Influence of chlorambucil, a bifunctional alkylating agent, on DNA replication and histone gene expression in HeLa S3 cells.

We have examined the influence of chlorambucil, a bifunctional alkylating agent that inhibits cell proliferation, on DNA replication and histone gene expression in exponentially growing HeLa S3 cells. During the period of treatment with chlorambucil (up to 3 days), neither transcription nor translation in general appeared to be affected, but the incorporation of [14C]thymidine into DNA was reduced to 15% of control values by the third day. The appearance of newly synthesized histones and non-histone proteins on chromatin was inhibited with a time course similar to that for inhibition of DNA synthesis. However, the representation of histone messenger RNA sequences in various cellular compartments did not appear to be affected by chlorambucil treatment, in contrast to the loss of histone messenger RNA sequences from polyribosomes following the more rapid inhibition of DNA and histone synthesis by 1-beta-D-arabinofuranosylcytosine or hydroxyurea. The possibility is considered that chlorambucil interferes with histone gene expression at posttranscriptional or posttranslational levels. We also conclude that the inhibition of DNA synthesis by chlorambucil is most probably an indirect effect, a result of the inhibition of cells in the G2 phase of the cell cycle.