Regulation of human histone gene expression: transcriptional and posttranscriptional control in the coupling of histone messenger RNA stability with DNA replication

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Abstract
The extent to which transcription and posttranscriptional regulation contributes to the coupling of histone gene expression and DNA replication was examined during the cell cycle in synchronized HeLa S3 cells. Rates of transcription were determined in vitro in isolated nuclei. A 3-5-fold increase in cell cycle dependent histone gene transcription was observed in early S phase, prior to the peak of DNA synthesis. This result is consistent with a previous determination of histone mRNA synthesis in intact cells [Plumb, M., Stein, J., and Stein, G. (1983) Nucleic Acids Res. 11, 2391]. The transcription of these genes did not change appreciably after inhibition of DNA replication by hydroxyurea treatment, although Northern blot analysis indicated that cellular levels of histone mRNA decreased rapidly in the presence of the drug. Total cellular levels of histone mRNA closely parallel the rate of DNA synthesis as a function of cell cycle progression, reaching a maximal 20-fold increase as compared with non S phase levels. This DNA synthesis dependent accumulation of histone mRNA occurs predominantly in the cytoplasm and appears to be mediated primarily by control of histone mRNA stability. Changes in nuclear histone mRNA levels were less pronounced. These combined observations suggest that both transcriptional regulation and posttranscriptional regulation contribute toward control of the cell cycle dependent accumulation of histone mRNA during S phase, while the stability of histone mRNA throughout S phase and the selective turnover of histone mRNAs, either at the natural termination of S phase or following inhibition of DNA synthesis, are posttranscriptionally regulated. The requirement of protein synthesis for histone mRNA turnover is consistent with the involvement of histone protein or a putative short-lived regulatory molecule in the stability of histone mRNAs during S phase and/or in the selective turnover of histone mRNAs in the absence of DNA synthesis.