Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Polyribosomes”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 55 records · Page 3Linked to original sources
PubMed1973

Influence of auxin and incubation on the relative level of polyribosomes in excised soybean hypocotyl.

The influence of incubation and auxin (2,4-D) on polyribosome level in soybean hypocotyl was studied.A marked drop in the relative level of polyribosomes in excised apical or meristematic tissue (0 to 5 millimeters below the cotyledons) occurred during incubation. The addition of auxin to the incubation medium did not affect polyribosome level. A similar decrease in polyribosome level occurred in excised elongating tissue (5 to 15 millimeters below cotyledons) during incubation. Auxin, however, caused a small but highly reproducible stabilization of polyribosomes in this tissue. There was a rapid, but small, auxin-independent increase in polyribosomes of basal or nongrowing hypocotyl (from 20 to 40 millimeters below cotyledons) during incubation, followed by a larger auxin-dependent increase in polyribosomes. While auxin is known to cause an increase in total ribosomes during incubation of the excised basal hypocotyl, the observed transformation from monoribosomes to polyribosomes was not dependent on new ribosome synthesis.Protein synthetic activity (poly U-directed phenylalanine incorporation) of the 80S monoribosomes at low Mg(2+) levels increased during incubation of the excised basal hypocotyl. The increase in ribosome activity was biphasic (an initial auxin-independent phase followed by an auxin-dependent increase in activity) correlating with the biphasic increase in polyribosomes. The enhanced activity of 80S monoribosomes was related, at least in part, to an increase in the level of peptidyl-tRNA associated with the ribosome population. Removal of peptidyl-tRNA from the ribosomes reversed the auxin effect.The hypothesis is advanced that the increase in polyribosomes in response to incubation and to auxin is preceded by and dependent upon the activation of 80S monoribosomes. This activation is in addition to a requirement for continued RNA synthesis, at least in part mRNA, for the transition from monoribosomes to polyribosomes.

Journal Article↗
PubMed1975

Free and membrane-bound chloroplast polyribosomes Chlamydomonas reinhardtii.

Over half of the chloroplast ribosomes isolated from growing cultures of Chlamydomonas reinhardtii are bound to chloroplast thylakoid membranes if completion of nascent polypeptide chains is prevented by chloramphenicol. The free chloroplast ribosomes are recovered in homogenate supernatants, and presumably originate from the chloroplast stroma. Only about 10% of these free chloroplast ribosomes are polyribosomes, even under conditions when 70% of free cytoplasm ribosomes are recovered as polyribosomes. The nonionic detergent Nonidet P-40 liberates atypical polyribosomes (Type I), from membranes, which require both ribonuclease and proteases for complete conversion to monomeric ribosomes. Thus Type I particles are held together by mRNA but are also held together by peptide bonds. These Type I polyribosomes probably are not bound to intact membrane, but might be bound to some protein-containing sub-membrane particle. The Type I polyribosomes are dissociated to ribosomal subunits by puromycin and high salt, and contained 0.2 to 1 nascent chain per ribosome. If membranes are treated with Nonidet and proteases at the same time, polyribosomes which are digested to monomeric ribosomes by ribonuclease alone (Type II) are obtained. Type II polyribosomes are smaller than Type I, and probably represent the true size distribution of polyribosomes on the membranes. At least 50% of the membrane-bound ribosomes are polyribosomes, since that much membrane bound chloroplast RNA is recovered as Type I or Type II polyribosomes.

Cell Fractionation↗
PubMed1979

Studies on the intracellular segregation of polyribosome-associated messenger ribonucleic acid species in the lactating guinea-pig mammary gland.

