3H-orotic Acid Research Articles

The de novo and salvage pathways for the synthesis of pyrimidine residues of RNA predominate in different locations within the mouse duodenal epithelium.

RNA synthesis was examined by radioautography in mouse duodenal epithelium using 3H-uridine as a tracer of the salvage pathway and 3H-orotic acid as a tracer of the de novo pathway. The incorporation of the two precursors was estimated by counting silver grains in light-microscopic and electron-microscopic radioautographs at successive levels of crypt and villus. With both precursors, silver grains were found over all epithelial nuclei, but in numbers varying by location. Thus, after 3H-uridine injection, the number of grains was high over nucleolus and nucleoplasm in the base of the crypt, declined gradually in the middle and top of the crypt, and was low along the villus. After 3H-orotic acid, the number of grains was fairly low throughout, but peaked over the nucleoplasm in lower villus cells. The 3H-uridine reaction over nucleolus and nucleoplasm in crypt cells was interpreted as synthesis by the salvage pathway of ribosomal RNA and heterogeneous RNA, respectively, whereas the 3H-orotic acid reaction over the nucleoplasm of some villus cells indicated that these cells synthesized heterogeneous RNA by the de novo pathway.

Radioautographic comparison of RNA synthesis patterns in the epithelial cells of mouse pyloric antrum following 3H‐uridine and 3H‐orotic acid injections

RNA synthesis was examined in the epithelial cells of the mouse pyloric antrum using radioautography 20 min after injection of either 3H-uridine or 3H-orotic acid. The epithelium of the mouse antrum was known to invaginate into blind tubular units composed of mucous cells arranged from base to top into a gland, an isthmus, and a pit. These were subdivided into segments and, after radioautography, silver grains were counted over cell nuclei in each segment. Following 3H-uridine injection, silver grains were present over all nuclei but were more abundant over those of the isthmus than of the gland or the pit. When nuclei were examined in the electron microscope, nucleoplasmic as well as nucleolar silver grains were more numerous in the isthmus than in the pit or gland. Following 3H-orotic acid injection, silver grains were again present over all nuclei; but maximal incorporation appeared to be in pit cell nuclei where, by electron microscopy, it was mainly assigned to the nucleoplasm. When the incorporation was calculated per whole nucleus, however, it was less in pit cell than in isthmal cell nuclei. Even so, the proportion of label in pit cell nuclei was much greater than after 3H-uridine injection. The interpretation of these findings is based on the fact that isthmal cells are immature, whereas cells migrating from the isthmus to become gland or pit cells show increasing differentiation. The immature cells of the isthmus incorporate both uridine and orotic acid more effectively than do the differentiated cells of pit and gland. Since silver grain counts over nuclei provide an index of the rate of RNA synthesis, this synthesis proceeds more actively in the isthmus than in the pit or gland. This is true of ribosomal RNA synthesis, as shown by nucleolar grain counts, and of other RNA's synthesis, as shown by nucleoplasmic grain counts. It seems, however, that while uridine is involved in the synthesis of all types of RNA, orotic acid is mainly implicated in the synthesis of the heterogeneous RNA from which the messenger RNA arises.

The effect of 5-fluoroorotic acid on the early labelling of nucleotides and RNA in whole liver and of RNA in subnuclear fractions in rats given 3H-orotic acid.

Male white rats were given either orotic acid or 5-fluoroorotic acid 60 min, and 3H-orotic acid 3 1/2 min before sacrifice. Liver nucleotides were analyzed by isotachophoresis. The (F)UTP pool increased to the same extent in both groups and showed the same specific labelling. The RNA synthesis, as measured by nmol of UTP entering RNA-UMP/g wet liver tissue was significantly lower in fluoroorotic acid treated rats. In the nucleolar fraction there was an increase in the RNA/DNA ratio, a decrease in the specific RNA labelling and an essentially unaltered specific labelling of RNA/microgram/DNA. The effect on the specific labelling of RNA/microgram DNA was the same in the nucleolar fraction and the nucleolar-free pellet, suggesting that the transcription of RNA in these two fractions proceeded in a similar way. There was a slight increase in the specific RNA labelling of the supernatant subnuclear fraction, containing low-molecular weight RNA. Incorporation into UDP-hexoses and UDP-N-acetylhexosamine was unchanged.

