Studies on nuclear ribosomes I. Association of DPN-pyrophosphorylase with nuclear ribosomes in normal and neoplastic tissues

The nucleus introduced

Now is an opportune moment to address the confluence of cell biological form and function that is the nucleus. Its arrival is especially timely because the recognition that the nucleus is extremely dynamic has now been solidly established as a paradigm shift over the past two decades, and also because we now see on the horizon numerous ways in which organization itself, including gene location and possibly self-organizing bodies, underlies nuclear functions.

Localization of reverse-transcriptase in interferon-treated mouse cells chronically infected with Moloney leukemia virus

Interferon treatment of mouse cells chronically infected with Moloney leukemia virus (3T3/MLV) resulted in 97 per cent inhibition of infective virus release. The intracellular localization and distribution of virus reverse-transcriptase and group specific (gs) antigen were determined in interferon treated and control cells. Cytoplasm of infected cells was fractionated by isopycnic centrifugation on discontinuous sucrose gradients. Fractions were analysed for their chemical composition and characterized by the activity of membranal marker enzymes. The association and levels of viral antigens were determined in each fraction. Fractions enriched with 5' nucleotidase, specific enzyme marker for plasma membrane, were also enriched with viral proteins. In interferon treated cells, intracellular accumulation of viral proteins was specifically localized in the plasma membrane. Threefold increase in reverse-transcriptase level was the maximal accumulation found in purified plasma membranes. Intracellular enzyme levels in interferon treated cells were in accordance with the amount of cell associated infective virus particles. The small accumulation of viral proteins and infective virus particles was not sufficient to account for the great reduction in virus yield observed in the supernatants of the interferon treated cells. A possible role for interferon in modification of plasma membrane associated with virus assembly is postulated.

Chromatin-associated RNA: differential extraction and characterization

Mammalian cells in different states of cytodifferentiation exhibited different RNA-synthesizing and processing patterns that could be used as markers for phenotypic variability. Inherent in these patterns was an RNA class which was differentially extracted from the cellular homogenate by elevating the temperature and pH of the buffer used in the phenol procedure. This class of RNA was initially designated fraction B (chromatin-associated RNA). In the characterization of fraction B, human myeloma cells labeled for 3 and 24 h were fractionated into subcytoplasmic and subnuclear components and the [3H]-RNA was differentially extracted. After 3 and 24 h labeling 84% and 73%, respectively, of the labeled RNA in the chromatin was extracted in fraction B. Only 10-20% of the polysomal RNA was extracted in fraction B with little enrichment in poly(A) RNA. These and other observations suggested that fraction B was a subpopulation of heterogeneous nuclear RNA which was tightly bound to the chromatin complex.

Isolation of two distinct classes of polysomes from a nuclear fraction of rat liver

Isolated rat liver nuclei were washed with Triton-X-100 in the presence of liver cell sap. This treatment liberated a fraction of polysomes which were isolated by differential centrifugation and were designated "outer membrane polysomes." The outer membrane polysomes synthesized protein in vivo. Shortly after injection of orotic acid-(14)C, the RNA of outer membrane polysomes had a higher specific activity than that of cytoplasmic polysomes. It was postulated that outer membrane polysomes may be an intermediate in the transfer of newly synthesized RNA from the nucleus to the cytoplasm. In other experiments, Triton-washed rat liver nuclei were lysed in the presence of deoxycholate and deoxyribonuclease. A ribonucleoprotein fraction was isolated from the lysate by differential centrifugation. This fraction contained "intranuclear ribosomes," which sedimented like partially degraded polysomes in sucrose gradients. This degradation could be partially prevented if intranuclear ribosomes were purified by sedimentation through heavy sucrose. The resulting pellets were termed "intranuclear polysomes" because they contained some undergraded polysomes. Intranuclear polysomes were highly radioactive after a brief pulse with orotic acid-(14)C, but did not appear to synthesize protein rapidly in vivo. Intranuclear polysomes may represent the initial stage of assembly of polyribosomes in the nucleus.

Ribosomal RNA synthesis and processing in a particulate site in the HeLa cell nucleus

A particulate fraction has been isolated from detergent-prepared HeLa cell nuclei. The fraction consists largely of organelles that resemble the nucleoli of intact cells. The 45S RNA that is precursor to 28S and 18S ribosomal RNA is associated with the fraction. The 32S RNA that is labeled after the 45S RNA and is the apparent precursor to 28S RNA is also associated with the fraction. The nucleoplasm contains 28S RNA that behaves as an intermediate between the 32S nucleolar RNA and the 28S cytoplasmic RNA.

The requirement for bivalent cations in formation of nicotinamide-adenine dinucleotide by nicotinamide mononucleotide adenylyltransferase of pig-liver nuclei

1. The requirement for bivalent cations in catalysis of NAD formation from ATP and NMN in the presence of NMN adenylyltransferase of pig-liver nuclei was studied. Rates of NAD formation in the presence of the activating cations Cd(2+), Mn(2+), Mg(2+), Zn(2+), Co(2+) and Ni(2+) were approximately a linear function of heats of hydration of the corresponding ions. Ba(2+), Sr(2+), Ca(2+), Cu(2+) and Be(2+) did not activate the enzyme; Be(2+) inhibited the reaction in the presence of Mg(2+) and, to a greater extent, in the presence of Ni(2+). 2. Michaelis constants for NAD formation, measured in a coupled assay with NMN adenylyltransferase and alcohol dehydrogenase at pH8.0 and 25 degrees , in the presence of 3mm concentrations of the unvaried reactants, were 88+/-7mum-ATP, 42+/-4mum-NMN and 85+/-4mum-Mg(2+). The results at this pH and at pH7.5 were consistent with mechanisms in which Mg(2+)-ATP complex is a reactant and free ATP a competitive inhibitor. 3. Formation of nicotinamide-hypoxanthine dinucleotide from NMN and ITP in the presence of the transferase was also more rapid with Ni(2+) and Co(2+) than with Mg(2+).