Ribosome Population Research Articles

Affinity labelling of 23-S ribosomal RNA in the active centre of Escherichia coli peptidyl transferase.

The radioactively labelled puromycin analogue, 5′‐O‐(N‐bromoacetyl‐p‐aminophenyl‐[32P]‐phosphoryl)3′‐N‐l‐phenylalanyl puromycin aminonucleoside, has been used as an affinity label to identify components of Escherichia coli ribosomes that are in the active centre for the peptidyl transferase function of the ribosome.The affinity label as an acceptor substrate at 0°C in the “fragment reaction” assay of peptidyl transferase was only about 10% as efficient as puromycin, although both compounds had the same Km (approximately 0.1 mM). Preincubation of 70‐S ribosomes with the affinity label at 27°C led to irreversible inactivation of the peptidyl transferase activity (pH optimum 8.8) and extensive covalent attachment of radioactivity to the ribosomes (up to 25 molecules of affinity label per ribosome). All the inactivation of the enzyme, but only a small proportion of the covalent labelling (up to 2 molecules per ribosome), could be prevented by the presence of chloramphenicol, an A′‐site‐specific inhibitor of peptidyl transferase. The chloramphenicol‐insensitive affinity labelling, designated nonspecific, was shown to be distributed over many ribosomal proteins in both the 50‐S and 30‐S ribosomal subunits.The specific, chloramphenicol‐sensitive, affinity labelling was shown to be attached to the 23‐S component of ribosomal RNA by polyacrylamide gel electrophoresis. Comparison of the stoichiometry of the specific RNA affinity labelling with the percentage inactivation of peptidyl transferase revealed a linear correlation between the two quantities, such that 2.1 ± 0.2 moles of affinity label per 23‐S RNA corresponded to complete inactivation.As previously reported [Harris, R. J., Greenwell, P. & Symons, R. H. (1973) Biochem. Biophys. Res. Commun. 55, 117–124], incubation of the covalent ribosome‐affinity label complex with the donor substrate, acetyl‐l‐[3H]leucylpentanucleotide, caused acetyl‐[3H]leucine to become covalently attached to ribosomal RNA. It has now been shown that this attachment was also on the 23‐S RNA and that it was linearly correlated both with the extent of 32P‐affinity labelling of the RNA, and with the percentage inactivation of peptidyl transferase activity. However, only approximately 0.5 % of the ribosome‐affinity label complexes were able to effect acetyl‐[3H]leucine transfer.The above results are consistent with a two‐site model for the ribosome, if affinity labelling of the peptidyl transferase acceptor (A′) and donor (P′) sites follows a hypothetical mechanism in which the affinity label binds to (and subsequently reacts with) first the A′ site, and then the P′ site, in an ordered and strongly positive cooperative manner. The hypothesis explains the linear 2:1 correlation between specific affinity labelling and loss of peptidyl transferase activity, and also the small proportion of ribosome‐affinity label complexes able to effect peptidyl transfer. The latter is considered to be the population of ribosomes labelled only in the A′ site, because P′ site labelling lags slightly behind that of the A′ site. The results also suggest that 23‐S ribosomal RNA has a direct functional role in the binding of the 3′‐CCA terminal triplets of tRNA to peptidyl transferase.

Appearance of a kidney-specific ribosomal protein during mouse embryonic development

Ribosomes were prepared from adult mouse liver and kidney and the protein components examined by SDS polyacrylamide gel electrophoresis. A kidney specific protein was identified and was found to be associated with the large ribosomal subunit. In vitro labelling of 11- and 14-day embryonic kidneys and subsequent analysis of the ribosomal proteins indicated a change in the ribosomal population during development. The kidney specific protein was synthesized during the first four days of kidney organogenesis.

