Cell cycle-dependent expression of canonical histone proteins enables newly synthesized DNA to be integrated into chromatin in replicating cells. However, the molecular basis of cell cycle-dependency in the switching of histone gene regulation remains to be uncovered. This study reports the identification and biochemical characterization of a molecular switcher, HERS (histone gene-specific epigenetic repressor in late S phase), for nucleosomal core histone gene inactivation in Drosophila. HERS protein is phosphorylated by a cyclin-dependent kinase (Cdk) at the end of S-phase. Phosphorylated HERS binds to histone gene regulatory regions and anchors HP1 and Su(var)3-9 to induce chromatin inactivation through histone H3 lysine 9 methylation. These findings illustrate a salient molecular switch linking epigenetic gene silencing to cell cycle-dependent histone production (Ito, 2012).

Histones are fundamental components of chromatin that maintain and regulate appropriate chromatin conformation to support all genomic DNA-dependent processes, including transcription, replication, repair, and mitosis). It is now accepted that histone proteins play a prime role in epigenetic regulation, serving as substrates for chromatin modifications (Ito, 2012).

The genes encoding canonical histones (H1, H2A, H2B, H3, and H4) are present in multiple copies and transcriptionally inactive in the quiescent cell state. Unlike the vast majority of the genes transcribed by RNA polymerase II, multiple copies of the histone genes are organized in gene clusters. In Drosophila, the histone gene cluster is composed of about 100 copies of tandemly arranged nucleosomal core (H2A, H2B, H3, and H4) and linker (H1) histone gene cassettes. The histone gene cluster is localized at a subnuclear compartment, histone locus body (HLB) that is presumed to contain factors essential for coordinated regulation of all histone gene copies (Nizami, 2010; White, 2011). In early S phase of the cell cycle, histone genes are activated to supply histone proteins for the integration of newly synthesized DNA into nucleosomes (De Koning, 2007; Groth, 2007). Coordinated protein production is required for all of the canonical core histones to produce optimal amounts of histone octamers in response to cellular requirements. Therefore, regulatory sequences appear to be common in the nucleosomal core histone gene loci to ensure their synchronized expression (Ito, 2012).

The cell cycle-dependent expression of canonical histones might be achieved through conserted action of cell-cycle, phase-specific nuclear factors at multiple levels of regulation. Recent studies have shown that the nuclear protein ataxia-telangiectasia locus (NPAT) plays a key role in the induction of histone genes. NPAT associates with histone loci and is functionally activated in early S phase by cyclin-dependent kinase 2 (Cdk2)/cyclin E (CycE), a kinase complex known to promote G1/S transition. Recently, fly homeotic gene mxc was identified as Drosophila NPAT ortholog (Rajendra, 2010; White, 2011). At the end of S phase, histone gene transcription is rapidly turned down and histone mRNAs are destabilized, ending the histone protein production. Although it has been assumed that dephosphorylation of NPAT is critical for histone gene silencing, the mechanisms of transcriptional suppression of histone genes remain to be identified (Ito, 2012).

During genetic screening of transcriptional coregulators in Drosophila, a critical factor has been identified that switched off nucleosomal core histone gene expression at the end of S phase through epigenetic inactivation of chromatin. Therefore, this factor was designated histone gene-specific epigenetic repressor in late S-phase (HERS) and presumed to function as a repressor of canonical core histone gene loci (Ito, 2012).

In contrast to NPAT, HERS protein abundance appears to be cell cycle-dependent because HERS was undetectable at the G1/S transition and only emerged in late S phase. Cdk-dependent phosphorylation of HERS is required for its association with core histone gene regulatory regions through sequence-specific DNA binding. This study has identified Cdk1/CycA as a prospective kinase complex that may phosphorylate HERS in vivo. However, because it has not been confirmed whether the Cdk1 inhibitor that was used here does not affect Drosophila Cdk2 or Cdk1/CycB activities, the possibility cannot be excluded that these Cdk complexes may also phosphorylate HERS. Owing to the cell cycle-specific expression and phosphorylation of HERS, its localization on the HIS-C appears to be limited to the completion of replication in the late S phase. This is further supported by HERS localization on the histone gene loci in the polytene chromosomes of salivary glands, in which DNA replication is completed and histone genes are inactive. Moreover, phosphorylation also significantly enhances HERS protein stability. Therefore, it is conceivable that dephosphorylation terminates HERS action and targets the protein for degradation (Ito, 2012).