1. Free and membrane-bound polyribosomes were isolated and the associated mRNA species characterized by cell-free protein synthesis, RNA-complexity analysis and polyribosome run-off in vitro. 2. Of the recovered polyribosomal RNA 85% was associated with membrane-bound polyribosomes and contained 87--93% of the total milk-protein mRNA species as assessed by cell-free protein synthesis or RNA-complexity analysis. 3. RNA-complexity analysis showed that the abundant (milk-protein mRNA assumed) species constituted 55% of the post-nuclear poly(A)-containing RNA population, the remainder consisting of a moderately abundant population (18%) and a low abundance population (27%). Calculations suggest that each population contained up to 2, 48 and 5000 different species respectively. 4. RNA-complexity analysis of the free polyribosomal poly(A)-containing RNA demonstrated that all the species in the post-nuclear fraction were present, though in different proportions, the abundant, moderately abundant and low-abundance groups representing 38, 30 and 32% of this population. 5. RNA-complexity analysis of the membrane-bound polyribosomal poly(A)-containing RNA revealed a more limited population, 72% consisting of the abundant (milk-protein mRNA) species, and 28% a population of up to 900 RNA species. 6. Polyribosome run-off confirmed that milk-protein mRNA was associated with the membrane-bound and free polyribosomes, but represented only a small fraction of the total protein synthesized by the latter. 7. Comparative analysis of milk proteins synthesized in mRNA-directed cell-free systems, or by run-off of free and of membrane-bound polyribosomes, is consistent with the interpretation that in vivo the initiation of protein synthesis occurs on free polyribosomes, followed by the attachment of a limited population to the endoplasmic reticulum. After attachment, but before completion of peptide synthesis, the detachable N-terminal peptide sequence of one of these(pre-alpha-lactalbumin) is removed. 8. The results are discussed in terms of the mechanisms involved in the intracellular segregation of mRNA species in the lactating guinea-pig mammary gland.

Animals↗
PubMed2001

Kidney ischaemia-reperfusion injury and polyribosome structure.

BACKGROUND: Inhibition of protein synthesis and polyribosome disaggregation are the early events in cell injury provoked by various pathogenic mechanisms, including energy depletion. Polyribosome disaggregation might be expected to occur during ischaemia-reperfusion injury due to ischaemic energy depletion, but also due to detrimental effects of reactive oxygen species on various macromolecules and cellular structures. METHODS: Mouse kidney ischaemia-reperfusion injury was provoked by temporary clamping of the renal artery. The polyribosome sedimentation pattern was analyzed by sucrose density centrifugation of kidney postmitochondrial supernatant. RESULTS AND CONCLUSIONS: Ischaemia for 5 min in the mouse kidney provoked polyribosome disaggregation and an increase of monomer ribosome fraction which was augmented during 10-360 min of reperfusion. Recovery of polyribosome aggregates appeared between 6 and 24 h of reperfusion. Cycloheximide pretreatment prevented only polyribosome disaggregation caused by ischaemia and not that caused by reperfusion. This indicates different mechanisms of polyribosome disaggregation during ischaemia and reperfusion. It probably occurs in the former due to inhibition of initiation of translation, resulting in accumulation of unprogrammed monomer ribosomes, and in the latter due to the splitting of mRNA and breakdown of polyribosomes. Reperfusion did not increase ribonuclease activity in kidney cytosol, but increased the tissue concentration of malonaldehyde, indicating an augmentation in oxygen free radical generation. Possibly these may have caused a non-enzymatic breakdown of polyribosomes. However, pretreatment with allopurinol did not prevent polyribosome breakdown during ischaemia-reperfusion injury.

Allopurinol↗
PubMed1976

[Content of polyribosomes and poly-A-containing RNA in rat liver during hepatocarcinogenesis induced by diethylnitrosamine].

The contents of total polyribosomes and the ratio of free to membrano-bound polyribosomes as well as the number of poly(A)-containing RNA in cytoplasm and in two classes of polyribosomes of the liver were studied in rats chronically treated with diethylnitrosamine. A decrease in the number of total polyribosomes and changes in the ratio of free to membrane-bound polyribosomes toward a decrease of the latter were found within the whole period of carcinogenesis. The absolute amount of the free polyribosomes per 1 gr of the liver tissue did not differ significantly from the normal values but the number of the membrane-bound polyribosomes in the treated animals was 2-2,5 times as low. The amount of poly(A)-containing RNA in the postmitochondrial fraction increased and decreased in free polyribosomes. A redistribution of the templates by free and membrane-bound polyribosomes in cytoplasm of the hepatocytes during hepatocarcinogenesis due to the disturbances in the equilibrium between these two classes of polyribosomes is suggested.

Animals↗
PubMed1986

Polyribosomes associated with synaptic specializations on axon initial segments: localization of protein-synthetic machinery at inhibitory synapses.

