Chegini N, Gregory S P, Hilder V A, Pocklington M J, Maclean N
J Submicrosc Cytol. 1981 Jul;13(3):291-308.
Nuclei from Xenopus erythrocytes have been isolated under varying ionic conditions and their morphology studied by light and electron microscopy. The 200-A beaded chromatin fibre observed in vivo is maintained in media containing MgCl2 or CaCl2, but not in those containing Mn2+ as the sole divalent cation. In the absence of these alkaline-earth metal ions, 100-A filaments predominate. This suggests that the native 200-A fibre in isolated nuclei is also stabilized by specific divalent cations. Gross chromatin condensation within the nucleus occurs independently of whether the chromatin is in the form of 100-A or 200-A fibres, but requires the presence of divalent cations. Lower concentrations of MgCl2 or CaCl2 are necessary as compared with MnCl2. In the presence of sufficient divalent cations, decreasing KCl concentration causes peripheral condensation, the condensation of chromatin towards the nuclear membrane. Monovalent cation alone does not appear capable of inducing peripheral condensation. The transcriptional capacities of the different morphological types identified have been studied using E. coli RNA polymerase as a probe. Neither the degree of chromatin condensation nor the state of higher-order coiling have a significant effect on the rate of transcription of the DNA. These results are discussed with regard to further in vitro studies on eukaryotic gene activity.
在不同离子条件下分离出非洲爪蟾红细胞的细胞核，并通过光学显微镜和电子显微镜研究其形态。在体内观察到的200埃串珠状染色质纤维在含有MgCl₂或CaCl₂的培养基中得以维持，但在以Mn²⁺作为唯一二价阳离子的培养基中则不然。在没有这些碱土金属离子的情况下，100埃的细丝占主导。这表明分离细胞核中的天然200埃纤维也由特定的二价阳离子稳定。细胞核内染色质的总体浓缩独立于染色质是以100埃还是200埃纤维的形式存在，但需要二价阳离子的存在。与MnCl₂相比，需要较低浓度的MgCl₂或CaCl₂。在存在足够二价阳离子的情况下，降低KCl浓度会导致周边浓缩，即染色质向核膜的浓缩。单独的一价阳离子似乎不能诱导周边浓缩。使用大肠杆菌RNA聚合酶作为探针研究了所鉴定的不同形态类型的转录能力。染色质浓缩程度和高级盘绕状态对DNA转录速率均无显著影响。结合对真核基因活性的进一步体外研究对这些结果进行了讨论。
