Alternative Lengthening of Telomeres in the Budding Yeast Naumovozyma castellii

The enzyme telomerase ensures the integrity of linear chromosomes by maintaining telomere length. As a hallmark of cancer, cell immortalization and unlimited proliferation is gained by reactivation of telomerase. However, a significant fraction of cancer cells instead uses alternative telomere lengthening mechanisms to ensure telomere function, collectively known as Alternative Lengthening of Telomeres (ALT). Although the budding yeast Naumovozyma castellii (Saccharomyces castellii) has a proficient telomerase activity, we demonstrate here that telomeres in N. castellii are efficiently maintained by a novel ALT mechanism after telomerase knockout. Remarkably, telomerase-negative cells proliferate indefinitely without any major growth crisis and display wild-type colony morphology. Moreover, ALT cells maintain linear chromosomes and preserve a wild-type DNA organization at the chromosome termini, including a short stretch of terminal telomeric sequence. Notably, ALT telomeres are elongated by the addition of ∼275 bp repeats containing a short telomeric sequence and the subtelomeric DNA located just internally (TelKO element). Although telomeres may be elongated by several TelKO repeats, no dramatic genome-wide amplification occurs, thus indicating that the repeat addition may be regulated. Intriguingly, a short interstitial telomeric sequence (ITS) functions as the initiation point for the addition of the TelKO element. This implies that N. castellii telomeres are structurally predisposed to efficiently switch to the ALT mechanism as a response to telomerase dysfunction.