In vivo rates of deaggregation of polyribosomes in normal and regenerating liver after the injection of pactamycin

The in vivo injection of 400 μg of pactamycin into normal and partially hepatectomized mice resulted in the rapid deaggregation of their liver polyribosomes and accumulation of ribosomal monomers. When nascent polypeptide chains were labelled in vivo, the accumulated monomers were free of protein radioactivity. The previous injection of cycloheximide prevented the deaggregation of polyribosomes. The proportion of polyribosomes relative to the total ribosome population was measured in normal and regenerating livers from 10–60 s after the injection of pactamycin. Their rate of deaggregation was the same in both situations and it was such as could be expected from the in vivo rate of translation measured by other means elsewhere. These results are taken as further evidence that the efficiency with which polyribosomes are utilized for protein synthesis is the same in normal and regenerating livers.

Caution in the interpretation of plant ribosome studies.

Published methods of ribosome isolation from plant tissues result in a partial and selective recovery of the total ribosome population. The enrichment of polyribosomes obtained may give a false impression of the change in translational capacity of the tissue under study.

Protein-synthesising activity of free and membrane-bound ribosomes in vitro and their differential sensitivity to protein synthesis inhibitors

1. Introduction Membrane-bound ribosomes of rat liver were found to be more sensitive to various protein synthesis inhibi- tors introduced in vivo than free ribosomes [ 11. Also in cell-free protein synthesis, the attached ribosomes were shown to behave in some aspects differently from the free ones [2-51, indicating a close relationship of the bound ribosome with the membrane. These studies describe the behaviour of free and membrane-bound ribosomes in a cell-free system towards inhibitors of some of the steps in protein synthesis. Membrane-bound ribosomes were generally found to be more sensitive towards these inhibitors, especially to cycloheximide. After solubilisation of membranes by Triton X-100, this greater sensitivity to the inhibitors was lost. 2. Methods The fractions were prepared from the livers of male albino rats weighing approx. 150 g, which were starved for 16-l 8 hr before an experiment. The livers were homogenised (0.007” cleavage) in 2 vol of 0.25 M sucrose in TKM buffer (50 mM Tris-HC 1, pH 7.6 at 20°C; 25 mM KC 1; 5 mM magnesium acetate) con- taining 1 mM dithiothreitol. The post-mitochondrial supernatant was prepared by centrifuging the homoge- nate for 10 min at 10 OOOg. It was then fractionated using a discontinuous sucrose gradient consisting of 1.35 M sucrose and 2.0 M sucrose in TKM buffer [6]. The cell sap fraction was passed through a column (1.8 X 50 cm) of Sephadex G25 equilibrated with 326 0.25 M sucrose-l mM dithiothreitol in TKM buffer. Membrane-bound ribosomes were collected from the 1.35-2.0 M sucrose interface, homogenised in glass apparatus with 2.5 vol of 2 M sucrose in TKM buffer and l/ 10 of the volume of the cell sap, overlaid with 0.25 M sucrose/TKM and centrifuged for 1 hr at 135 000 g. The floated membrane-bound ribosomes were collected and homogenised. The RNA/protein ratio for this fraction was 0.15-0.21. The free riboso- ma1 fraction was prepared as follows. The 2 M sucrose layer, after removal of rough membranes, was diluted with 3 vol of TKM buffer and centrifuged for 1 hr at 135 000 g. The pellet thus obtained consists of the total population of free ribosomes. The ribosomal samples (0.1 ml; 50- 100 1.18 of RNA) were incubated at 37°C for 30 min in a final volume of 0.5 ml containing the following: 0.1 ml of Sephadex G25-treated cell sap (approx. 2.3 mg of protein); 2 mM ATP; 0.25 mM GTP; 5 mM creatine-phosphate; 25 pg of creatine-kinase; 0.105 $i of [ l4 C] -L-valine (280 mCi/mmole); 50 mM Tris-HCl, pH 7.6 at 20°C; 75 mM KC1 ; 5 magnesium acetate and 3 dithiothreitol. Inhibitors were dissolved in TKM buffer and added at concentrations indicated. In some experiments, Sephadex G25-treated post-mitochondrial supernatant (0.2 ml) was incubated as above instead of the ribosomes and cell sap. Where indicated, a solubilisation of membranes by Triton X-100 was performed as follows: 11% (w/v) Triton X-100 in TKM buffer was added to subcellular fractions to a final concentration of 1% and after approximately 2 hr on ice, the treated fractions were added to cell-free incubations so that the resulting concentration of Triton X-l 00 in the system was 0.2%. The untreated