Hormonal stimulation of 3H-orotic acid incorporation into RNA by serum-free cultured hepatocytes

Insulin and dexamethasone greatly stimulate the incorporation of 3H-orotic acid into RNA. Such a stimulation is associated to an increase in the uptake of the labelled precursor into the acid soluble fraction as well as in the specific radioactivity of the nucleoside plus nucleotide pool suggesting that hormone supplementation does not affect RNA synthesis by cultured cells. The lack of effect of insulin and dexamethasone on the level of total RNA polymerase activity in nuclei isolated from cultured hepatocytes is in line with this assumption. The hormone stimulated uptake of orotic acid is dependent on protein synthesis since it is completly abolished by cycloheximide.

Binding sites for glucocorticoids in cytosols from the ventral prostate and seminal vesicle of rats.

Binding sites for 3H-dexamethasone (Dex) were demonstrated in cytosols from the ventral prostate and seminal vesicle of rats only if dithiothreitol (DTT) was present in the incubation mixture. The binding observed in the presence of DTT was high affinity (Kd=1-5 nM) and low capacity (Bmax=approx. 0.1 p mole/mg protein) and exhibited a binding specificity for glucocorticoids. The addition of molybdate (Mo) to the incubation mixture enforced the effect of DTT on the binding but the extent of the effect of Mo in cytosols of the ventral prostate differed from that in the seminal vesicle. The binding sites in the cytosols of these tissues were depleted after administration of Dex to animals. The depletion observed was not due to occupation of the binding sites by injected Dex and this was confirmed by the exchange assay. Addition of the cytosol from the seminal vesicle inhibited the 3H-Dex binding in the liver cytosol but the cytosol from the ventral prostate did not show an inhibitory effect. The binding sites in neither of these male accessory sex organs were modified markedly after the animals were castrated. Although the 3H-Dex binding sites observed in the cytoplasm of male accessory sex organs fit the definition proposed for the steroid hormone receptors, it is not clear whether these tissues are under the influence of glucocorticoid or not; the rate of incorporation of 3H-leucine and 3H-orotic acid into the acid-insoluble fraction from the ventral prostate is not influenced by the administration of Dex to animals.

Hydra viridis: transfer of metabolites between Hydra and symbiotic algae.

"Back transfer" of metabolites from food to endosymbiotic algae in the digestive cells of Hydra viridis was demonstrated. Brine shrimp nauplii labeled with tritiated precursors of protein and nucleic acids (DNA and RNA) were fed to light and dark grown hydras. The fate of the label after a single feeding with radioactive material in hydra and algal fractions was followed by scintillation counting and autoradiographic techniques. Labeled thymidine was incorporated into DNA in both light- and dark-grown hydras. Although the symbiosis persists indefinitely in hydras in darkness (7-10 days) the number of algae per cell is reduced. Tritiated orotic acid and tritiated uridine, RNA precursors, were incorporated into peptides and proteins, and to a lesser extent into simple sugars, oligosaccharides, and oligonucleotides in hydra and algal fractions. Thus the metabolites of the brine shrimp food are available to both partners. A decrease over time in label introduced as 3H-orotic acid and 3H-uridine and incorporated into hydra RNA is compensated for by an increase in label in the algae, implying competition for constant quantities of metabolites from the single feeding. Although food availability, light, number of algae per cell, and other factors influence the quantity and rate of nutrient transfer between the partners, in both light and dark grown hydras the amount of "back transfer" of metabolites to the symbiotic algae is impressive.

Ethanol and cycloheximide alter protein and RNA synthesis of Cox astrocytoma cells in culture.

