Processing and release of ribosomal RNA from isolated nuclei: analysis of the ATP-dependence and cytosol-dependence

Nucleocytoplasmic RNA transport

A number of closely related post-transcriptional facets of RNA metabolism show nuclear compartmentation, including capping, methylation, splicing reactions, and packaging in ribonucleoprotein particles (RNP). These nuclear 'processing' events are followed by the translocation of the finished product across the nuclear envelope. Due to the inherent complexity of these interrelated events, in vitro systems have been designed to examine the processes separately, particularly so with regard to translocation. A few studies have utilized nuclear transplantation/microinjection techniques and specialized systems to show that RNA transport occurs as a regulated phenomenon. While isolated nuclei swell in aqueous media and dramatic loss of nuclear protein is associated with this swelling, loss of RNA is not substantial, and most studies on RNA translocation have employed isolated nuclei. The quantity of RNA transported from isolated nuclei is related to hydrolysis of high-energy phosphate bonds in nucleotide additives. The RNA is released predominantly in RNP: messenger-like RNA is released in RNP which have buoyant density and polypeptide composition similar to cytoplasmic messenger RNP, but which have distinctly different composition from those in heterogeneous nuclear RNP. Mature 18 and 28S ribosomal RNA is released in 40 and 60S RNP which represent mature ribosomal subunits. RNA transport proceeds with characteristics of an energy-requiring process, and proceeds independently of the presence or state of fluidity of nuclear membranes. The energy for transport appears to be utilized by a nucleoside triphosphatase (NTPase) which is distributed mainly within heterochromatin at the peripheral lamina. Photoaffinity labeling has identified the pertinent NTPase as a 46 kD polypeptide which is associated with nuclear envelope and matrix preparations. The NTPase does not appear to be modulated via direct phosphorylation or to reflect kinase-phosphatase activities. A large number of additives (including RNA and insulin) produce parallel effects upon RNA transport and nuclear envelope NTPase, strengthening the correlative relationship between these activities. Of particular interest has been the finding that carcinogens induce specific, long-lasting increases in nuclear envelope (and matrix) NTPase; this derangement may underlie the alterations in RNA transport associated with cancer and carcinogenesis.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of cytosol on transport of RNA in vitro during cardiac hypertrophy

An enhanced RNA-transport activity was observed in vitro from nuclei obtained from animals with cardiac hypertrophy as compared with that of sham-operated controls. The 100 000 g supernatants obtained from hypertrophic hearts stimulated the RNA transport from nuclei of sham-operated controls, and this stimulation was maximum with 40% supernatant. Ca2+- and nucleic acid-dependent ATPase and alkaline phosphatase activities, which may be involved in an energy-dependent transport, were high in nuclei from hypertrophic hearts, and the nuclei of sham-operated animals showed higher activities of these enzymes after incubation with supernatant from hypertrophic hearts, which stimulates the RNA transport in vitro from nuclei of sham-operated animals.

RNA metabolism in isolated nuclei: processing and transport of immunoglobulin light chain sequences

Transport of prelabeled RNA from isolated myeloma nuclei is studied using conditions that permit RNA synthesis. Cytosol and spermidine are not required to maintain nuclear stability and inhibited RNA release. Omission of ATP or GTP decreased release 25 to 40%. The stimulatory effect of ATP or GTP is not due to hydrolysis of the triphosphates by the nuclear envelope NTPase, since addition of quercetin (an inhibitor of this NTPase) has no effect on the quantity of RNA released. The size distribution and percentage of poly A-containing species released from nuclei incubated with or without ATP or the other rNTPs are identical. Hybridization analysis of nuclear RNA before the transport assay revealed mature and precursor k light chain mRNA sequences. Following the transport assay, a significant fraction of k mRNA precursors is chased into mature k mRNA which is found both in nuclear-retained and released RNA.

