Control of cell cycle transcription during G1 and S phases

Abstract

The accurate transition from G1 phase of the cell cycle to S phase is crucial for the control of eukaryotic cell proliferation, and its misregulation promotes oncogenesis. During G1 phase, growth-dependent cyclin-dependent kinase (CDK) activity promotes DNA replication and initiates G1-to-S phase transition. CDK activation initiates a positive feedback loop that further increases CDK activity, and this commits the cell to division by inducing genome-wide transcriptional changes. G1-S transcripts encode proteins that regulate downstream cell cycle events. Recent work is beginning to reveal the complex molecular mechanisms that control the temporal order of transcriptional activation and inactivation, determine distinct functional subgroups of genes and link cell cycle-dependent transcription to DNA replication stress in yeast and mammals.

Figure 1. G1–S transcriptional activation

a | Schematic showing how the G1–S transcriptional programme, once initiated, is reinforced by a positive feedback loop. b | In mammalian cells, the transcriptional repressors RB, p107 and p130 (collectively known as pocket proteins) are bound to E2F transcription factors to repress expression during early G1. Pocket proteins either prevent activator E2F proteins (such as E2F1, E2F2 and E2F3) to activate transcription or function as co-repressors for repressor E2F proteins (such as E2F4). Phosphorylation of pocket proteins by cyclin D– cyclin-dependent kinase 4 (CDK4) and cyclin D–CDK6 probably releases them from the E2F transcription factors. This induces the transcription of G1–S target gene, including the gene encoding cyclin E. Cyclin E–CDK2 phosphorylates pocket proteins, thereby providing a positive feedback loop. c | Model depicting G1–S transcriptional activation in budding yeast. In early G1, transcription is inhibited by Whi5 binding to the SBF (SCB-binding factor) complex at target promoters. Cln3–Cdk relieves transcriptional inhibition by phosporylating Whi5, which induces its nuclear export and thereby G1–S transcription. Activation of transcription results in the accumulation of Cln1 and Cln2, which in complex with Cdk, further inactivate Whi5 through phosphorylation. This provides positive feedback that results in cell cycle commitment.

Figure 2. G1–S transcriptional repression

a | Inactivation of E2F-dependent cell cycle transcription involves multiple negative feedback mechanisms. b | In mammalian cells, G1 cyclin–cyclin-dependent kinase (CDK) (cyclin E–CDK2) together with S phase cyclin–CDK (cyclin A–CDK2) targets the S phase cyclin-specific inhibitor p27 for degradation. The subsequent increase in CDK2 activity results in phosphorylation and release of the activator E2F1, E2F2 and E2F3 transcription factors from gene promoters, thus inactivating transcription. In addition, the E2F targets E2F6, E2F7 and E2F8 accumulate when cells progress to S phase, and they repress transcription when bound to target promoters. The negative feedback mechanism involving the E2F target S phase kinase-associated protein 2 (SKP2), which has a role in targeting E2F1 for degradation via the SCF (SKP2–cullin 1–F-box protein) ubiquitin ligase pathway, has been omitted for simplicity. c | Activation of G1–S transcription in budding yeast results in the accumulation of ~300 gene products, including Nrm1, Cln1, Cln2, Clb5 and Clb6. Some of these proteins are directly or indirectly involved in turning off transcription, thereby forming a negative feedback loop. Cln1–Cdk and Cln2–Cdk prime the Clb–Cdk-specific inhibitor Sic1 for Clb-Cdk phosphorylation, which targets it for degradation (not shown). Clb–Cdk-dependent phosphorylation of SBF (SCB-binding factor) components releases SBF from promoters, and this leads to the inactivation of transcription. MBF (MCB-binding factor)-dependent transcription is inactivated through binding of the MBF-associated co-repressor Nrm1.

Figure 3. G1–S phase transcription and genome stability

The mechanism by which the DNA replication checkpoint maintains high levels of G1−S transcription in response to replication stress is conserved from yeast to humans. This mechanism involves the inactivation of a transcriptional repressors and/or co-repressors (Nrm1 and Yox1 in yeast and E2F6 in human cells) involved in an autoregulatory negative feedback loop. The downstream effector checkpoint protein kinase (Cds1 in fission yeast, Rad53 in budding yeast and checkpoint kinase 1 (CHK1) in mammals) inactivates the transcriptional repressors Nrm1, Yox1 and E2F6 through phosphorylation to maintain high levels of G1–S transcription. The DNA replication checkpoint protein kinases are conserved from yeast to humans, but the G1–S transcriptional repressors and transcriptional activators are not. MBF, MCB-binding factor.