Changes in 3H-thymidine incorporation into the DNA by histones from normal and tumor tissues

Attempts were made to isolate a factor from tumor tissue which, after injection, enhances the 3H-thymidine incorporation into the DNA of mice. This factor was in the cell nuclei and was extractable together with acid-soluble nuclear proteins (histones). Additional purification showed that this factor was either closely bound with the very lysine-rich histone (F1), or it was this histone itself. Acid-soluble nuclear proteins prepared from liver and thymus did not influence the incorporation of 3H-thymidine into DNA after injection. The very lysine-rich histone (F1) in these extracts was in an inactive form. It could be activated by separation from other acid-soluble nuclear proteins. Purified, the very lysine-rich histone (F1) enhanced the 3H-thymidine incorporation into DNA, regardless of its being prepared from tumor or nontumor tissues. By enhancing the incorporation of 3H-thymidine into DNA after injection, the very lysine-rich histone (F1) significantly differed from arginine-rich (F2aF3) and lysine-rich (F2b) histones which suppressed the 3H-thymidine incorporation.