Recombination at long mutant telomeres produces tiny single- and double-stranded telomeric circles

Mol Cell Biol. 2005 Jun;25(11):4406-12. doi: 10.1128/MCB.25.11.4406-4412.2005.

Abstract

Recombinational telomere elongation (RTE) known as alternate lengthening of telomeres is the mechanism of telomere maintenance in up to 5 to 10% of human cancers. The telomeres of yeast mutants lacking telomerase can also be maintained by recombination. Previously, we proposed the roll-and-spread model to explain this elongation in the yeast Kluveromyces lactis. This model suggests that a very small ( approximately 100-bp) circular molecule of telomeric DNA is copied by a rolling circle event to generate a single long telomere. The sequence of this primary elongated telomere is then spread by recombination to all remaining telomeres. Here we show by two-dimensional gel analysis and electron microscopy that small circles of single- and double-stranded telomeric DNA are commonly made by recombination in a K. lactis mutant with long telomeres. These circles were found to be especially abundant between 100 and 400 bp (or nucleotides). Interestingly, the single-stranded circles consist of only the G-rich telomeric strand sequence. To our knowledge this is the first report of single-stranded telomeric circles as a product of telomere dysfunction. We propose that the small telomeric circles form through the resolution of an intratelomeric strand invasion which resembles a t-loop. Our data reported here demonstrate that K. lactis can, in at least some circumstances, make telomeric circles of the very small sizes predicted by the roll-and-spread model. The very small circles seen here are both predicted products of telomere rapid deletion, a process observed in both human and yeast cells, and predicted templates for roll-and-spread RTE.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Chromosomes, Fungal / metabolism
  • DNA / metabolism
  • DNA / ultrastructure
  • DNA Replication
  • DNA, Circular / metabolism*
  • DNA, Circular / ultrastructure
  • DNA, Fungal / metabolism*
  • DNA, Fungal / ultrastructure
  • Kluyveromyces / genetics*
  • Mutation
  • RNA / genetics*
  • Recombination, Genetic*
  • Telomerase / genetics*
  • Telomere / genetics
  • Telomere / metabolism*

Substances

  • DNA, Circular
  • DNA, Fungal
  • telomerase RNA
  • RNA
  • DNA
  • Telomerase