Zahradka P, Elliot T, Hovland K, Larson D E, Saward L
St. Boniface General Hospital Research Centre, Department of Physiology, University of Manitoba, Canada.
Eur J Biochem. 1993 Oct 15;217(2):683-90. doi: 10.1111/j.1432-1033.1993.tb18294.x.
Maintaining murine 3T6 fibroblasts in serum-depleted medium for a period of three days results in a resting cell population that does not synthesize DNA. Histone mRNA levels, closely tied to the cell-proliferation rate, are low due to a reduced rate of synthesis. A comparison of histone gene transcription in vitro by nuclear extracts of quiescent or proliferative 3T6 cells showed that a 200-bp segment of the promoter was responsible for repressing gene activity when cells were in a G0 state. In the absence of the distal promoter region (-200 to -400), gene transcription remained high in quiescent cells, indicating the proximal promoter region (+1 to -200) was responsible for basal gene activity. Alterations in protein binding to the distal promoter region correlated with histone H4 gene activity, suggesting that repression of histone gene transcription is linked to the attachment of a specific nuclear protein. During G1, the histone H4 gene was efficiently transcribed in vitro, but an inability to process the histone pre-mRNA limited the cellular content of mature histone mRNA. This distinction between transcriptional (in G0) and post-transcriptional (in G1) mechanisms for modulating histone mRNA levels suggests that gene-regulatory factors are specifically activated in quiescent cells to reduce expression of non-essential genes.
将小鼠3T6成纤维细胞在血清饥饿培养基中培养三天会产生一个不合成DNA的静止细胞群体。由于合成速率降低，与细胞增殖速率密切相关的组蛋白mRNA水平较低。对静止或增殖的3T6细胞的核提取物在体外进行组蛋白基因转录的比较表明，当细胞处于G0状态时，启动子的一个200bp片段负责抑制基因活性。在没有远端启动子区域（-200至-400）的情况下，静止细胞中的基因转录仍然很高，表明近端启动子区域（+1至-200）负责基础基因活性。与远端启动子区域结合的蛋白质的变化与组蛋白H4基因活性相关，这表明组蛋白基因转录的抑制与一种特定核蛋白的附着有关。在G1期，组蛋白H4基因在体外有效转录，但无法加工组蛋白前体mRNA限制了成熟组蛋白mRNA的细胞含量。这种在转录（在G0期）和转录后（在G1期）调节组蛋白mRNA水平机制之间的区别表明，基因调节因子在静止细胞中被特异性激活，以减少非必需基因的表达。
