Different modes of de novo telomere formation by plant telomerases

The telomerase reverse transcriptase can recognize broken chromosome ends a nd add new telomeres de novo in a reaction termed 'chromosome healing'. Her e we investigate new telomere formation in vitro by telomerases from a vari ety of flowering plant species. Comparing the electrophoretic mobilities an d nucleotide sequences of the products, we uncovered three different modes of new telomere formation. The soybean telomerase, designated a Class I enz yme, only elongated DNA primers ending in telomeric nucleotides. Arabidopsi s and maize telomerases, designated Class II enzymes, efficiently extended completely non-telomeric sequences by positioning the 3' terminus at a pref erred site on the RNA template. Silene latifolia and sorghum telomerases co nstituted class III enzymes that elongated non-telomeric DNA primers by ann ealing them at alternative sites on the RNA template. For all enzymes, erro rs were prevalent during synthesis of the first two repeats, likely reflect ing lateral instability of the primer 3' terminus on the template during th e initial rounds of elongation. Class III telomerases, however, were five- to 13-fold more error prone than class II, generating more mistakes in dist al repeats added to the primers. This remarkable variability in enzyme-DNA interactions among plant telomerases does not reflect phylogenetic relation ships, and therefore implies that the telomerase active site can evolve rap idly.