Histone exchange in chromatin of hydroxyurea-blocked Ehrlich ascites tumour cells.

It is well established that DNA and histone synthesis are tightly coupled. Nevertheless, these two processes can be partially uncoupled by drugs specifically inhibiting protein or DNA synthesis, and also during n-butyrate-induced differentiation of Friend cells. The fate of the histones synthesized in the absence of DNA synthesis is unknown; they could be: (1) degraded without joining chromatin; (2) deposited on chromatin as extra histones; or (3) replace original chromatin histones. The only data concerning this problem are a recent report supporting the second possibility. We present evidence here in favour of the third possibility by showing that the histones synthesized in the absence of DNA synthesis enter chromatin and become organized in nucleosomes.