Otto Warburg Laboratory, Department of Vertebrate Genomics, Max Planck Institute for Molecular Genetics Berlin, Germany ; Department of Biology, Chemistry and Pharmacy, Free University Berlin Berlin, Germany.
Front Physiol. 2013 Jul 12;4:173. doi: 10.3389/fphys.2013.00173. eCollection 2013.
The Forkhead (Fkh) box family of transcription factors is evolutionary conserved from yeast to higher eukaryotes and its members are involved in many physiological processes including metabolism, DNA repair, cell cycle, stress resistance, apoptosis, and aging. In budding yeast, four Fkh transcription factors were identified, namely Fkh1, Fkh2, Fhl1, and Hcm1, which are implicated in chromatin silencing, cell cycle regulation, and stress response. These factors impinge transcriptional regulation during cell cycle progression, and histone deacetylases (HDACs) play an essential role in this process, e.g., the nuclear localization of Hcm1 depends on Sir2 activity, whereas Sin3/Rpd3 silence cell cycle specific gene transcription in G2/M phase. However, a direct involvement of Sir2 in Fkh1/Fkh2-dependent regulation of target genes is at present unknown. Here, we show that Fkh1 and Fkh2 associate with Sir2 in G1 and M phase, and that Fkh1/Fkh2-mediated activation of reporter genes is antagonized by Sir2. We further report that Sir2 overexpression strongly affects cell growth in an Fkh1/Fkh2-dependent manner. In addition, Sir2 regulates the expression of the mitotic cyclin Clb2 through Fkh1/Fkh2-mediated binding to the CLB2 promoter in G1 and M phase. We finally demonstrate that Sir2 is also enriched at the CLB2 promoter under stress conditions, and that the nuclear localization of Sir2 is dependent on Fkh1 and Fkh2. Taken together, our results show a functional interplay between Fkh1/Fkh2 and Sir2 suggesting a novel mechanism of cell cycle repression. Thus, in budding yeast, not only the regulation of G2/M gene expression but also the protective response against stress could be directly coordinated by Fkh1 and Fkh2.
叉头（Fkh）框转录因子家族从酵母到高等真核生物都是进化保守的，其成员参与许多生理过程，包括代谢、DNA 修复、细胞周期、应激反应、细胞凋亡和衰老。在芽殖酵母中，鉴定出四个 Fkh 转录因子，即 Fkh1、Fkh2、Fhl1 和 Hcm1，它们参与染色质沉默、细胞周期调控和应激反应。这些因子在细胞周期进程中影响转录调控，组蛋白去乙酰化酶（HDACs）在这个过程中起着至关重要的作用，例如，Hcm1 的核定位依赖于 Sir2 活性，而 Sin3/Rpd3 在 G2/M 期沉默细胞周期特异性基因转录。然而，目前尚不清楚 Sir2 是否直接参与 Fkh1/Fkh2 依赖性靶基因的调控。在这里，我们表明 Fkh1 和 Fkh2 在 G1 和 M 期与 Sir2 相关，并且 Fkh1/Fkh2 介导的报告基因激活被 Sir2 拮抗。我们进一步报道 Sir2 的过表达强烈影响细胞生长，这种影响依赖于 Fkh1/Fkh2。此外，Sir2 通过在 G1 和 M 期与 Fkh1/Fkh2 结合到 CLB2 启动子来调节有丝分裂周期蛋白 Clb2 的表达。我们最后证明 Sir2 在应激条件下也富集在 CLB2 启动子上，并且 Sir2 的核定位依赖于 Fkh1 和 Fkh2。总之，我们的结果表明 Fkh1/Fkh2 和 Sir2 之间存在功能相互作用，这表明了一种新的细胞周期抑制机制。因此，在芽殖酵母中，不仅 G2/M 基因表达的调控，而且对胁迫的保护反应都可以直接由 Fkh1 和 Fkh2 协调。
