Evidence for the persistence of a decondensed chromosome scaffold in the interphase nucleus.

Nuclei of in vitro cultured bovine liver cells, deprived of the membranes by Triton X-100, were treated with 2 M-NaCl and DNase. Changes in ultrastructure and protein composition were studied at successive steps during treatment. Electron micrographs of nuclei treated with 2 M-NaCl showed a peripheral lamina and an internal system of randomly coiled filaments embedded in a mass of DNA fibres. After partial removal of the DNA the filaments could be seen to serve as backbones for the DNA attachment. Artificial redistribution occurring during fixation with glutaraldehyde suggests that the salt-resistant filaments are not stably cross-bridged into a three-dimensional network. The existence of reversible cross-bridges in vivo cannot be excluded, however. From the available data it is inferred that the filaments represent a decondensed from of the chromosome scaffolds and play a basic role in the organization of the genome throughout the nuclear cycle.

Isolation of a residual protein structure from nuclei of the myxomycete Physarum polycephalum.

A nuclear framework structure has been obtained from isolated interphase nuclei of Physarum polycephalum by extraction with 2.5 M NaCl and subsequent digestion with DNase. Whole-mount electron micrographs showed a nuclear lamina containing residual pore structures associated with the fibrous internal matrix. The matrix was continuous with fibrillar remnants of the nucleolus. The structure was shown to consist of 2 major polypeptides of 23,000 and 36,500 Daltons as well as 30 to 40 minor polypeptides of various molecular weight classes. The 2 major polypeptides were also prominent in preparations of the residual nucleolar material, suggesting that matrix proteins are common to both structures. The predominance of low-molecular-weight polypeptides in Physarum nuclear matrix suggests that there may be significant differences in composition of nuclear structural proteins between lower and higher eukaryotes.

Change of the nuclear pore frequency during the nuclear cycle of Physarum polycephalum.

The change of the nuclear pore frequency during the nuclear cycle of synchronous plasmodia of Physaru- polycephalum was investigated. Counts were made on platinum-carbon replicas of isolated nuclei. Pore numbers varied markedly at any given time of the nuclear cycle, possibly due to the variable DNA contents of the nuclei. The average pore frequency per nucleus increased from 336 at 1 h after mitosis to 770 at 50 min before the subsequent mitosis. The results suggest a quantitative relation between the nuclear DNA content and pore frequency. This is compatible with the hypothesis that pore complexes serve as attachment sites for nuclear DNA.