End joining occurs rapidly, with only minimal processing of the DNA ends to render them liga table and limited polymerization. When D NHEJ fails, locally in repair proficient cells, and globally in mutants with defects in D NHEJ components, or in cells treated with DNA PK inhibitors, an alternative form of end joining operating as backup to D NHEJ becomes activated. B NHEJ utilizes selleckchem Lig3 and Parp1, but also histone H1 as a stabilizing factor and BCR/Abl as a regulatory component. Also components of the DNA end resection apparatus such as the MRN complex and CtIP are implicated in B NHEJ. B NHEJ contributes to important cellular functions. It robustly supports class switch recombination at the Ig locus, and V J recombination in B cells har boring mutant forms of Rag1 and Rag2 that release unrejoined ends for processing by pathways other than D NHEJ.

B NHEJ also supports telomere mainte nance. On the negative hand, B NHEJ is directly implicated in the formation of chromosome aberrations and thus also in carcinogenesis. B NHEJ shows dependence throughout the cell cycle that is fundamentally different from that of other DSB re pair pathways. It is well documented that D NHEJ operates throughout the cell cycle and homologous recom bination repair only during the S and G2 phase of the cell cycle, where a sister chromatid becomes available. In contrast, B NHEJ remains active throughout the cell cycle, like D NHEJ, but shows a marked enhancement du ring the G2 phase like HRR. An additional and probably more intriguing feature of B NHEJ is the strong growth state dependence it shows.

Thus, B NHEJ is mark edly compromised in cells that enter the plateau phase of growth. This effect has been recently reproduced in cultures deprived of serum. The reduction of B NHEJ activity in non cycling cells is profound and comparable to that observed for D NHEJ between Ku70/Ku80 or Lig4 mutants and wild type cells. It suggests important AV-951 regula tory mechanisms that remain to be elucidated. The present work is conceived as an attempt to elucidate parameters underpinning this response and focuses on chromatin con formation as a possible modulator of B NHEJ efficiency. Changes in chromatin conformation facilitate several DNA repair pathways and play a central role in DNA damage signaling. Histone H1 features as a stimulatory factor of B NHEJ in a biochemical screen and heterochromatin is thought to present a barrier that determines DSB repair pathway selection. Yet, the role of chromatin conformation and chromatin compactness in B NHEJ remains unknown, although it may partly underpin the marked efficiency fluctuations observed with cell cycle phase and growth state. Histone acetylation, together with DNA methylation, plays a crucial role in chromatin dynamics.