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At least 19 recordsLinked to original sources
PubMed1977

Differences in size, structure and function of free and membrane-bound polyribosomes of rat liver. Evidence for a single class of membrane-bound polyribosomes.

Free loosely bound and tightly bound polyribosomes were separated from rat liver homogenate by salt extraction followed by differential centrifugation, and several of their structural and functional properties were compared to resolve the existence of loosely bound polyribosomes and verify the specificity of the separation. The free and loosely bound polyribosomes have similar sedimentation profiles and polyribosome contents, their subunit proteins have similar electrophoretic patterns and their products of protein synthesis in vitro show a close correspondence in size and amounts synthesized. In contrast, the tightly bound polyribosomes have different properties from those of the free and loosely bound polyribosomes; their average size is significantly smaller; their polyribosome content is higher; their 60 S-subunit proteins lack two components and contain four or more components not found elsewhere; their products of protein synthesis in vitro differ in size and amounts synthesized. These observations show that rat liver membranes entrap a large fraction of the free polyribosomes at low salt concentrations and that these polyribosomes are similar to those of the free-polyribosome fraction and are different from those of the tightly bound polyribosome fraction in size, structure and function.

Animals↗
PubMed1981

[Polyribosomes loosely bound to endoplasmic reticulum membranes as an early step in the formation of a membrane-polyribosome complex].

Fractionation of polyribosomes tightly and loosely bound to the endoplasmic reticulum membranes in a sucrose density gradient was carried out. The obtained fractions (mono- and disomes, light and heavy polyribosomes) were analyzed in a homologous cell-free protein-synthesizing system in order to determine their total protein- and collagen-synthesizing activities. All the three fractions prepared by fractionation of the loosely bound polyribosomes were found to be actively involved in protein synthesis translation. In case of tightly bound polyribosomes only the fractions of light and heavy polyribosomes could actively synthesize the protein. Study of collagen synthesis in the given fractions revealed that in tightly bound polyribosomes this synthesis is localized in the light and have polyribosomal fractions, while in loosely bound ones only the fractions of mono- and disomes are responsible for active collagen synthesis. Based on the present-day concepts on the nature of bonds formed by interaction between the endoplasmic reticulum membranes and polyribosomes the polyribosomes loosely bound to the membranes cna be considered as an early step in the formation of a membrane-polyribosome complex.

Animals↗
PubMed1970

Regulation of polyribosome formation and protein synthesis in the uterus. Effect of administration of graded doses of oestradiol-17-beta and actinomycin D on the amino acid-incorporation activity in vitro and the cytoplasmic concentration in vivo of polyribosomes.

1. The effects of graded doses of oestradiol-17beta and actinomycin D, administered separately or together, on the amino acid-incorporation activity in vitro and the cytoplasmic concentration in vivo of uterine polyribosomes are described. Preparations of polyribosomes isolated from uteri of ovariectomized adult rats were determined for cytoplasmic concentration in vivo and assayed for [(14)C]leucine-incorporation activity in the cell-free system, exactly as described by Teng & Hamilton (1967b). 2. A minimal dose of 10mug of oestradiol-17beta administered for 10h was found to increase, by about 100%, both the amino acid-incorporation activity in vitro and the cytoplasmic concentration in vivo of the polyribosomes. A minimal dose of 250mug of actinomycin D administered for 10h was found to inhibit, by about 50%, the incorporation activity in vitro of the polyribosomes. All doses of the inhibitor administered for 10h failed to alter the cytoplasmic concentration in vivo of the polyribosomes. 3. A dose of 10mug of oestradiol-17beta restored to the control value the inhibitory effect of a dose of either 50 or 125mug of actinomycin D on the activity in vitro of the polyribosomes, at 10h after treatment with the inhibitor and the hormone. In these experiments, there was an increase of 60-100% in the cytoplasmic concentration in vivo of the polyribosomes. 4. A dose of 125mug of actinomycin D, administered to animals along with 10mug of oestradiol-17beta for 6-36h, abolished the hormone-induced enhancement of the incorporation activity in vitro, but did not prevent an increase of about 200% in the cytoplasmic concentration in vivo of the polyribosomes. However, treatment with 750mug of the inhibitor abolished both stimulatory effects of the hormone. 5. The results reported indicate that the stimulatory effects of oestradiol-17beta in vivo on the number and activity of the cytoplasmic polyribosomes in the uterus of the ovariectomized rat have different sensitivities to actinomycin D, but the primary molecular mechanisms responsible for the results are unknown. The major conclusion drawn is that the formation and appearance in the cytoplasm of newly formed polyribosomes are important features of the early action of oestrogen in the uterus.

