At least 60 ADP‐ribosylated variant histones are present in nuclei from dimethylsulfate‐treated and untreated cells.

The level of histone adenosine diphospho (ADP) ribosylation was studied in isolated nuclei from mouse myeloma cells in culture. The cells were treated with dimethylsulfate (DMS), a DNA‐methylating agent, and histones were analyzed using two‐dimensional gel electrophoresis. Seventeen or more bands probably representing mono‐to heptadeca (ADP‐ribosylated) histones could be visualized for each major variant histone. DMS treatment, by increasing the number of chromatin sites undergoing repair, greatly enhanced histone ADP‐ribosylation. When histones were labeled in a cell lysate rather than in isolated nuclei, mono‐ and oligo(ADP‐ribosylated) histone forms prevailed. The presence of approximately 87 ADP‐ribosylated variant histone forms in cell lysates and of approximately 170 in isolated nuclei is shown for the first time in this work. Previous studies show multiple ADP‐ribosylated forms for only histone H1. The theoretical number of variegated nucleosomes is thus much higher than previously thought, provided that histone‐histone contacts are not disrupted at up to a certain level of histone ADP‐ribosylation.