AbstractChronic ethanol ingestion or cycloheximide treatment results in alterations in the properties and synthesis of protein and RNA of polyribosomes in the whole brain. To analyze the effects on a homogeneous neural cell population, Cox astrocytoma (glioma) cells were grown in tissue culture media with 100 mM ethanol or 0.017 mg/ml of cycloheximide. When ethanol had been present for ten days, the cell densities remained unchanged but had markedly reduced RNA and protein contents. Furthermore, the ethanol treatment reduced the whole‐cell pulse‐labeling of RNA with (5–3H) orotic acid and protein with (14C) leucine in the postmitochondrial supernatant. These results suggest that chronic ethanol treatment reduced the whole cell synthesis of RNA and protein or increased their degradation. Analysis of the polyribosomes on sucrose density gradients showed that dense polyribosomal chains were decreased after the ethanol treatment, supporting the concept that the polyribosomes were degraded with an alteration in the metabolism of mRNA. The cell‐free incorporation of (14C) leucine into hot TCA precipitable protein by the purified polyribosomes in the presence of ATP, GTP, a heterologous source of soluble factors, and endogenous mRNA was also reduced following the ethanol treatment, further indicating that the previous chronic exposure to ethanol had inhibited the translation of mRNA. When the control cells were grown in the presence of cycloheximide for one hour prior to harvesting, the cell densities remained unchanged, but again, as with the ethanol treatment, the polyribosomal protein and RNA yields decreased. In contrast to ethanol, however, cycloheximide treatment caused increases both in the whole‐cell incorporation of labeled RNA and protein precursors into the supernatant fraction and in the cell‐free incorporation of (14C) leucine into protein. These results suggest that, like the ethanol effects, cycloheximide reduces the total polyribosomes, but unlike the ethanol effects, the remaining polyribosomes have stable mRNA and rapidly incorporate radioactive amino acids, even more than untreated controls. The one‐hour cycloheximide treatment also caused an increase in the ratio of dense polyribosomes to monosomes plus 40s and 60s ribosomal subunits of control cells. In addition, it increased the incorporation of the labeled precursor into protein in the polyribosomal region of the sucrose gradients of both control and ethanol treated cells, suggesting that cycloheximide inhibited the termination step of protein synthesis. When cycloheximide was present for 24 hours prior to harvesting, the ethanol treated cells, in contrast to the controls, still had increased cell‐free incorporation of amino acid into protein, indicating that the stimulatory effects of cycloheximide are prolonged to 24 hours when ethanol is present. Thus, while the ethanol treatment in general inhibits polyribosomal biogenesis in the cells, it alters the complex of stimulating and then inhibiting effects of cycloheximide by preserving cycloheximide's stimulating effects on the amino acid incorporation activities of the polyribosomes.

The effect of short term incubation in vitro on the RNA content of regenerating mouse epidermis.

The effect of short term incubation in vitro on RNA content in regenerating and normal epidermis has been investigated. Regenerating mouse epidermis was incubated for three hours, either attached to its underlying dermis or by itself, in either buffered sucrose or Roswell Park Memorial Institute culture medium at 26 degrees C or at 37 degrees C. There is a significant loss of RNA when regenerating epidermis is incubated without being attached to its underlying dermis, at either 26 degrees C or 37 degrees C, although there was little loss of DNA, good incorporation of (3H) orotic acid into RNA, as well as good preservation of epidermal histological details. In contrast, when regenerating epidermis was incubated attached to its dermis, little loss of RNA occurred. Similarly, incubating normal epidermis attached to its dermis results in no loss of RNA. These conditions also result in no significant loss of DNA, good incorporation of (3H) orotic acid into RNA, and preservation of epidermal histological details.

Ribonucleic acid metabolism during planarian regeneration.

A method for the extraction of total RNA of the planarian Polycelis tenuis is described. This technique has been applied to the study of RNA synthesis in the course of regeneration. Synthesis of RNA begins 7 hrs after sectioning and proceeds in two phases. The first phase, from 7 to 21 hr is characterized by an increased rate of (3H) orotic acid incorporation into RNA up to the 18th after sectioning followed by a decrease between 18 and 21 hrs. The rate of precursor incorporation then had risen again in the second phase to the 30th hr and finally decreased slowly until completion of regeneration. Studies of the pool of nucleotide precursors, show that the observed variations of orotic acid incorporation into RNA reflect variations in RNA synthesis. Electrophoretic analysis of RNA labelled for 2 hrs at various times during regeneration shows that the product formed during the first phase of regeneration is mainly rRNA and low molecular weight RNA. The second phase of the regeneration process, from the 24th hr, is characterized by the appearance of both ribosomal and polydisperse RNAs. These heterogeneous RNA species are detected up to the 46th hr of regeneration, after which time rRNA again becomes predominent. The significance of the two phases of RNA metabolism of which this is the first observation during planarian regeneration is discussed in connection with other biochemical events which have been described during regeneration processes in planarians and other animals.

In vitro effects of aflatoxin B1 on the uptake of 14C-orotic acid into kidney, liver and muscle tissue of the Mongolian gerbil, Meriones unguiculatus.

