Mrel 1p nuclease activity is dispensable for telomeric rapid deletion

Telomeric rapid deletion (TRD) is an intrachromatid recombination process that truncates over-elongated telomeres to the genetically determined average telomere length. We have proposed that TRD is initiated by invasion of the 3' G-rich overhang into centromere-proximal telomere sequence, forming an intermediate that leads to excision of the distal telomere tract. TRD efficiency is dependent on Mrel 1p and Rad50p, two members of the widely conserved Mrel 1p/Rad50p/Xrs2p (MRX) complex. To investigate the role of Mrellp in TRD, we conducted a structure/function analysis by testing the TRD rate and precision of mutations within known functional domains. We analyzed 12 alleles that disrupt different Mrel 1p activities. Surprisingly, mutations in essential residues of the nuclease domain do not inhibit TRD, effectively ruling out nuclease activity as the source of the Mrel 1p requirement. Interestingly, loss of Exolp alone or loss of Exolp in an Mre11 nuclease deficient background does not eliminate TRD, suggesting the presence of an additional nuclease. Second, deletion of DNA binding sites A (residues 410-420) and B (residues 644-692) actually enhances the TRD rate. Even deletion of both DNA binding domains does not abrogate TRD, although its kinetics and precision are variable. This suggests altered DNA binding or a conformational defect in the MRX complex may affect the rate of TRD product formation and indicates that the DNA binding sites formally act as repressors of TRD. Remarkably, the H213Y allele (nuclease motif IV) confers an extraordinarily rapid kinetics, with the vast majority of elongated telomeres deleted imprecisely in a single round of subculturing. In striking contrast, the P162S allele that confers dissolution of the complex also exhibits the null phenotype. These data suggest that Mrel 1p can act as a positive and negative regulator of TRD in context of the MRX complex that is essential for TRD.