STRUCTURE OF THE VASCULAR PARENCHYMA IN THE STEM OF LYCOPODIUM LUCIDULUM

At maturity the vascular cylinder of the stem of Lycopodium lucidulum contains two distinct types of parenchyma cells, one which is always associated with sieve cells, the other with tracheids. The remaining parenchyma cells have characteristics intermediate between the two extremes. The most conspicuous feature of the sieve cell‐associated parenchyma cell is the very dense appearance of its protoplast, due to a high ribosome population and absence of large vacuoles. The large, ramifying nuclei of these cells have numerous connections with the endoplasmic reticulum (ER). The tracheid‐associated parenchyma cells, which are light in appearance, contain many small vacuoles and a relatively small ribosome population. These cells also contain relatively small nuclei and considerable ER cisternae. The parenchymatous elements which have characteristics intermediate between sieve cell‐ and tracheid‐associated parenchyma may or may not be contiguous to the sieve cells or tracheids. An intergradation in wall thickness occurs among parenchyma cells of the vascular cylinder, the thicker‐walled cells being adjacent to the sieve cells, the thinner‐walled ones next to the tracheids. An intergradation also occurs in the frequency of plasmodesmata between the various parenchyma cells. The closer parenchyma cells are to the sieve cells the greater the number of connections between them. No plasmodesmata were found between the tracheid‐associated parenchyma cells.

Molecular-weight determinations and stoichiometric measurements of 40-S and 60-S ribosomal proteins of the cellular slime mold Dictyostelium discoideum.

The stoichiometry of ribosomal proteins of the cellular slime mold Dictyostelium discoideum was investigated. The average number of copies per mole ribosomal subunit of each of 34 proteins from 40-S subunits and 48 proteins from 60-S subunits was measured on the basis of the molecular weights and the mass ratio of proteins estimated by combining two disc electrophoretic systems with urea-polyacrylamide and dodecylsulfate-polyacrylamide gels. According to the stoichio-metric classification of Escherichia coli ribosomal proteins suggested by Weber[1], D. discoideum ribosomal proteins were divided into at least four groups; fractional (F), unit (U), fractional repeated (FR) and repeated (R) proteins with the average number of copies of 0.10–0.69, 0.70 to 1.30, 1.31–1.70 and more than 1.71 per mole subunit, respectively. The data indicated that 40-S and 60-S proteins from isolated ribosomal subunit populations contain many F-, R- and FR-proteins besides U-protein. We concluded that the isolated ribosomal subunits of D. discoideum are structurally heterogeneous.

Ribosomal protein S21 is required for full activity in the initiation of protein synthesis.

The role of the 30 S ribosomal protein S 21 in ribosome function, specifically in relation to the initiation of polypeptide synthesis, was studied. Reconstituted particles using 16 S RNA and all purified 30 S proteins, but omitting S21. The reconstituted particles ([-S 21] particles) sedimented at 30 S and contained all the 30 S proteins except S 21 in approximately the same concentration as control reconstituted particles ([Σ Si] particles). In several functional assays including AUG-directed fMet-tRNA binding, [-S 21] particles were only 33 to 44% as active as complete particles. The addition of pure S 21 to [-S 21] particles showed 2 to 3 fold stimulation in AUG-directed fMet-tRNA binding using purified initiation factors IF 1 and IF2, assayed either in the presence or absence of 50 S subunits. Maximum stimulation was observed when approximately 0.5 to 1.0 mole of S 21 per mole of [-S 21] particles were added. We conclude that, contrary to the report of van Duin et al. (1972), S 21 does not inhibit AUG-directed fMet-tRNA binding, and that S 21 is required for full activity of the ribosomes in the initiation of polypeptide synthesis. Thus, the results published by van Duin et al. cannot be taken to support the postulated heterogeneity of the ribosome population in vivo. Possible reasons for the discrepancy between our conclusion and that of van Duin et al. are discussed.

Dissociability and tRNA content of monosomes produced by dimethylnitrosamine and starvation.

