreased from 50% in seedli ngs to 35% in leaves. The difference may reflect increased susceptibility o f the G-overhangs to nuclease attack in adult leaves, an event that could a ct as a precursor for the catabolic processes accompanying leaf senescence.