Aflatoxins are a mixture of highly toxic metabolites produced by certain strains of the mold Aspergillu8 ~avu~ a contaminant of groundnuts and other grains. The chemical structures for these potent hepatotoxic and hepatocarcinogenic agents have been studied extensively and aflatoxin B I (AFB I) has been found to be the most abundant and toxic metabol~te (B~TLER 1966). A major area of study involving the aflatoxins has dealt with their mechanism of action. Often severe inhibition of protein synthesis by the cell can be measured by uptake and incorporation of various labeled precursors of protein and nucleic acids (SMITH 1963, CLIFFORD & REES 1966, 1967). One investigator reported inhibition Of nuclear and nucl~olar RNA synthesis (30% and 85%, respectively) as measured by 14C-orotic acid incorporation in Other work with rats showed rat livers (LEFARGE et al. 1966). i~ no inhibition of incorporation of C-orotic acid in in vivo studies of the nucleotide pool, but immediate inhibition in in vitro studies. However, the authors did find inhibition of nucleotide precursors into nuclear RNA and postulated inhibition of RNA polymerase as a possible explanation (CLIFFORD & REES 1967). Later work subsequently explored this hypothesis corroborating it for RNA polymerase and implicating metabolites of AFB I as the active intermediate forms responsible for the inhibitmon (AKINRIMIST et al. 1974). A metabolite of AFB I was also suggested as the actual inhibitor of RNA synthesis in another work where the incorporation of RNA precursors, 14C-orotic acid and 3H-orotic acid, was markedly inhibited by low levels of AFH I and AFGI, but not AFB 2 and AFG 2 (MCINTOSH et al. 1976). Most of the literature involving measurement of uptake and incorporation of precursors has dealt m~inly with the effects on rats and few recent studies have used 14C-orotic acid. It was the purpose of this study to determine the effects of AFB_ on the uptake of 14C-orotic acid into kidney, liver and muscle tissue of the Mongolian gerbil, Meriones unguicula~s. This animal appears to be less sensitive to the toxin than the rat (GODY & NEAL 1976, LLEWELLYN & THOMEN 1978).

Effect of age on RNA synthesis by rat hepatocytes

The incorporation of ( 3H) orotic acid into RNA by hepatocytes from rats of various ages was studied. A 37% decrease in the incorporation of ( 3H) orotic acid into RNA was observed by hepatocytes isolated from 30-month-old rats compared to 12-month-old rats. There was no age-related difference in the incorporation of ( 3H) orotic acid into acid-insoluble material by hepatocytes. The incorporation of ( 3H) orotic acid into UTP by hepatocytes was determined by high pressure liquid chromatography. The ( 3H) orotic acid was rapidly converted to UTP by hepatocytes, and the specific activity of the UTP reached a steady state after 10 min of incubation. There was no significant difference in the specific activity of the UTP in hepatocytes isolated from rats ranging in age from 6 to 25 months. Therefore, the age-related decrease in the incorporation of ( 3H) orotic acid into RNA by hepatocytes is not due to an age-related difference in the uptake or conversion of ( 3H) orotic acid into UTP.

Effect of ginseng saponins on nuclear ribonucleic acid (RNA) metabolism. I. RNA synthesis in rats treated with ginsenosides.

The effect of ginsenosides purified from Ginseng (the roots of Panax ginseng C. A. MEYER) on nuclear ribonucleic acid (RNA) synthesis was investigated. Four hours after intraperitoneal injection of each of the ginsenosides (-Rb1, -Rc, and -Rg1) at a dose of 5 mg per 100 g body weight, incorporation of 3H-orotic acid into nuclear RNA of rat liver was determined. The ginsenosides showed different activities : Rb1 increased and Rc decreased the incorporation, and Rg1 did not affect it. Furthermore, the effect of ginsenoside-Rb1 and -Rc on RNA polymerase activity was investigated 3 hr after the intraperitoneal administration. Rb1 enhanced and Rc repressed the enzyme activity, the results being consistent with those on the incorporation study. Both purified ginsenosides did not show any effect on RNA polymerase activity when added in vitro.