Some features of nucleo-cytoplasmic RNA transport from isolated nuclei

Messenger RNA is released preferentially from isolated rat liver nuclei in the presence of the ATP-generating system and cytosol. The release is suppressed by spermidine, while cytoplasmic RNase inhibitor was ineffective and PCMB like some other thiol-blocking agents inhibitory. Cytoplasmic SOD added to the system strongly suppressed RNA release. A similar effect could be obtained by anaerobiosis due to addition of SMP. In both cases the inhibition is reversed by cyanide. In contrast to normal liver where the generation of superoxide radicals takes place almost exclusively in microsomes and is coupled with the oxidation of NADPH, in mouse ascites hepatoma 22a the generation of superoxide radicals occurs mainly in the nuclear envelope and is coupled wih the oxidation of both NADPH and NADH and inhibited by cyanide.

In vitro ribosomal ribonucleoprotein transport upon nuclear expansion

The interdependence of nuclear rRNA release and nuclear size is investigated in macronuclei isolated from Tetrahymena. Nuclei are induced to contract and to expand, without any structural disintegration of the nuclear envelope, by final Ca2+/Mg2+ (3:2) concentrations of 5 and 1.5 mM, respectively. Upon expansion, the average volume of nuclei increases from 600 +/- 42 to 811 +/- 76 micron3. Concomitantly, nuclei begin to release RNA following saturation kinetics. This RNA release stops immediately upon nuclear contraction. Similar to the in vivo situation, only advanced rRNA processing products are released in the form of ribosomal precursor particles, as identified in detail by polyacrylamide gel electrophoresis and rate zonal and isopycnic density gradient centrifugation. Three particle ty9es are released having average buoyant densities of 1.495, 1.470, and 1.532 g/cm3, exhibiting average sedimentation coefficients of 62, 62, and 35 S, and containing the immediate precursor to the 25S rRNA, 26S rRNA, and 17S rRNA, respectively. Tje rRNP release if ATP independent and noncoincident with the release of endogenous nuclear Pi, though it is Be2+ sensitive. Our data are compatible with the views that nuclear expansion is the prerequisite rather than the cause for the rRNP release and that nuclear pore complex associated ATPases play only, if at all, a minor role in nucleocytoplasmic exchange of rRNP.

Influence of nucleotides, cations and nucleoside triphosphatase inhibitors on the release of ribonucleic acid from isolated rat liver nuclei

The reasons underlying reported discrepancies in the effects of ATP, ADP, adenosine 5'-[beta gamma-methylene]triphosphate, AMP + PPi, P-chloromercuribenzoate and F- on RNA efflux from isolated rat liver nuclei and on nuclear envelope nucleoside triphosphatase activity were investigated. The stimulatory effect of ADP was attributed to myokinase activity associated with the nuclei; this activity was eluted on repeated washing with nuclear incubation medium. In the absence of Ca2+ and Mn2+, ATP, adenosine 5'[beta gamma-methylene]triphosphate and AMP +PPi were found to promote release of both DNA and RNA. In the presence of 0.5 mM-Ca2+ and 9.3 mM-Mn2+, only ATP promoted RNA efflux to a significant extent. In the absence of spermidine, Ca2+ and Mn2+, nuclei released large quantities of DNA and RNA into the medium; this effect was promoted by p-chloromereuribenzoate. In the presence of the three cations, however, p-chloromercuribenzoate inhibited RNA efflux. F- caused a slight leakage of DNA from nuclei. The results are discussed in terms of models for the effects of ATP and analogues on RNA efflux and nuclear stability.

The role of ATP in the transport of rapidly-labeled RNA from isolated nuclei of rat liver in vitro

To understand the mechanism of the action of ATP on the in vitro transport of the rapidly-labeled RNA from isolated nuclei, the fate of ATP during the incubation as well as the effect of ATP, its analogues and other ribonucleoside triphosphates on the transport was examined and the following results were obtained. (1) More than 97% of added ATP remained acid soluble. No polyadenylation of the rapidly-labeled RNA in the released fraction by added ATP occurred although new polyadenylate segments smaller than 10 S were synthesized. (2) The addition of an ATP-generating system to the reaction mixture restored the initial rate of the release of the rapidly-labeled RNA from isolated nuclei. (3) Among the ribonucleoside triphosphates tested, ATP was most effective in stimulating the release. GTP was about 2/3 as effective as ATP. UTP showed some effect, but CTP showed no effect. EDTA was also non-effective. (4) When no ATP-generating system was added to the reaction mixture, AMP failed to mimic the effect of ATP. However, the combination of AMP and pyrophosphate could take the place of ATP. (5) Both AMP-CPP and AMP-PCP, the ATP analogues, showed the equal degree of their effect on the release, regardless of the position of the methylene bond. From these results, the principal role of ATP in the in vitro transport systems seemed to be its interaction with isolated nuclei to dissociate a structure which retains the rapidly-labeled RNA in the nucleus.

