Background: Vegetatively dividing cells of Saccharomyces cerevisiae carrying a mutation in RAD50 grow significantly more slowly in rich medium and are sensitive to DNA damage inflicted by X-ray or chemical mutagens. RAD50 function is essential for the formation and repair of meiosis-specific double-strand breaks and chromosome stability.

Results: We present evidence for two new phenotypes associated with the rad50delta mutant; shortened telomeres and cell senescence. Comparison of TG1-3 telomeric sequences in an isogenic pair of RAD50 and rad50delta haploid strains showed that they were considerably shortened in the latter. Although rad50delta mutation conferred cell enlargement and slow growth, cell doubling was faster but caused an increase in the frequency of cell death. Telomeres were restored to the wild-type size in hemizygous RAD50/rad50delta and rad50S/rad50delta strains; however, they showed a significant increase in rad50S/rad50S diploid with a concomitant rise in cell viability. Telomeres were stabilized in hemizygous RAD50/rad50delta and rad50S/rad50delta diploids during prolonged growth, suggesting that even a half-dosage of RAD50 is sufficient to conserve the telomere size during successive cell divisions. Furthermore, cells bearing the rad50delta mutation revealed abnormalities in nuclear segregation and, in the presence of hydroxyurea, displayed phenotypes consistent with defects in S-phase checkpoint control.

Conclusion: This report presents evidence of the involvement of a gene relevant to recombinational repair in the maintenance of telomeres. We conclude that the phenotypes displayed by yeast rad50delta cells have intriguing similarities among the human cell lines representing DNA repair-deficient chromosome instability syndromes.

Author: Kironmai KM

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