Previous studies have revealed a selective association between polyribosomes and axospinous synapses in a variety of brain regions. The present study evaluates whether polyribosomes are also associated with the symmetrical and presumably inhibitory synaptic connections found on the initial segment of axons of some neurons in the CNS. The initial segments of pyramidal neurons in the sensorimotor cortex of the monkey and of granule cells in the hippocampus of the rat were examined. The initial segments of these cell types are contacted by GABAergic terminals that form symmetrical synaptic connections. In the present study, these initial segments were found to contain polyribosomes that tended to be selectively localized beneath the synaptic specializations. Both the synaptic connections and the polyribosomes were localized to the initial segment; after the point at which the axon became myelinated, neither synapses nor polyribosomes were found. The association between polyribosomes and synapses was also suggested by the position of the polyribosomes with respect to the synapse. In each cell type, the majority of the polyribosomes that were present in the initial segments appeared to be localized preferentially beneath synaptic sites, although some polyribosomes were also present in the core of the axon. These data reveal that polyribosomes are not peculiar to spine synapses, but rather are ubiquitous components of the subsynaptic region of many types of synapses. We propose that neurons may regulate their innervation by positioning protein-synthetic machinery (and appropriate mRNA molecules) at particular locations in order to construct particular types of synapses at defined positions on the postsynaptic cells' receptive surface.

Animals↗
PubMed1980

[Content and properties of different classes of membrane-bound polyribosomes during embryogenesis in chickens].

The quantitative ratio, chemical composition and sedimentation properties of three polyribosomal fractions (free polyribosomes and polyribosomes tightly and loosely bound to the endoplasmic reticulum) isolated from chicken embryos of different age were studied. The fraction of polyribosomes loosely bound to the membranes was obtained from the total preparation of membrane-bound polyribosomes by treatment with high ionic strength solutions. It was found that the polyribosomes loosely bound to the membranes and free polyribosomes possess similar chemical composition and sedimentation properties. All the fractions studied were characterized by a much higher polyribosomal content as compared to the 80S monomers. In the course of embryogenesis the amount of polyribosomes loosely bound to the membranes is decreased, whereas that of tightly bound ones is increased. These data are indicative of an enhanced synthesis of secreted proteins, particularly collagen, on tightly bound polyribosomes during embryogenesis in chicken.

Animals↗
PubMed1971

Polyribosomal attachment to rat liver and hepatoma endoplasmic reticulum in vitro. A method for its study.

A system for study and measurement of the attachment in vitro of exogenous polyribosomes to membranes has been presented. Its main features are use of low temperature, post-microsomal supernatant, pyrophosphate and citric acid to remove ribosomes from the surface of rough endoplasmic reticulum, and a method for quantitative separation of unattached from membrane-associated polyribosomes. The following were found. (1) Rough endoplasmic reticulum, from which ribosomes had been removed by treatment with pyrophosphate and citrate, bound over 50% of added polyribosomes, whereas the untreated (or control) rough and smooth endoplasmic reticulum and the smooth endoplasmic reticulum treated with pyrophosphate-citrate did not bind polyribosomes. (2) The polyribosome-binding capacity of rough endoplasmic reticulum stripped of its ribosomes decayed upon storage of the membranes at 0-4 degrees C. The half-life of this decay was about 6 days whereas that of the polyribosome-binding capacity of hepatoma stripped rough endoplasmic reticulum was about 1.5 days. (3) Preparations of stripped rough endoplasmic reticulum after reassociation with polyribosomes in vitro were quite similar to preparations of native rough endoplasmic reticulum as viewed with the electron microscope. Evidence is presented to support the contention that association of polyribosomes with membranes was the result of polyribosomal reattachment to the membranes rather than trapping of the polyribosomes between vesicles of the membranes.

Animals↗
PubMed1989

Transcortin biosynthesis and intracellular distribution of rat liver polyribosomes synthesizing transcortin.