Age-dependence of nuclear RNA processing

Following a 3 hour in vivo labelling of cytoplasmic RNA in rat liver with orotic acid-6-14C under conditions where ribosomal RNA synthesis was suppressed, the proportion of labelled messenger-like RNA released to the cytoplasm which contained polyadenylate (poly(A)) tracts was about 3.0 times higher in the livers of juvenile (50 day) as compared to adult (180 day) rats. This discrepancy was confirmed in a cell-free system which consisted of isolated prelabelled nuclei in fortified cytosol. Thus under conditions where approximately 80% of the released labelled RNA was messenger-like, the proportion of polyadenylated labelled RNA transported to the homologous cytosol was 3.4-fold greater in the systems derived from juvenile as compared to adult rat liver. Through comparisons of homologous and heterologous systems it was determined that the age-dependent change in the metabolism of polyadenylated messenger RNA resides in the nucleus and not in the cytoplasm. This change, furthermore does not involve the known ATP-dependence of nuclear RNA release. Rather it must involve other age-dependent changes in the processing or transport of polyadenylated messenger RNA.

Proflavine sensitivity of RNA processing in isolated nuclei

The intercalating agent proflavine inhibits the processing and subsequent release of preformed messenger RNA and ribosomal RNA from isolated liver nuclei to surrogate cytoplasm. The direct effect of proflavine on these processes, as monitored in a reconstituted cell-free system, supports the theory that base-paired segments (i.e. hairpin loops) in the precursor RNA's are involved as recognition sites in nuclear RNA processing.

Characterization of ribonucleoprotein particles released from isolated nuclei of regenerating rat liver in two different in vitro systems

The ribonucleoprotein particles released from isolated nuclei of regenerating rat liver in two in vitro systems were studied and the following results were obtained. 1. When the isolated nuclei of regenerating rat liver labeled in vivo with [14C] orotic acid were incubated in medium containing ATP and an energy-regenerating system (medium I) release of labeled 40-S particles was observed. Analysis of these 40-S particles showed that they contained heterogeneous RNA but no 18 S or 28 S ribosomal RNAs and their buoyant density in CsCl was 1.42-1.45 g/cm3, suggesting that they were nuclear informosome-like particles released during incubation. 2. When the same nuclei were incubated in the same medium fortified with dialyzed cytosol, spermidine and yeast RNA (medium II), release of labeled 60-S and 40-S particles was observed. Using CsCl buoyant density gradient centrifugation, two components were found in the labeled ribonucleoprotein particles released from nuclei in this medium. The labeled 60-S particles were found to contain 28-S RNA as the main component and their buoyant density in CsCl was 1.61 g/cm3, suggesting that they were labeled large ribosomal subunits. The labeled 40-S particles contained both 18 S RNA and heterogeneous RNA and they formed two discrete bands in CsCl, at 1.40 and 1.56 g/cm3, suggesting that they contained small ribosomal subunits and nuclear informosome-like particles. 3. These results clearly indicate that addition of dialyzed cytosol, spermidine and low molecular yeast RNA to medium I causes the release of ribosomal subunits or their precursors from isolated nuclei in the in vitro system.

