Restricting the ligation step of non-homologous end-joining

Regulation of non-homologous end joining via post-translational modifications of components of the ligation step

DNA double-strand breaks are the most serious type of DNA damage and non-homologous end joining (NHEJ) is an important pathway for their repair. In Saccharomyces cerevisiae, three complexes mediate the canonical NHEJ pathway, Ku (Ku70/Ku80), MRX (Mre11/Rad50/Xrs2) and DNA ligase IV (Dnl4/Lif1). Mammalian NHEJ is more complex, primarily as a consequence of the fact that more factors are involved in the process, and also because higher chromatin organization and more complex regulatory networks exist in mammals. In addition, a stronger interconnection between the NHEJ and DNA damage response (DDR) pathways seems to occur in mammals compared to yeast. DDR employs multiple post-translational modifications (PTMs) of the target proteins and mutual crosstalk among them to ensure highly efficient down-stream effects. Checkpoint-mediated phosphorylation is the best understood PTM that regulates DDR, although recently SUMOylation has also been shown to be involved. Both phosphorylation and SUMOylation affect components of NHEJ. In this review, we discuss a role of these two PTMs in regulation of NHEJ via targeting the components of the ligation step.

Timing is everything: cell cycle control of Rad52

Regulation of the repair of DNA double-strand breaks by homologous recombination is extremely important for both cell viability and the maintenance of genomic integrity. Modulation of double-strand break repair in the yeast Saccharomyces cerevisiae involves controlling the recruitment of one of the central recombination proteins, Rad52, to sites of DNA lesions. The Rad52 protein, which plays a role in strand exchange and the annealing of single strand DNA, is positively regulated upon entry into S phase, repressed during the intra-S phase checkpoint, and undergoes posttranslational modification events such as phosphorylation and sumoylation. These processes all contribute to the timing of Rad52 recruitment, its stability and function. Here, we summarize the regulatory events affecting the Rad52 protein and discuss how this regulation impacts DNA repair and cell survival.