DNA-Protein Cross-Link Formation in Nucleosome Core Particles Treated with Methyl Methanesulfonate.

N7-Methyl-2'-deoxyguanosine (MdG) is the major damage product in DNA produced by methylating agents, but it often thought to be nontoxic and nonmutagenic. MdG is chemically unstable. An abasic site (AP) is the major product produced from MdG under physiologically relevant conditions. AP formation is frequently considered to be responsible for the cytotoxic effects of MdG, but the reaction is suppressed in nucleosome core particles (NCPs). Recently, it was discovered that histone proteins form reversible DNA-protein cross-links (DPCs) with MdG in reconstituted NCPs, as well as in methylmethanesulfonate (MMS) treated cells. In this study, the formation and reactivity of MdG in MMS treated NCPs was examined at single nucleotide resolution. Sequences consisting of three or more consecutive dGs are more reactive with MMS. The efficiency and selectivity of MdG formation by MMS is largely unaffected within a NCP, although reactivity at several dGs is ∼1.5-2.5-fold higher in NCPs. DPC formation from MdG (DPCMdG) predominates over AP at all positions within the NCP. With few exceptions, DPCMdG yield is strongly dependent upon the accessibility of the major groove containing MdG to lysine-rich histone N-terminal tails. These data indicate that histone-MdG DPC formation will depend upon DNA sequence and translational position within an NCP.