Factors causing release of ribosomal subunits from isolated nuclei of regenerating rat liver in vitro

Incubation medium II causes release of ribosomal subunits from isolated prelabeled nuclei of regenerating rat liver in vitro (Sato, T., Ishikawa, K. and Ogato, K. (1976) Biochim. Biophys. Acta 000, 000-000). The effects of individual components of this medium on release of subunits were studied and the following results were obtained. 1. Dialyzed cytosol was effective in causing release of total labeled RNA, but its effect on release of labeled ribosomal subunits was rather lower than that of low molecular yeast RNA. Spermidine inhibited the release of total labeled RNA as well as that of labeled ribosomal subunits. 2. Low molecular yeast RNA was the most effective component for inducing release of labeled ribosomal subunits. Homologous ribosomal RNA was as effective as yeast RNA. Cytoplasmic ribosomes, prepared by washing with solution of high salt concentration, and their subunits were also effective. 3. Transfer RNA was not so effective as yeast RNA and ribosomal RNA and even after heat treatment it had little effect. 4. Among the homopolyribonucleotides tested, polyuridylic acid had a strong effect but polyadenylic acid, polycytidylic acid and polyinosinic acid had no effect. 5. The effects of yeast RNA and polyuridylic acid in causing release of labeled ribosomal subunits were dependent upon their concentrations in the reaction mixture. The characteristics of the factors which cause release of labeled ribosomal subunits in vitro are discussed on the basis of the results.

A model for cytoplasm-governed gene regulation

A model of cytoplasm-governed transcription is presented. The nuclear membrane has a selective permeability towards nuclear pre-mRNA molecules which are provided with group-specific non-translated "passwords". RNA transcription on the chromatin proceeds under a dual control. One of them is gene regulation according to the Britten-Davidson and Georgiev models. The other is cytoplasm-governed regulation mediated through the selective transport of mRNA from nucleus to cytoplasm. Pre-mRNA molecules which are not "in immediate demand" by the cytoplasm and therefore accumulating the nucleus repress their own synthesis by end-product inhibition. The interrelationship of the two types of regulation in the course of cell development is discussed.

Processing of RNA by cytoplasmic extract. Synthesis and processing of RNA by isolated HeLa cell nuclei incubated with cytoplasmic extract

Nuclei are isolated from HeLa cells by a low speed centrifugation procedure. These nuclei can carry out RNA synthesis at 30 degrees. The effect of cytoplasmic extract on the transport and processing of RNA is measured. In the presence of cytoplasmic extract the precursor to 4S RNA is processed to 4S RNA and an RNA species OF 7-9S is transported outside the nuclei.

Effect of phenobarbitone on the nucleocytoplasmic transport of ribonucleic acid in vitro

The transport of nucleic acids from the nucleus to the cytoplasm is a potential site for modification of normal cellular processes by drugs and hormones. In this study the effect of phenobarbitone on nucleocytoplasmic transport of ribosomes was measured in an assay system in vitro. The transport of radioactive ribosomes from isolated rat hepatic nuclei to unlabelled post-microsomal supernatant was measured in rats treated with 80 mg of phenobarbitone/kg body wt. or saline 3h before death. With either treatment, transport was linear with time, and dependent on temperature and the presence of ATP. However, phenobarbitone treatment increased transport of ribonucleoproteins over saline-treated animals nearly twofold. The effect of phenobarbitone was mediated through the cytosol, but was not the result of altered stability of the RNA transported to the cytosol. Cycloheximide (5 mg/kg body wt.) given 3.5 h before death inhibited the stimulation of transport by phenobarbitone. The data indicate that phenobarbitone increased the transport of RNA by stimulating the synthesis of cytosol factors that regulate transport of RNA from the nucleus.

Nature of the facilitated messenger ribonucleic acid transport from isolated nuclei

Cytoplasmic macromolecules were previously identified which regulate both qualitatively and quantitatively the release of messenger-like RNA from isolated nuclei. These macromolecules are now shown to be denatured at 45-50 degrees C and their synthesis is sensitive to pactamycin or cycloheximide. The putative regulatory proteins are essentially quantitatively precipitated with high specificity from the cytosol by streptomycin at a concentration 10-fold higher than that used to precipitate RNA. The nuclear concentration-dependence of RNA transport from successive samples of nuclei strongly suggests that the regulatory factors are recycled. Quantitative changes in the sequences transported at various dilutions of the cytosol suggest that not all the different classes of the putative regulatory macromolecules are present in an effective concentration at any one dilution.