Transcortin biosynthesis in rat has been examined using liver slices technique. The incorporation of 14C-labeled amino acids into the anti-transcortin-precipitable material of liver slices has been measured and compared with that of serum transcortin. It was shown that liver synthesized transcortin with an apparent mol. wt of 66 kDa on SDS-electrophoresis which co-migrated with authentic rat serum transcortin. In order to determine an intracellular distribution of transcortin synthesizing polyribosomes, the binding character of [125J]anti-transcortin-IgG to free and membrane-bound rat liver polyribosomes has been studied. It was shown that after incubation of [125J]anti-transcortin-IgG with liver membrane-bound polyribosomes, the radioactivity was associated with the discrete polyribosome fraction in heavy polyribosome region. In similar experiments the radioactivity of [125J]anti-transcortin-IgG bound to the free polyribosomes was distributed throughout the polyribosome region. The results of our experiments obviously demonstrated that serum transcortin was synthesized exclusively on membrane-bound polyribosomes of rat liver; free polyribosomes were devoid of detectable antigenic material able to bind antibodies to transcortin.

Animals↗
PubMed1975

Initiation factors in protein synthesis by free and membrane-bound polyribosomes of liver and hepatoma.

The activity of initiation factors obtained from free and membrane-bound polyribosomes of liver and of transplantable H5123 hepatoma of rats was investigated by using an assay of protein synthesis in vitro in which poly (U)-directed polyphenylalanine synthesis was measured. Initiation factors of membrane-bound polyribosomes prepared by using the anionic detergent deoxycholate exhibited less activity in incorporating [14C]phenylalanyltRNA into polypetides than did initiation factors of free polyribosomes. However, when membrane-bound polyribosomes were prepared after using the non-ionic detergent Triton X-100, no significant differences in activities in polyphenylalanine synthesis were observed between the initiation factors of free and membrane-bound polyribosomes. These results suggest that Triton X-100 is preferable to deoxycholate in the isolation of of initiation factors from polyribosomes. Initiation factors, prepared by using Triton X-100, of free polyribosomes of hepatoma exhibited greater activity in the stimulation of polyphenylalanine synthesis than did the initiation factors of free or membrane-bound polyribosomes of host livers or of membrane-bound polyribosomes of hepatomas.

Animals↗
PubMed1983

Preparation and properties of a complex from rat liver of polyribosomes with components of the cytoskeleton.

Gel filtration with 1% agarose (Bio-Gel A-150m) separates polyribosomes bound to microsomal membranes from 'free' polyribosomes when these fractions are prepared by standard centrifugal techniques. However, when polyribosomes contained in an unfractionated postmitochondrial supernatant are run on an identical column, over 90% of the total polyribosomes are present as aggregates, designated 'membrane-cytomatrix', which are eluted in the column void volume. Polyribosomes are not released from these aggregates on removal of microsomal phospholipids by treatment of postmitochondrial supernatant with 1% Triton X-100, a neutral detergent. The aggregates are disrupted by the usual ultracentrifugation techniques used in subcellular fractionation. After treatment of membrane-cytomatrix with Triton X-100 to remove phospholipids and membrane proteins, 58% of the polyribosomes still remain associated with protein-containing complexes in the form of a cytomatrix and are not 'free'. Preparations of both membrane-cytomatrix and cytomatrix are capable of sustained protein synthesis. Sodium dodecyl sulphate/polyacrylamide-gel electrophoresis revealed that the cytoskeletal proteins actin and myosin are present in the cytomatrix. Incubation of cytomatrix preparations with the actin-depolymerizing agent deoxyribonuclease I caused release of the polyribosomes. Polyribosome release by deoxyribonuclease I was prevented by prior incubation with phalloidin, which is known to stabilize F-actin. Thus polyribosomes are associated with cytoskeletal elements in rat liver, and this association is dependent on polymeric forms of actin.

Animals↗
PubMed1984

Differences in sequence content of nuclear and cytoplasmic polyribosomal RNA from adenovirus-infected cells.

