text: Chromatin relaxation is one of the earliest cellular responses to DNA damage. Several experiments have been performed on the recruitment kinetics of proteins involved in the response to DNA damage. The relaxation appears to be initiated by PARP1, whose accumulation at DNA damage is half complete by 1.6 seconds after DNA damage occurs. This is quickly followed by accumulation of chromatin remodeler Alc1, which has an ADP-ribose–binding domain, allowing it to be quickly attracted to the product of PARP1. The maximum recruitment of Alc1 occurs within 10 seconds of DNA damage. About half of the maximum chromatin relaxation, presumably due to action of Alc1, occurs by 10 seconds. PARP1 action at the site of a double-strand break allows recruitment of the two DNA repair enzymes MRE11 and NBS1. Half maximum recruitment of these two DNA repair enzymes takes 13 seconds for MRE11 and 28 seconds for NBS1.
  Another process of chromatin relaxation, after formation of a DNA double-strand break, employs γH2AX, the phosphorylated form of the H2AX protein.  The histone variant H2AX constitutes about 10% of the H2A histones in human chromatin. γH2AX (phosphorylated on serine 139 of H2AX) was detected at 20 seconds after irradiation of cells (with DNA double-strand break formation), and half maximum accumulation of γH2AX occurred in one minute. The extent of chromatin with phosphorylated γH2AX is about two million base pairs at the site of a DNA double-strand break.
  γH2AX does not, by itself, cause chromatin decondensation, but within seconds of irradiation the protein "Mediator of the DNA damage checkpoint 1" (MDC1) specifically attaches to γH2AX. This is accompanied by simultaneous accumulation of RNF8 protein and the DNA repair protein NBS1 which bind to MDC1 as MDC1 attaches to γH2AX. RNF8 mediates extensive chromatin decondensation, through its subsequent interaction with CHD4 protein, a component of the nucleosome remodeling and deacetylase complex NuRD. CHD4 accumulation at the site of the double-strand break is rapid, with half-maximum accumulation occurring by 40 seconds after irradiation.
  The fast initial chromatin relaxation upon DNA damage (with rapid initiation of DNA repair) is followed by a slow recondensation, with chromatin recovering a compaction state close to its pre-damage level in ~ 20 min.