Amino Acids↗
PubMed1991

Selective localization of polyribosomes beneath developing synapses: a quantitative analysis of the relationships between polyribosomes and developing synapses in the hippocampus and dentate gyrus.

Previous studies have revealed that polyribosomes are selectively localized beneath post-synaptic sites on central nervous system (CNS) neurons, and are particularly prominent during periods of synapse growth. The present study evaluates whether polyribosomes are most prominent at a consistent time in the developmental history of the synapse, or instead at a consistent time in the life of the organism regardless of the state of synaptic maturation (suggesting a globally acting factor). We compare the time course of synaptogenesis and the association between polyribosomes and developing synapses in three regions that develop at different rates: the external and internal blades of the dentate gyrus, and the CA1 region of the hippocampus proper. Each region was examined electron microscopically at 1, 4, 7, 10, 15, 20, 28 and over 120 days of age, evaluating: (1) synapse density (the number of synaptic profiles/area of neuropil), (2) the width of the neuropil layers, (3) the proportion of synapses with underlying polyribosomes, and (4) the number of polyribosome-containing synapses/area of neuropil. As anticipated on the basis of the differences in cytogenesis, the time course of synaptogenesis was different in the three regions. In the external blade of the dentate gyrus, synapse density increased in a nearly linear fashion between birth and 15 days of age, and then continued to increase at a somewhat slower rate until 28 days of age. Synapse development in the internal blade was delayed by several days in comparison to the external blade. In CA1, synapse density increased slowly between 1 and 7 days, and then at a rapid rate between 7 and 28 days of age. In all three regions, the proportion of synapses with underlying polyribosomes was highest between 1 and 7 days of age, and then decreased as synapse density increased. However, the peak in the number of polyribosome-containing synapses/unit area of neuropil occurred at different times in the three regions (4-7 days of age in the external blade of the dentate gyrus and in CA1, and 20 days of age in the internal blade). In addition to further defining the relationship between polyribosomes and developing synapses, the present study provides a data base on the time course of synapse development in the hippocampus and dentate gyrus, which will be useful for comparisons with other measures.

Aging↗
PubMed1979

Polyribosome size analysis. Measurement of number-average polyribosome sizes.

The analysis of translational efficiencies of specific mRNAs requires a determination of the polyribosome size. The appropriate value to use in such calculations is the number-average size. A method is described for accurately measuring the number-average size of total and of specific protein synthesizing polyribosomes using isokinetic sucrose density gradients and 125I-labeled antibodies. By this method, we demonstrated that albumin synthesizing polyribosomes from a serum albumin secreting mouse hepatoma cell line exist over a broad range from trimers to 20-mers (mean 6-10). The specificity of antibody interaction with polyribosomes was demonstrated using cells not synthesizing mouse serum albumin, and by demonstrating that 125I-anti ovalbumin does not bind to mouse hepatoma polyribosomes. Treatment of the mouse hepatoma cells with 1 MUM cycloheximide shifted practically all of the monomers into polyribosomes resulting in an increase in the number-average size of the albumin synthesizing polyribosomes. Cycloheximide treatment, however, did not eliminate the size heterogeneity in the albumin synthesizing polyribosomes.

Albumins↗
PubMed1980

Changes in polyribosomes and poly(A)-containing polyribosomal RNA in the uterus of the ovariectomized rat in response to oestradiol-17 beta treatment.

Polyribosome formation and the characteristics of polyribosomal poly(A)-containing RNA from uteri of ovariectomized rats responding to a single dose of oestradiol-17 beta was investigated. The mean proportion of polyribosomes in the atrophic uterus was 65%. In response to 10 micrograms of oestradiol-17 beta/100 g body mass, the amount of polyribosomes increased to 88% 24 h after stimulation. Thereafter the proportion of polyribosomes decreased to a value of 48% at 72h. The pattern of amino acid incorporation in oocytes from Xenopus laevis injected with these polyribosomes was similar to the changes in polyribosome formation and degradation. The polyribosomal poly(A)-containing RNA from the controls consisted of a heterogeneous population of RNA with sedimentation values between 5S and 25S. The hormone stimulation resulted in an increase in both the amount and the size (13S to 35S) of the RNA.