DIMETHYLNITROSAMINE is an hepatotoxic agent which disaggregates liver polysomes into monosomes1,2. Previous investigations have suggested that polysome disaggregation results from alkylation and subsequent fragmentation of mRNA in polysomes3, producing complexed ribosomes. But, fragmentation has been ruled out by the findings that monosomes produced by dimethylnitrosamine dissociate into subunits in solutions containing high concentrations of KCl4,5 and that pretreatment of animals with cycloheximide prevents polysome disaggregation by dimethylnitrosamine6. In bacteria the single ribosome population contains both complexed and runoff ribosomes which can be distinguished by their dissociability7 and tRNA content8. Therefore, we have compared these properties in monosomes produced by dimethylnitrosamine and by starvation, to determine whether they are runoff ribosomes. Our results show that in spite of their similar dissociability, monosomes produced by dimethylnitrosamine and starvation differ in tRNA content.

Protein synthesis of the sponge Geodia cydonium: characterization of the system.

The ribosomal population of the sponge Geodia cydonium has been examined. The monosomes have a sedimentation constant of 80 S, the sizes of the subunits are approximately 60 S and 45 S respectively. The polyribosomes contain up to 40 ribosomal units. Cell free protein synthesizing systems (cell homogenate as well as reconstituted system) have been prepared and characterized with respect to Mg(2+), KCI and ATP concentrations, temperature, pH and time course of the reaction. In the cell-free system and in the cellular system the protein biosynthesis is inhibited by chloramphenicol. It is not affected by cycloheximide.

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.

Characteristics of membrane-bound and free hepatic ribosomes from insulin-deficient rats. I. Acute experimental diabetes mellitus.

Membrane-bound and free ribosomes were prepared by discontinuous density gradient centrifugation from livers of rats 2-3 days after receiving alloxan (75 mg/kg) or streptozotocin (100 mg/kg). Hepatocytes from these animals were also examined by electron microscopy and subjected to quantitative morphometric analysis. The results indicated that the two populations of hepatic ribosomes respond differently to acute insulin deficiency. There was an overall reduction (P < 0.001) in total number of bound ribosomes per volume cytoplasm: the remaining bound ribosomes underwent a shift to smaller-sized ribosomal messenger RNA (mRNA) aggregates (P < 0.02); and the proteinsynthetic activity of these bound ribosomes was less than normal (P < 0.02) when protein synthesis was directed by endogenous mRNA. However, there was no difference between bound ribosomes from livers of normal and diabetic rats when protein synthesis was directed by polyuridylic acid. In contrast, free ribosomes were unchanged in number and degree of ribosomal mRNA aggregation, but displayed a significantly increased rate of in vitro protein synthesis (P < 0.01) as compared to normal controls. This increased protein-synthetic activity occurred when amino acid incorporation was directed by endogenous mRNA or polyuridylic acid. These changes in structure and function of bound and free hepatic ribosomes were prevented by the concomitant administration of insulin. The decrease in protein-synthetic activity of bound hepatic ribosomes from acutely diabetic rats seems to be secondary to marked disruption and disaggregation of the rough endoplasmic reticulum (RER) with production of smaller ribosomal mRNA aggregates which incorporate less amino acids into protein. Increased protein synthetic activity of free ribosome appears to be related to the ability of these ribosomes to copy mRNA more efficiently.

Ribosome-membrane interaction and protein synthesis.

ABSTRACT The role of secretion of proteins for the attachment of ribosomes to membranes has been well established. That another function must exist for membrane-ribosome interaction is suggested by observations on: (a) the active synthesis of proteins on membrane-bound ribosomes of predominantly non-protein secreting cells, and b) the massive proliferation of membrane-bound ribosomes during active growth and development of both secretory and non-secretory tissues. Literature on functional and compositional differences between membrane-bound and free ribosomes is reviewed and it is proposed that a major function of ribosome-membrane interaction is to effect a topological segregation of different populations of ribosomes synthesizing different classes of proteins.

