Costa, A. B.; Aid Saada, A.; Pham, T.; Fan, J.; Lever, R.; Zhao, X.; Lobachev, K. S.

Palindromic sequences are a potent source of genomic instability that can lead to cancer and hereditary diseases in humans. Molecular evidence shows that palindrome instability results from the formation of secondary structures, such as hairpins and cruciforms, which are cleaved by structure-specific nucleases. However, the mechanisms underlying cruciform formation and cleavage at palindromic sequences in eukaryotic cells remain incompletely understood. Here, we describe a pathway for secondary structure formation and chromosomal breakage at palindromes involving DNA helicase Mph1, Rad51 recombinase, Rad54 ATPase DNA strand remodeler, Rtt107 scaffold protein, and the multifunctional Smc5/6 complex. Deletion or mutation of any of these components results in a similar reduction in double-strand breaks at an Alu palindrome and a significant decrease in chromosomal rearrangements. We propose that Mph1, Rad51, and Rad54 work together at stalled replication forks to generate cruciform structures via fork remodeling, while Smc5/6 and Rtt107 mark the cruciforms, indicating an appropriate substrate for nuclease cleavage. As members of this pathway are conserved in humans, the uncovered mechanisms may underlie genomic instability at palindrome sites involved in disease etiology.