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Nuclear matrix, hnRNA, and snRNA in friend erythroleukemia nuclei depleted of chromatin by low ionic strength EDTA

A novel and rapid procedure is described for the preparation of chromatin‐depleted nuclei (CDN) from Friend erythroleukemia cells under conditions that avoid the use of high salt concentrations. By this procedure isolated nuclei that had previously been incubated with DNase I to partially digest DNA were washed twice in 2 mM EDTA to extract the chromatin. The resulting structures contained 1% of DNA, 65% of total RNA, 60‐80% of hnRNA, 74% of snRNA, 29% of protein, and 2% of histones contained in isolated nuclei. Electron microscopy revealed intact, spherical structures similar in diameter to isolated nuclei and consisting of dense networks of fibrils 50‐100 A thick surrounded by distinct nuclear laminae. Although no morphological evidence was found for residual nucleoli, C23, a nucleolus‐specific phospho‐protein, remained centrally localized in CDN, while Sm antigen specific for snRNPs was diffusely localized but absent from central regions. Addition of 2 mM MgCl2 to CDN resulted in the reformation of morphologically distinguishable residual nucleoli. These studies suggest that nucleolar morphology is, in part, dependent upon divalent cations and components unique to the nucleolar matrix and demonstrate that little randomization of nuclear and nucleolar matrix fibrils occurs during CDN isolation in the absence of divalent cations.