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Molecular and Cellular Biology, December 2003, p. 8820-8828, Vol. 23, No. 23
0270-7306/03/$08.00+0     DOI: 10.1128/MCB.23.23.8820-8828.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Yeast Mre11 and Rad1 Proteins Define a Ku-Independent Mechanism To Repair Double-Strand Breaks Lacking Overlapping End Sequences

Jia-Lin Ma,1 Eun Mi Kim,1 James E. Haber,2 and Sang Eun Lee1*

Department of Molecular Medicine, Institute of Biotechnology, The University of Texas Health Science Center at San Antonio, San Antonio, Texas 78245,1 Rosenstiel Center and Department of Biology, Brandeis University, Waltham, Massachusetts 024542

Received 1 August 2003/ Returned for modification 19 August 2003/ Accepted 27 August 2003

End joining of double-strand breaks (DSBs) requires Ku proteins and frequently involves base pairing between complementary terminal sequences. To define the role of terminal base pairing in end joining, two oppositely oriented HO endonuclease cleavage sites separated by 2.0 kb were integrated into yeast chromosome III, where constitutive expression of HO endonuclease creates two simultaneous DSBs with no complementary end sequence. Lack of complementary sequence in their 3' single-strand overhangs facilitates efficient repair events distinctly different from when the 3' ends have a 4-bp sequence base paired in various ways to create 2- to 3-bp insertions. Repair of noncomplementary ends results in a set of nonrandom deletions of up to 302 bp, annealed by imperfect microhomology of about 8 to 10 bp at the junctions. This microhomology-mediated end joining (MMEJ) is Ku independent, but strongly dependent on Mre11, Rad50, and Rad1 proteins and partially dependent on Dnl4 protein. The MMEJ also occurs when Rad52 is absent, but the extent of deletions becomes more limited. The increased gamma ray sensitivity of rad1{Delta} rad52{Delta} yku70{Delta} strains compared to rad52{Delta} yku70{Delta} strains suggests that MMEJ also contributes to the repair of DSBs induced by ionizing radiation.

* Corresponding author. Mailing address: Department of Molecular Medicine, Institute of Biotechnology, The University of Texas Health Science Center at San Antonio, 15355 Lambda Dr., San Antonio, TX 78245. Phone: (210) 567-7273. Fax: (210) 567-7269. E-mail: lees4{at}uthscsa.edu.

Molecular and Cellular Biology, December 2003, p. 8820-8828, Vol. 23, No. 23
0022-538X/03/$08.00+0     DOI: 10.1128/MCB.23.23.8820-8828.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.



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