Synthesis of liver ferritin on free and membrane-bound polyribosomes of different sizes

Much evidence supports the thesis that secreted proteins are made on membrane-attached ribosomes, whereas proteins retained within cells are generally synthesized by ribosomes lying free in the cytoplasm [1 ]. Ferritin, a retained protein found in many tis- sues, consists of an outer shell of apoferritin subunits surrounding a core of iron salts. Studies in our labo- ratory [2] and by Redman [3] agree in showing pre- ferential synthesis of apoferritin by the free ribosomes of rat liver; after incubation with labeled amino acids, free ribosomes show many more nascent chains of apoferritin than total or membrane-attached ribosomes, whereas the reverse is true for albumin. Nevertheless, Redman observed that, a few minutes after intact rats were injected with [14C]leucine, only 75% of the radioactivity in the nascent apoferritin of the liver was recovered in the free ribosome population, the re- maining 25% being associated with membrane-bound ribosomes. Contamination with free ribosomes could account for about 10% of this. Puro and Richter [4] also used in vivo injection of labeled leucine and found that labelwas incorporated into nascent liver apofer- ritin about 20-30% as well by microsomal ribosomes as by free ribosomes. Contamination of their micro- some fraction with free ribosomes did not exceed 4--8%. While these studies indicate preferential syn- thesis of ferritin on free liver ribosomes, they leave doubts about these as the exclusive site of synthesis. The size of polysomes synthesizing a given protein depends on the number of amino acid residues for which the messenger codes. Although horse spleen apoferritin appears to have 24 identical subunits with a mol. wt. of 18 500, we [5] have recently observed by gel electrophoresis that rat liver ferritin contains two types of subunits with molecular weights of 18 500 and 12 000 respectively. We have therefore examined the free and membrane-bound ribosomes of rat liver for the presence of polysomes consistent with synthesis of these two subunits by reacting each population of ribosomes with ferritin antibody coupled to 125 I. On free polysomes, the antibody was bound by trimers and tetramers, consistent with a peptide, having a mol. wt. of 12 000, whereas the ferritin antibody attached to pentamers in the mem- brane-bound ribosome population, consistent with a product of 17 000 molecular weight. We conclude that free and membrane-bound ribosomes contribute different subunits to liver ferritin. 2. Materials and methods 125 Iodine (sodium salt, carrier free) was obtained from New England Nuclear. Lactoperoxidase was purchased from Calbiochem., and heparin from Nu- tritional Biochemicals. Other chemicals were reagent grade. Rabbit antiserum against horse spleen ferritin was generously supplied by Dr. Linder (M.I.T.). Most of the media were made up in TKM buffer (50 mM Tris-HC1, pH 7.8; 25 mM KC1; 5 mM MgCI2) contain- ing heparin to inhibit ribonuclease.

The biosynthesis of fibroin: II. The synthesis of fibroin in a crude cell-free system

The properties of a crude cell-free system able to incorporate amino acids into fibroin are described. Fibroin is characterized by its composition and its sedimentation properties on sucrose gradients. Two populations of membrane-bound ribosomes can be characterized by the utilization of puromycin and detergents for releasing the newly made chains. One population produces fibroin, which remains associated with the membrane fraction, the other produces different proteins which are released into the cell sap.

Protein synthesis and distribution of ribosomal elements in ovarian oocytes and early developmental stages of Drosophila melanogaster

Protein synthesis has been investigated in cell-free preparations from mature ovarian oocytes, unfertilized and fertilized eggs, and early embryos of Drosophila melanogaster. Preparations from unfertilized eggs have a specific activity that is 5- to 6-fold higher than the activity of fractions from ovarian oocytes. There is an additional small increase in activity of preparations from fertilized eggs. The specific activity that is rapidly attained in the fertilized egg remains essentially constant for 2 to 2.5 h after fertilization, decreases sharply during blastoderm formation, and again increases during gastrulation. The activities of unfertilized eggs decline slightly during the first 2 h after oviposition, and then decrease more sharply. About 35 % of the ribosomes in preparations from both unfertilized and fertilized eggs sediment in the polyribosome region of sucrose density gradients, whereas no polyribosomes could be detected in preparations from ovarian oocytes. In both ovarian oocytes and fertilized eggs, less than 1 % of the ribosome populations were present as subunits. Additional ribonucleoprotein material of buoyant densities different from those of ribosomal subunits or ribosomes was found throughout the sucrose gradients. About 3.5 % of the ribosomes were found to be membrane-bound in preparations from both unfertilized and fertilized eggs.

