Alternative Lengthening of Telomeres is dependent on <i>RAD52</i> and <i>RAD51</i> gene function in the budding yeast <i>Naumovozyma castellii</i>

Telomerase is a specialized ribonucleoprotein enzyme responsible for elongation of telomeres, the DNA ends of eukaryotic chromosomes. In the absence of telomerase activity, telomeres progressively shorten, eventually triggering a DNA damage response that leads the cell to halt the cell cycle, a process known as replicative senescence. In budding yeast, the telomerase core enzyme consists of Est2, the catalytic subunit that acts as a reverse transcriptase, in complex with the non-coding telomerase RNA template (TLC1). Saccharomyces cerevisiae cells lacking any of these two components exhibit a severe growth crisis, which only few cells manage to survive by activating Alternative Lengthening of Telomeres (ALT) pathways. In contrast, telomerase-deficient strains of the budding yeast Naumovozyma castellii can bypass senescence, by rapidly activating an ALT mechanism that allows the cells to maintain short and stable telomeres. The process involves a spreading of the telomere-adjacent TelKO element, leading to homogenization of the subtelomeric regions. In this study we investigated the genetic requirements of the ALT mechanism in N. castellii, by analyzing deletion mutants of several genes involved in DNA recombination. By applying a quantitative growth assay, we demonstrate that the establishment of the ALT cells requires RAD52 and RAD51 gene function, while the RAD50 and RAD59 genes are not essential. This implies that the N. castellii ALT mechanism is dependent on homologous recombination with a similar genetic basis as the type I pathway in S. cerevisiae, and shows that subtelomeric regions are playing a key role in the effective activation of ALT.