Rearrangement of nucleosome structure during excision repair in xeroderma pigmentosum (group A) human fibroblasts.

Rearrangements of chromatin structure during excision repair were examined in xeroderma pigmentosum (XP; complementation group A) human fibroblasts treated with the small-molecule alkylating agent methyl methanesulfonate (MMS). In agreement with past reports, we observed normal levels of repair synthesis in these cells during the first 12 h after exposure to 1.5 mM MMS, in contrast to the near zero incorporation of repair patches following exposure to 12 J/m2 u.v. light. Our results indicate that the relative nuclease sensitivity of newly repaired regions in MMS-treated nuclease sensitivity of newly repaired regions in MMS-treated XP (group A) cells is quantitatively similar to that of newly repaired regions in MMS-treated normal human fibroblasts. This enhanced sensitivity is accompanied by a marked under-representation of repair-incorporated nucleotides in isolated nucleosome core DNA. Pulse-chase experiments demonstrated that these regions rapidly undergo rearrangements in chromatin structure, and both the rate and extent of these rearrangements are similar (but not identical) to those observed in normal cells. This was also the case for the rate and extent of ligation of repair patches, as measured by the sensitivity of these regions to exonuclease III digestion. If the changes in nuclease sensitivity of newly repaired regions in DNA reflect an unfolding of nucleosome structure during excision repair, then these results indicate that the activity associated with this unfolding is present in XP (group A) cells.