Structural transitions of chromatin in isolated Xenopus erythrocyte nuclei.  I. The effects of ions. 
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.