Subtelomeric TelKO Elements Stabilize Short Telomeres in Telomerase-Lacking Yeast

When yeast cells lose telomerase, the enzyme that normally replenishes the protective caps at the ends of chromosomes, most lineages face a slow slide toward senescence. Yet the budding yeast Naumovozyma castellii manages this crisis with unusual efficiency, switching on a recombination-based backup system known as the Alternative Lengthening of Telomeres, or ALT, pathway and continuing to grow with barely a hiccup. A new study published in Current Genetics reveals how these telomerase-negative cells pull it off: a subtelomeric DNA element called TelKO spreads to every chromosome end, where it appears to lock the shortened telomeres into a stable, functional configuration.

Each passage corresponds to roughly 20 to 25 generations, and the experiments extended across up to 16 streaks. Using terminal restriction fragment analysis, a technique that cuts genomic DNA with the restriction enzyme HinfI and probes the terminal fragments, the researchers watched the telomeric signal shrink from the wild-type smear of about 600 base pairs down to a sharp, distinct band of roughly 350 base pairs within the first few streaks, corresponding to about 60 to 80 generations after telomerase disruption.

What happened next was the striking part. That shortened band did not drift or blur as generations accumulated. It stayed put, narrow and uniform, through extensive further passaging. When the team examined ten independent colonies picked from individual streaks of both diploid and haploid ALT strains, every colony showed the same distinct telomeric band, indicating that the terminal structure established early in each lineage is faithfully inherited and maintained genome-wide. Occasional exceptions appeared as ladder patterns of bands with regular increments, the signature of rare, dramatic telomere elongation events, but these were sporadic and sometimes vanished without a trace in subsequent passages.

To find out exactly which DNA sequences sat at the chromosome tips, the researchers used Telomere-PCR, a method that tails the genomic DNA ends and amplifies the terminal sequences for sequencing. This uncovered two length variants of the TelKO element: a 445 base pair version, TelKO445, and a 220 base pair version, TelKO220, which is a truncated form of the longer one. Both variants proved capable of maintaining ALT telomeres, and within any given ALT lineage, one specific variant predominated. In one diploid strain, the band pattern even shifted cleanly from the TelKO220-associated size to the TelKO445-associated size during passaging, suggesting that one variant had replaced the other across most telomeres.

The location of these elements at the actual chromosome ends was confirmed with a BAL-31 nuclease assay, which digests DNA inward from the termini. Telomeric signals and TelKO-hybridizing signals disappeared on a similar time course in both wild-type and ALT strains, demonstrating that the TelKO elements remain terminally positioned even after the ALT pathway has taken over telomere maintenance.

PacBio whole-genome sequencing provided the most complete picture. In the wild-type strain, the team could retrieve telomere-adjacent sequence from 18 of the 20 chromosome ends and found a variable landscape: seven ends lacked TelKO elements entirely, while the rest carried single copies of either TelKO220 or TelKO445, with some partial extensions of the longer variant. In an established ALT strain, by contrast, every chromosome end had acquired the same TelKO220 variant, including ends that had lacked any TelKO element in the wild type. Even finer detail emerged here: the wild-type strain carries two alleles of TelKO220 differing by a single nucleotide, but only one of these alleles was present at all telomeres of the ALT strain. The subtelomeres had undergone a thorough homogenization.

The sequencing also explained the ladder patterns. Some ALT telomeres contained long tandem arrays built from a 325 base pair higher-order repeat unit, in which the TelKO220 element is flanked by telomeric sequences and short 13 base pair segments identical to the end of the TelKO element. This composite unit was completely identical across arrays at different chromosome ends, which accounts for the uniform band increments in the TRF ladders and implies that the elongation mechanism repeatedly copies the very same sequence block. Different ALT strains showed ladders with different increment sizes, indicating that each population uses its own characteristic repeat unit.

Why would a subtelomeric sequence stabilize a telomere? The clue lies in the telomere-binding protein Rap1, which in N. castellii normally recognizes a 12 base pair minimal binding site within the double-stranded telomeric repeats. The TelKO element has a slight TG-rich strand bias, reaching a combined thymine and guanine content of 82 percent in its TelKO220 portion, and it is scattered with short interstitial telomeric sequences. In electrophoretic mobility shift assays, recombinant Rap1 bound to four oligonucleotides spanning the TelKO region, though with far lower affinity than to canonical telomeric sequence. Competition experiments showed that 400 to 4000 times molar excess of the subtelomeric competitors was needed to challenge Rap1 binding to a labeled telomeric probe, with quantification indicating roughly 30 to 90 times more signal remaining in the shifted band compared with the telomeric competitor. Interestingly, the strongest subtelomeric binder, ST-4, contained only a 6 base pair interstitial telomeric sequence, hinting that flanking TG-rich stretches help accommodate the protein’s known spatial flexibility.

The authors propose a model in which these low-affinity Rap1 binding sites become accessible as telomeres shorten and the high-affinity telomeric sites dwindle. Cooperative binding by Rap1 molecules on the remaining short telomeric stretch could then stabilize Rap1 recruitment to the TelKO element, extending the protective telomeric chromatin cap inward into the subtelomeric region. Consistent with the plausibility of cap formation, previous work from the same group showed that ALT strains retain telomeric single-stranded 3′ overhangs with a preferred permutation similar to wild-type cells. The researchers caution that they cannot exclude the possibility that the shortened telomeres are stable independently, but they note that short telomeres are highly unstable across many species, making the chromatin-cap hypothesis an attractive explanation.

The findings carry broader resonance because ALT is not a yeast curiosity. About 10 to 15 percent of human tumor types rely on ALT pathways rather than telomerase reactivation to keep proliferating, and therapeutic approaches targeting ALT are under development. In the fission yeast Schizosaccharomyces pombe, amplification of subtelomeric elements similarly stabilizes linear chromosomes in rare telomerase-deficient survivors, and in Saccharomyces cerevisiae, strains engineered to carry Y’ elements on all telomeres bypass the senescence crisis altogether. The N. castellii work adds a particularly clean example of how the sequence content of subtelomeres can dictate which recombinational route a cell takes and how stably the resulting telomeres are maintained. Because the TelKO variants act as natural tags, the authors anticipate that this system will serve as a tractable model for dissecting the molecular mechanics of ALT telomere dynamics, including the possible roles of extrachromosomal circular DNA templates and break-induced replication in spreading repeat arrays from one chromosome end to another.

Subject of Research: Subtelomeric TelKO element spreading stabilizes shortened telomeres in telomerase-negative budding yeast

Article Title: Subtelomeric elements provide stability to short telomeres in telomerase-negative cells of the budding yeast Naumovozyma castellii

Article References: Jaiswal, R. K., Garibo Domingo, T., Grunchec, H., Singh, K., Pirooznia, M., Elhaik, E., & Cohn, M. (2025). Subtelomeric elements provide stability to short telomeres in telomerase-negative cells of the budding yeast Naumovozyma castellii. Current Genetics, 71(1), Article 19. https://doi.org/10.1007/s00294-025-01325-w

Image Credits: AI Generated

DOI: 10.1007/s00294-025-01325-w

Keywords: telomeres, ALT pathway, Naumovozyma castellii, subtelomeres, TelKO element, Rap1, telomerase, recombination, yeast genetics, genome stability, Subtelomeric, elements