RNA molecules from nuclear and cytoplasmic polyribosomes of adenovirus-infected HeLa cells were compared by hybridization to analyse the sequence content. Nuclear polyribosomes were released by exposure of intact detergent-washed nuclei to poly(U) and purified. Cytoplasmic polyribosomes were also purified from the same cells. To show that nuclear polyribosomes contain ribosomes linked by mRNA, polyribosomes were labelled with methionine and uridine in the presence of actinomycin D in adenovirus-infected cells. Purified nuclear polyribosomes were treated with EDTA under conditions which dissociate polyribosomes into ribosomes and subunits with a simultaneous release of mRNA, and sedimented. The treatment dissociated these polyribosomes, releasing the mRNA from them. Radiolabelled total RNA from each polyribosome population was fractionated in sucrose gradients into several pools or hybridized to intact adenovirus DNA to select virus-specific RNA. Sucrose-gradient-fractionated pool-3 RNA (about 28S) and virus-specific RNA were then hybridized to fragments of adenovirus DNA cleaved by restriction endonucleases SmaI, HindIII and EcoRI by the Southern-blot technique and by filter hybridization. The results showed that nuclear RNA contained sequences, from about 0 to 18 map units, which were essentially absent from cytoplasmic RNA. Furthermore, the amount of virus-specific RNA for a particular sequence was also different in the two populations.

Adenoviridae Infections↗
PubMed1989

Ferritin mRNA is found on bound as well as on free polyribosomes in rat heart.

Free and endoplasmic reticulum-bound polyribosomes from rat heart were examined for their ferritin mRNA content. A procedure for separation and purification of the two ribosome populations that produced good yields of homogeneous mono- and polyribosomes with no contaminating ultrastructures and gave distinctive sedimentation profiles in 15-50% sucrose gradients was developed. 14C-labeled free and bound polyribosomes added to heart preparations indicated that only 3% of free and 5.5% of bound polyribosomes cross-contaminated the bound and free fractions, respectively. RNA from both polyribosome populations hybridized with [32P]cDNA for rat ferritin. The extent of hybridization with mRNA from endoplasmic reticulum (ER)-derived polyribosomes was much greater than what could be accounted for by cross-contamination with free polyribosomes. This indicates that heart ferritin is synthesized not only on free polyribosomes for internal use in iron storage but also on ER-bound polyribosomes, where it may be destined for secretion into the plasma.

Animals↗
PubMed1967

Biochemical and physicochemical characterization of collagen-synthesizing polyribosomes.

Synthesis of collagen on polyribosomes has been demonstrated in vitro in chick embryo corium by radioisotope incorporation, zone centrifugation through sucrose gradients, and analytical ultracentrifugation. Collagen synthesis was associated with polyribosomes ranging in size, as reflected by their sedimentation constants, from about 180S to approximately 1600S. Most of the newly formed collagen, hydroxyproline, was present on the largest polyribosome aggregates ( approximately 350-1600S), but small polyribosomes ( approximately 180-200S) also contained collagen. On the basis of the proline-(14)C/hydroxyproline-(14)C ratios and the disrupting effect of collagenase, the proposal is made that the 350-1600S polyribosomes from this tissue are involved predominantly in collagen synthesis. The large polyribosomes are disrupted extensively by collagenase but only partially by ribonuclease and trypsin. Therefore, it appears that they are stabilized by the interaction of newly forming collagen chains. Evidence is presented consistent with the hypothesis that these large polyribosomes are formed by the aggregation of small polyribosomes (180-200S) through the interaction of collagen polypeptides. It is suggested that these small polyribosomes might be involved in the synthesis of subunits of the collagen alpha chain.

Animals↗
PubMed1970

Light-induced Development of Polyribosomes and the Induction of Nitrate Reductase in Corn Leaves.

Nitrate reductase activity was induced by nitrate in green corn (Zea mays) leaves in either light or darkness. The induction process required oxygen in darkness but not in light. A light treatment was required before the enzyme could be induced in etiolated leaves.The capacity for nitrate reductase induction by nitrate was positively correlated with the level of cytoplasmic polyribosomes under a variety of experimental conditions. (a) Light-grown leaves contained high levels of polyribosomes (84% of the total population, most of which were of the 80 S type); similarly high levels of nitrate reductase activity were induced. (b) The level of polyribosomes and the ability to form nitrate reductase activity rapidly decreased in light-grown leaves following transfer to an anaerobic environment in the dark; both parameters were maintained at a high level when light-grown leaves were kept in the light under anaerobic conditions. (c) The ability of light-grown leaves, previously placed in darkness under nitrogen to dissociate polyribosomes to monoribosomes, to form nitrate reductase activity again correlated with the level of reformed polyribosomes following transfer of the leaves back to light. (d) Etiolated leaves contained a low level of cytoplasmic polyribosomes (27%), and nitrate reductase activity was induced following exposure to light only after a lag of 2 to 4 hours. During this lag period there was a marked increase in the level of polyribosomes.The ability of leaves to form nitrate reductase activity and the level of polyribosomes also correlated with the level of in vitro incorporation of amino acids into protein by the isolated ribosome preparations. Thus, the apparent requirement of light for nitrate reductase induction in etiolated leaves seems not to be specific. Rather an influence of light upon the development of an active protein-synthesizing apparatus as evidenced by the state of polyribosomes is indicated.The results also show that energy from photosynthetic phosphorylation can be used to maintain cytoplasmic polyribosomes (and thus to drive cytoplasmic protein synthesis), at least under anaerobic conditions.

