Cross-linking of chromosomal non-histone proteins to DNA by UV radiation and some antitumor drugs.

Antibodies reacting specifically with HeLa cell chromatin can be elicited by immunization with dehistonized HeLa chromatin preparations. The nature of these chromatin-associated antigens was investigated by cross-linking with UV irradiation or by in vitro exposure of chromatin to 1-methyl-1-nitrosourea (MNU) or 1,3-bis-(2-chloroethyl)-1-nitrosourea (BCNU). With the exception of 1-methyl-1-nitrosourea the described treatment of chromatin (native or dehistonized) significantly increased its immunological reactivity. Dissociation of the chromosomal proteins from DNA by concentrated salt-urea solutions essentially abolished the immunological reactivity of the residual chromatin pellets. The immunological activity was found in the supernatant protein fraction after its reconstitution with purified human placenta DNA. UV irradiation or alkylation of chromatin cross-linked the active proteins to DNA and prevented their dissociation. It is concluded that the immunologically cell-specific antigens in HeLa chromatin exist as closely associated complexes of chromosomal protein(s) with DNA.