Animals↗
PubMed1973

Polyribosomes from Peas: II. Polyribosome Metabolism during Normal and Hormone-induced Growth.

Polyribosomes as large as 10-mers (strands of messenger RNA bearing 10 ribosomes) were isolated from etiolated pea (Pisum sativum L. var. Alaska) stem tissue during all stages of development when methods were used which essentially eliminated ribonuclease activity during extraction. Actively growing tissue, harvested from the apical 10 mm, yielded many large polyribosomes and a low (<20%) proportion of monosomes. Similar tissue, allowed to age by applying lanolin to decapitated apices, showed a progressive decrease in number of larger polyribosomes and an increase in the proportion of monosomes. Hormone treatments, which prolonged growth and delayed aging, delayed the loss in large polyribosomes and the increase in proportion of monosomes. Growth-stimulating hormones, added to previously aged tissue, stimulated the production of many large polyribosomes in pre-existing cells.It is suggested that (a) large polyribosomes occur in all regions of the pea stem, (b) changes in polyribosome distribution appear to precede changes in growth rate, (c) loss of larger polyribosomes is closely related to a decrease in mRNA templates followed more gradually by loss of ribosomes, (d) hormone-stimulated continuation of growth is accomplished through maintenance of available mRNA.Methods are described, involving detailed analysis of polysome distribution, which, although they cannot be used to measure changes in initiation of ribosomes on to mRNA, do permit measurement of the amount of polysomal-associated mRNA present in tissues at different stages of growth. These analyses lead to the further suggestion that hormone stimulation of growth of previously nongrowing tissue is accomplished primarily through an increase in available mRNA prior to synthesis of ribosomes.

Journal Article↗
PubMed1967

Regulation of polyribosome formation and protein synthesis in the uterus. Effect of ovariectomy and administration of oestradiol-17beta on the amino acid-incorporation activity in vitro and the cytoplasmic concentration in vivo of polyribosomal preparation.

1. The hormonal regulation of cytoplasmic protein synthesis in the uterus is described. Polyribosomal preparation from uteri of normal or ovariectomized rats was isolated by procedure 3 and assayed for [(14)C]leucine-incorporation activity in the cell-free system, as described by Teng & Hamilton (1967). 2. Ovariectomy of normal animals caused, 3 weeks after surgery, a 50-60% increase in the amino acid-incorporation activity in vitro of uterine polyribosomal preparation, but a 90-95% decrease in the cytoplasmic concentration in vivo of the preparation. 3. Administration of 10mug. of oestradiol-17beta to ovariectomized rats at zero time caused, 10-12hr. later, a 100% stimulation in amino acid-incorporation activity in vitro of the uterine polyribosomal preparation. From 12hr. to 36hr. after hormone administration, the activity in vitro of the preparation decreased. If a second dose of hormone was administered at 36hr., the activity in vitro of the preparation continued to decrease, and approached at 48hr. and 72hr. the lower activity observed for the preparation from normal animals. The cytoplasmic concentration of polyribosomal preparation increased by 600-700% under these experimental conditions. If a second dose of oestradiol-17beta was not administered at 36hr., the initially elevated cytoplasmic concentration of the preparation decreased by 50% from 36hr. to 72hr., and the activity in vitro of the preparation was not fully depressed to the ;normal' value. 4. Pretreatment of ovariectomized animals with actinomycin D or cycloheximide abolished 80-90% of the stimulatory effects of hormone treatment on the amino acid-incorporation activity in vitro and cytoplasmic concentration in vivo of uterine polyribosomal preparation. 5. Two major conclusions are drawn from the results reported: that during early oestrogen action new polyribosomes having amino acid-incorporation properties different from those of the old ones appear and accumulate in the cytoplasm of the uterus; and that the regulation of cytoplasmic protein synthesis in the organ by oestrogen is of an indirect nature, with dual effects of the hormone on genetic transcription resulting in turn in a regulation of the rate and amount of genetic translation.

Journal Article↗