CHANGES IN LABELLING OF RIBONUCLEIC ACID SPECIES FRACTIONATED FROM THE RAT VENTRAL PROSTATE IN RESPONSE TO TESTOSTERONE

ABSTRACT The total RNA isolated at various times up to 24 h after testosterone administration from the ventral prostate of castrated rats, was labelled either by injecting 3H-orotic acid directly into the ventral prostate 6 h before the animals were killed, or by incubating prostatic tissue in vitro with 3H-uridine for 20 to 60 min. The isolated RNA was separated into tRNA, ribosomal RNA (Q1 RNA) and two DNA-like RNA fractions (Q2) and TD RNA) by chromatography on methylated albumin kieselguhr (MAK) columns, and the fractions were further analysed by sucrose gradient centrifugation. Testosterone given into castrated animals for 12 h, stimulated the labelling of all main fractions. The radioactivity of TD RNA after a 60-min incubation period in vitro with 3H-uridine was approximately twice that seen in the castrated rat, while there was a 3.1- and 2.2-fold increase in the radioactivity of the Q1 and Q2 RNA fractions respectively. Kinetics of incorporation of 3H-uridine into different RNA fractions revealed that the hormone facilitated the labelling of the TD RNA fraction relatively more than that of the Q2 fraction. The injection of 3H-orotic acid into the ventral prostate labelled the Q1 RNA preferentially. More than 60 % of the recovered radioactivity was found in Q1 RNA (as 18 and 28 S). Testosterone increased markedly (9.4-fold) the labelling of this fraction. It was concluded that testosterone has an activatory effect on the production of ribosomal RNA, and the bulk of the testosterone effect on the total RNA labelling is to be found in this fraction. Furthermore, it seems likely that testosterone also stimulates both the synthesis and processing of DNA-like RNA. When actinomycin D was given 2 h before the hormone administration in a dose of 25 μg per 100 g of body weight, there was no noticeable increase in the labelling of any fraction above the level seen in the untreated castrated rat. There is evidence that testosterone exerts some effects on the labelling of proteins with radioactive amino acids and 14C-glucose metabolism in the absence of that fraction of the total RNA synthesis which is sensitive to a low dose (25 μg per 100 g of body weight) of actinomycin D (Isotalo & Santti 1972). In this way it may be concluded that the major changes of the RNA synthesis after testosterone administration are likely to be secondary to the protein synthesis and glucose metabolism, or the hormone exerts its anabolic effect on prostatic cells at different sites and by different modes of action, each of which can be operated independently.

Messenger ribonucleoprotein complexes isolated with oligo(dT)-cellulose chromatography from kidney polysomes

As an initial step towards understanding the role of mRNP complexes in translational regulation during compensatory renal hypertrophy, characteristics of polysome-associated mRNP isolated by affinity chromatography were studied. Renal mRNP contained 15–30% of the counts after a 1 hr pulse with 3H-orotic acid; it sedimented mainly between 10S and 100S and had a buoyant density of 1.42–1.44 g/cm 3. RNA derived from the mRNP sedimented between 5S and 40S on sucrose density gradients, with the greatest radioactivity in the region of 15S. After labeling with 3H-adenine for 1 hr, up to 17% of the radioactivity present in the mRNP-associated RNA was resistant to digestion by pancreatic and T1 ribonucleases. The mRNP protein moiety contained six polypeptides with molecular weights 69,000, 75,000, 80,000, 100,000, 109,000, and 118,000 daltons, which were undetected in the material not binding to oligo(dT)-cellulose.

Micro-autoradiographic studies on host-parasite interactions

Tritium labeled uredospores of Uromyces phaseoli were produced be feeding the host, Phaseolus vulgaris, with 3H-orotic acid. These spores were allowed to germinate on and to penetrate into a bean leaf. 24 hrs after inoculation, the bean rust had formed the first haustorium. All fungal structures, including the fungus walls, were heavily labeled. No label could be detected in the cells that had come into contact with the hyphae. In the infected host cell, the haustorium was labeled heavily, but the sheath around the haustorium and the host cell remained free of label. These results indicate that no detectable amounts of label leach from the bean rust into the host at this stage of infection although it is known that the rust takes up many metabolites. Since the sheath remains free of label and all fungal structures are evenly labeled, it is concluded that the sheath is formed by the host.

Intrahippocampal injection of chemicals: Analysis of spread

Using intrahippocampal injection as an example, the present paper describes the spreading of solutions of chemicals in the brain after topical cerebral injection (TCI). Detection of radioactivity in different brain regions after intrahippocampal injection of 3H-orotic acid revealed a distribution pattern that cannot be explained by radial diffusion of the substance from the site of injection. The EEG flattering induced by application of KCl (spreading depression) was used as an additional index of the distribution of a chemical after injection into the hippocampus. The biochemical and electrophysiological experiments showed two ways of spread to be of importance to the distribution pattern under the experimental conditions chosen: (a) spreading by transport up the outside of the implanted microcannula, and (b) flow back of the substance into the ventricular system with subsequent diffusion into adjacent brain structures. This type of distribution is almost independent of the physicochemical properties of the substance injected. Therefore, when using TCI, it should be taken into consideration the the substance might spread into other brain areas.

