Telomerase- and Rad52-Independent Immortalization of Budding Yeast by an Inherited-Long-Telomere Pathway of Telomeric Repeat Amplification

Abstract

In the absence of telomerase, telomeres erode, provoking accumulation of DNA damage and death by senescence. Rare survivors arise, however, due to Rad52-based amplification of telomeric sequences by homologous recombination. The present study reveals that in budding yeast cells, postsenescence survival relying on amplification of the TG 1-3 telomeric repeats can take place in the absence of Rad52 when overelongated telomeres are present during senescence (hence its designation ILT, for inherited-long-telomere, pathway). By growth competition, the Rad52-independent pathway was almost as efficient as the Rad51-and Rad52-dependent pathway that predominates in telomerase-negative cells. The ILT pathway could also be triggered by increased telomerase accessibility before telomerase removal, combined with loss of telomere protection, indicating that prior accumulation of recombination proteins was not required. The ILT pathway was dependent on Rad50 and Mre11 but not on the Rad51 recombinase and Rad59, thus making it distinct from both the type II (budding yeast ALT [alternative lengthening of telomeres]) and type I pathways amplifying the TG 1-3 repeats and subtelomeric sequences, respectively. The ILT pathway also required the Rad1 endonuclease and Elg1, a replication factor C (RFC)-like complex subunit, but not Rad24 or Ctf18 (two subunits of two other RFC-like complexes), the Dnl4 ligase, Yku70, or Nej1. Possible mechanisms for this Rad52-independent pathway of telomeric repeat amplification are discussed. The effects of inherited long telomeres on Rad52-dependent recombination are also reported.

Telomere Maintenance and Survival in Saccharomyces cerevisiae in the Absence of Telomerase and RAD52

“…We noted that S288C est2D rad52D colonies survived at an increased frequency if S288C est2D cells had been propagated in culture to generate type II survivors prior to deletion of RAD52, compared with removal of both genes simultaneously (p,0.01). This result is in accord with a recent finding by Grandin and Charbonneau (2009) that the generation of type II survivors permits survival upon the subsequent deletion of RAD52 (see Note added in revision).…”

Telomerase reverse transcriptase-dependent telomere equilibration mitigates tissue dysfunction in mTert heterozygotes

“…The impact of the initial telomere length on subsequent telomere dynamics is not without precedent. In S. cerevisiae strains that possess longer telomeres, rare cells can survive the loss of telomerase and RAD52, this survival does not occur in populations with shorter telomeres (Grandin and Charbonneau, 2009;LeBel et al, 2009). Our data suggest that the response to prolonged telomerase heterozygosity could similarly be influenced by the initial telomere length.…”

Polymerase Epsilon Is Required To Maintain Replicative Senescence

“…Budding yeast cells from most-genetic backgrounds do not form ALTOS survivors in the absence of Rad52. Instead, they enter senescence and remain senescent (13). Yet, telomerase-and ALTOS-negative cells can escape from senescence when nucleases like Exo1 or helicases like Sgs1 are inactivated (18,28).…”

“…We introduce the term "ALTOS" to describe the outcome of any pathway able to maintain/elongate (sub) telomeres in the absence of telomerase. ALTOS survivors could be either Rad52 dependent or Rad52 independent (13,17). Budding yeast cells from most-genetic backgrounds do not form ALTOS survivors in the absence of Rad52.…”