Journal Article↗
PubMed1977

Identification and Precipitation of the Polyribosomes in Chlamydomonas reinhardi Involved in the Synthesis of the Large Subunit of d-ribulose-1,5-bisphosphate Carboxylase.

The size classes of polyribosomes involved in the synthesis of ribulose-1,5-bisphosphate carboxylase large subunit were determined by binding radioiodinated specific antibodies to polyribosomal preparations from Chlamydomonas reinhardi. Antibodies specific to the denatured large subunit and to the native enzyme bound primarily to small polyribosomes (N = two to five ribosomes). The binding of antibodies to small polyribosomes was unexpected since the large subunit is a large polypeptide (molecular weight 55,000) coded for by a corresponding large mRNA (12-14S). Control experiments showed that this unexpected pattern of antibody binding was not a result of messenger RNA degradation, "run-off" of ribosomes from polyribosomes, or adventitious binding of the completed enzyme to a selected class of polyribosomes. In addition, polyribosomes bearing nascent large subunit chains have been immunoprecipitated from small polyribosome fractions. A large RNA species that can direct the synthesis of large subunit in vitro was extracted from small polyribosomes.

Journal Article↗
PubMed1976

Fractionation on the microscale of brain polyribosomes by electrophoresis on acrylamide gels.

A method is described for brain polyribosome fractionation by acrylamide gel electrophoresis. Brain polyribosomes were run in 2.0% gels in quartz capillaries of 800 mum inner diameter where the gels were supported by capillary force. The gels could then be ultraviolet-scanned in situ. Amounts of brain polyribosomes as small as 10-10(-3) A260nm unit could be analysed by this method. The method was checked by running a macroscale-prepared brain polyribosome sample. The various electrophoretic bands obtained showed a favourable A260nm: A280 ratio. A short RNase treatment caused the disappearance of the slowly migrating bands and the emergence of a predominant band migrating faster than the dimer. The various polyribosomal bands were then identified by comparison with the mobility of polyribosome fractions taken from a sucrose gradient fractionation. Finally, the electrophoretic pattern of brain polyribosomes compared favourably with the pattern obtained by the classic method of sucrose gradient sedimentation. The electrophoretic fractionation of polyribosomes prepared from one rat hippocampus (80 mg) is presented.

Animals↗
PubMed1980

Induction of rat liver metallothionein mRNA and its distribution between free and membrane-bound polyribosomes.

Total, membrane-bound and free polyribosomes were purified from livers of Zn2+-treated and control rats. Polyadenylated RNA was separated from the polyribosomal RNA extracts by oligo(dT)--cellulose chromatography and translated in a wheat-germ cell-free translation system. Newly synthesized 35S-labelled metallothionein was isolated from the other [35S]methionine-labelled translation products by activated-thiol--Sepharose 4B chromatography. The purity of the 35S-labelled metallothionein product was substantiated by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. Zinc administration resulted in an elevation of metallothionein mRNA activity to 11% of the total polyribosomal mRNA activity. The vast majority of biologically active metallothionein mRNA was localized in the free polyribosomal pool, at least 94% and 97% in control and zinc-treated rats respectively. The increase in the percentage of polyribosomal mRNA coding for metallothionein after zinc administration was 3-fold, whether measured directly in total polyribosomal mRNA or as a combination derived from membrane-bound and free polyribosomal mRNA. These data indicate that the induction of metallothionein mRNA by zinc involves only free polyribosomes and suggest that the function of metallothionein is limited to intracellular processes.

Animals↗