Coupling of DNA synthesis and histone synthesis in S phase independent of cyclin/cdk2 activity.

David M Nelson,Timothy J Yen,J Wade Harper,Tianlin Ma,Xiaofen Ye,Gary D Kao,Hidelita Santos,Caitlin Hall,Peter D Adams

doi:10.1128/mcb.22.21.7459-7472.2002

David M Nelson, Timothy J Yen + Show 7 more

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https://doi.org/10.1128/mcb.22.21.7459-7472.2002

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Abstract

DNA and histone synthesis are both triggered at the beginning of S phase by cyclin/cdk2 activity. Previous studies showed that inhibition of DNA synthesis with hydroxyurea or cytosine arabinoside (AraC) triggers a concerted repression of histone synthesis, indicating that sustained histone synthesis depends on continued DNA synthesis. Here we show that ectopic expression of HIRA, the likely human ortholog of two cell cycle-regulated repressors of histone gene transcription in yeast (Hir1p and Hir2p), represses transcription of histones and that this, in turn, triggers a concerted block of DNA synthesis. Thus, in mammalian cells sustained DNA synthesis and histone synthesis are mutually dependent on each other during S phase. Although cyclin/cdk2 activity drives activation of both DNA and histone synthesis at the G1/S transition of cycling cells, concerted repression of DNA or histone synthesis in response to inhibition of either one of these is not accompanied by prolonged inhibition of cyclin A/cdk2 or E/cdk2 activity. Therefore, during S phase coupling of DNA and histone synthesis occurs, at least in part, through a mechanism that is independent of cyclin/cdk2 activity. Coupling of DNA and histone synthesis in S phase presumably contributes to the prompt and orderly assembly of newly replicated DNA into chromatin.

Highlights

Progression through the cell cycle is driven by the sequential and periodic activation of cyclin/cdk complexes [23, 27]
Efficient pre-mRNA processing requires the U7 snRNP, whose binding to a purinerich sequence downstream of the stem-loop is facilitated by stem-loop binding protein (SLBP) [13, 16], a heat-labile factor (HLF) [15], and novel zinc finger protein hZFP100, which interacts with the U7 snRNP and SLBP [14]
After prolonged treatment with HU, DNA synthesis and histone synthesis are both repressed but cyclin/cdk2 activity is elevated. These results provide the first experimental evidence to suggest that human HIRA, like Hir1p and Hir2p in yeast, is a cell cycle-regulated repressor of histone gene transcription (Fig. 1 to 3)

Summary

Introduction

Progression through the cell cycle is driven by the sequential and periodic activation of cyclin/cdk complexes [23, 27]. The simultaneous activation of DNA synthesis and histone synthesis by cyclin/cdk activity at the G1/S transition ensures a certain level of coordination between them, in view of the inevitable impact of chromatin structure on the fidelity of nuclear processes, these processes are likely to be regulated in a very closely coordinated and concerted manner throughout S phase Consistent with this idea, the size of the free histone pool in S phase in mammalian somatic cells is very small and hydroxyurea (HU)-mediated inhibition of DNA synthesis in mammalian cells triggers a rapid and concerted destabilization of histone mRNAs, indicating that sustained histone synthesis is dependent on ongoing DNA replication [4, 25, 62, 68]. Whether cyclin/cdk, in addition to its role in simultaneous activation of these events at the G1/S transition, plays a direct role in coordination of these events in S phase is not known

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