Mdt1 Facilitates Efficient Repair of Blocked DNA Double-Strand Breaks and Recombinational Maintenance of Telomeres

Abstract

DNA recombination plays critical roles in DNA repair and alternative telomere maintenance. Here we show that absence of the SQ/TQ cluster domain-containing protein Mdt1 (Ybl051c) renders Saccharomyces cerevisiae particularly hypersensitive to bleomycin, a drug that causes 3′-phospho-glycolate-blocked DNA double-strand breaks (DSBs). mdt1Δ also hypersensitizes partially recombination-defective cells to camptothecin-induced 3′-phospho-tyrosyl protein-blocked DSBs. Remarkably, whereas mdt1Δ cells are unable to restore broken chromosomes after bleomycin treatment, they efficiently repair “clean” endonuclease-generated DSBs. Epistasis analyses indicate that MDT1 acts in the repair of bleomycin-induced DSBs by regulating the efficiency of the homologous recombination pathway as well as telomere-related functions of the KU complex. Moreover, mdt1Δ leads to severe synthetic growth defects with a deletion of the recombination facilitator and telomere-positioning factor gene CTF18 already in the absence of exogenous DNA damage. Importantly, mdt1Δ causes a dramatic shift from the usually prevalent type II to the less-efficient type I pathway of recombinational telomere maintenance in the absence of telomerase in liquid senescence assays. As telomeres resemble protein-blocked DSBs, the results indicate that Mdt1 acts in a novel blocked-end-specific recombination pathway that is required for the efficiency of both drug-induced DSB repair and telomerase-independent telomere maintenance.

We thank Nora Tenis and Tricia Lo Liang for assistance with blot analyses; Trevor Lithgow and his laboratory for access to the Singer yeast dissection microscope; Brenda Andrews, Lorraine Symington, Greg Cost, Rodney Rothstein, and Jim Haber for yeast strains; and Ana Traven and Monique Smeets for comments on the manuscript.

This work was supported by grants from the National Health and Medical Research Council of Australia to J.H. and a grant from the Harold and Cora Brennen Benevolent Trust for purchase of PFGE equipment.