Differential synthesis of RNA in self- and cross-pollinated styles of Petunia hybrida L.

Styles of Petunia hybrida were pollinated (selfed or crossed) and pollentubes were allowed to grow in the presence of 3H-orotic acid. After 24 h RNA was extracted using the cold phenol method and separated on a tandem polyacrylamide gel. Results show that: 1. the synthesis of RNA is more enhanced after cross- than after selfpollination and 2. a restricted number of RNA peaks is responsible for this difference; this RNA has a molecular weight different from r-RNA and t-RNA and could be messenger-like of nature. Results indicate a differential gene expression correlated with rejection or acceptance of the pollentubes by the style.

Effects of phenoxybenzamine on early stages of liver regeneration in partially hepatectomized rats

The changes in regenerating rat liver during the first 24 hr after 70 per cent hepatectomy have been studied using phenoxybenzamine, an adrenergic blocker. At 5 mg/kg, given before operation, the drug did not alter the increased uptake of 3H orotic acid into the acid-soluble intracellular fraction of the liver detected at 1 hr after hepatectomy, but the increase in 3H orotic acid incorporation into nuclear RNA at this time was only 40 per cent of that in untreated partially hepatectomized rats. Induction of ornithine decarboxylase (EC 4.1.1.17) was delayed by about 2 hr, but DNA synthesis was unaffected. When 5 mg/kg phenoxybenzamine was given between 5 and 10 hr after partial hepatectomy, DNA synthesis was altered. Given 9 hr after operation the drug delayed the onset of peak DNA synthesis by 3 hr, from 21.5 to 24.5 hr. The induction of nuclear histone phosphokinase was also delayed.

Pattern and activity of polyribosomes from the integument of blowfly larvae during development: Stimulation of polyribosome formation with β-ecdysone

The profiles and activity of polysomes obtained from the integument of larvae of the blowfly Calliphora erythrocephala during the late third instar as well as of ligated, and ligated ecdysone-treated larvae have been investigated. Polysomes are almost absent in 6–7-day-old animals, appear in 8-day-old larvae and are maximally developed in white prepupae. Incorporation of orotic acid into RNA of polysomes shows a similar dependence on larval stage, maximal incorporation being observed in white prepupae where two groups of polysomes are highly labelled. Leucine incorporation into protein also increases with progressing age, most of the incorporated radioactivity being found as soluble protein and only small amounts connected with polysomes. Preparations from ligated larvae show characteristics similar to those from 6–7-day-old larvae. Injection of ecdysone into ligated animals maximally stimulates within 3 h the formation of polysomes. Five hours after hormone administration the polysomal peaks diminish. Incorporation of 3H-orotic acid into RNA is stimulated within 1 h after hormone administration. Most of the radioactivity is found in the 30–60S region of the gradient, corresponding in part to RNA in form of RNP-particles, as shown by CsCl centrifugation. Three hours after ecdysone injection incorporation of orotic acid into RNA is maximal. Labelled RNA is found in significant amounts on the same groups of polysomes which are also labelled in white prepupae, whereas radioactivity in the 30–60S region shows a decline. Five hours after hormone administration, incorporation of orotic acid into RNA, although still being above control values, decreases both in the polysomal as well as in the 30–60S region. Part of the labelled RNA appearing in the polysomal region is found there in the form of RNP-particles with a buoyant density in CsCl centrifugation of 1.46.

The ribonucleoprotein particles from nuclear residual proteins of calf thymus

Ribonucleoprotein particles have been isolated from calf thymus nuclear residual proteins prepared by combined NaCl extractions and DNase treatment. The residual nucleoprotein particles contain 91.3% protein, 7.4% RNA, and 1.3% DNA. The size range of the residual particles is 115–276 × 20–50 Å, and the average sedimentation coefficient, 33S. The residual particles contain a 5S RNA with a base composition similar to that of the nuclear ribosomes. The protein moiety of the residual particles is heterogeneous when examined by polyacrylamide gel electrophoresis. Chemical analyses of the residual particles show that the proteins have a high acidic amino acid content. Double labeling of the residual particles' proteins and RNA by incubation of isolated calf thymus nuclei with 3H-orotic acid and 14C-glutamic acid revealed a high metabolic activity of the residual RNA and low incorporation of labeled glutamic acid into protein. These properties are in contrast to those of the nuclear ribosomes.