Nuclease sensitivity of repair-incorporated nucleotides in chromatin and nucleosome rearrangement in human cells damaged by methyl methanesulfonate and methylnitrosourea

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Abstract
We have examined both the initial nuclease sensitivity and subsequent nucleosome rearrangement of newly repaired regions of chromatin in human diploid fibroblasts treated with methyl methanesulfonate (MMS) and methylnitrosourea (MNU). We initially examined the effect of these two alkylating agents on DNA replicative systhesis. The results indicate that immediately following damage by MMS or MNU, at a concentration of 2 mM, the level of replicative systhesis is 20–25% of the level in untreated cells. In the MMS-treated cells, this suppression of replicative synthesis is short lived and by 15 h after damage the level of replicative synthesis is −3-fold greater than that in untreated cells. This ‘latent stimulation’ of replicative synthesis was not observed in the cells treated with 2 mM MNU, although the level of replicative synthesis in these cells did approach the level of untreated cells at later times. When these contributions were corrected for, it was found that the nucleotides incorporated by repair synthesis are initially (i.e., immediately following repair synthesis) both staphylococcal nuclease and DNase I sensitive, and are underrepresented in isolated nucleosome core DNA. Using methods previously described by us, we show that the relative nuclease sensitivity of these regions is quantitatively similar to that of newly repaired DNA following damage by u.v. radiation. Furthermore, the relative nuclease sensitivity of newly repaired DNA is initially high regardless of the time after damage that repair occurs (at least for 13 h after damage). This feature is also similar to u.v. induced repair synthesis. Finally, pulse-chase experiments demonstrated that following repair synthesis induced by MMS or MNU rearrangements of chromatin structure take place, and both the rate and extent of these rearrangements are similar to that observed for cells treated with u.v. radiation or bulky chemical carcinogens. Thus, our results indicate that the excision repair induced by these two small alkylating agents is associated with the same overall chromatin structural features as the excision repair of DNA damage induced by u.v. radiation and ‘u.v.-mimetic’ chemicals.