The control of the form and function of the ribosomes in androgen-dependent tissues by testosterone

1. The ribosome content of the rat ventral prostate gland is controlled by the concentrations of circulating androgens and the polyribosomal complement of the total population of ribosomes is acutely dependent on androgenic stimulation. After the administration of testosterone to castrated rats in vivo, there is a pronounced increase in the amounts of heavy (150-240S) polyribosomes. 2. These results are consistent with a pronounced increase in the mRNA and rRNA content of the prostate gland after the administration of testosterone in vivo. 3. From studies conducted both in vitro, the heavy prostate polyribosomes formed after androgenic stimulation are particularly active in protein synthesis. 4. The androgen-stimulated increase in the formation of prostate polyribosomes has a mandatory requirement for sustained RNA and protein synthesis. 5. Since the androgen-mediated increase in prostate polyribosomes may also be suppressed by the concomitant administration of certain anti-androgenic steroids in vivo, the response in polyribosome formation is probably initiated by the binding of a metabolite of testosterone, 5alpha-dihydrotestosterone, in the prostate gland. 6. The relevance of these findings to the pronounced increase in protein synthesis in androgen-dependent tissues after hormonal stimulation is discussed.

Cyclic blockade of initiation sites by streptomycin-damaged ribosomes in Escherichia coli: an explanation for dominance of sensitivity

In bacterial extracts streptomycin is known not only to inhibit ribosomal activity but also to cause gradual release of ribosomes from polysomes. Nevertheless, we now find that after streptomycin has virtually halted protein synthesis in cells of Escherichia coli K12 a substantial (though reduced) level of polysomes persists. These polysomes are evidently maintained by turnover rather than by static blockade, for in streptomycin-treated cells [ 3H]uracil pulses are rapidly incorporated in the polysomal messenger RNA; moreover, if the synthesis of RNA or the formylation of methionyl-transfer RNA is blocked the polysome level decreases rapidly. Streptomycin thus appears to cause a cycle of ribosomal initiation, blockage of chain extension, gradual release, and reinitiation. The resulting cyclic blockade of initiation sites can account for the dominance of streptomycin sensitivity over resistance in str s str r † † Abbreviations used: Str, streptomycin; str s, streptomycin sensitive; str r, streptomycin resistant. heterozygotes. In confirmation of this model, the inactive resistant ribosomes in treated heterozygotes were found to resume activity if the cells were lysed and excess messenger was provided. These findings further suggest that in sensitive cells damage to only a fraction of the ribosomal population by streptomycin may be sufficient to block protein synthesis.

Possible role of ribosomal bodies in the control of protein synthesis in pre-vitellogenic oocytes of the lizard Lacerta sicula Raf

In the pre-vitellogenic oocytes of the hibernating lizard Lacerta sicula, a fraction of the ribosomal population crystallizes in the form of ribosomal bodies (RB). In these oocytes the rate of protein synthesis is 35–75 % lower than in the oocytes of the summer animals in which the RB have dissolved. The isolated RB are able to respond to Poly(U) and are also active in the endogenous protein synthesis in a cell-free system. This contrasts with the in vivo situation, where they appear to be completely inactive in protein synthesis. The possibility that RB represent a regulatory mechanism of protein synthesis is discussed.

Mammalian brain ribosomes are behaviourally and structurally heterogeneous.

STUDIES of the protein components of isolated ribosomal sub-units, primarily in E. coli, have made it evident that the population of ribosomes from even single-celled organisms is heterogeneous1–4. The functional significance of ribosomal heterogeneity is not clear; studies of ribosomes from higher organisms have suggested that the ribosomes differ in functional characteristics (such as activity in in vitro systems) depending on the age of the organism or organ5, whether the organ from which they are derived is in an active or repressed state6 and whether the ribosomes are “free” or bound to membranes7–8. There is also evidence that some ribosomes may be “message specific”9. All these observations support the not unlikely hypothesis that the heterogeneity of ribosomal populations provides important control functions in various metabolic and synthetic processes.