Comparison of the protein-synthesizing machinery in the skeletal muscle of normal and dystrophic Bar Harbor mice

1. Although the total weight of leg muscle increased with the age of a normal mouse the DNA and RNA content per leg did not change significantly. 2. The weight of leg muscle from a dystrophic mouse was only about 45% of that from a normal mouse but the DNA and RNA contents were the same and hence similar DNA/RNA ratios were obtained. 3. The total ribosome contents of normal and dystrophic mice were the same on a whole-leg basis, and for both the free ribosomes were about 60% of the total. However, comparison with similar data from liver suggested that some loss of ribosomes occurred during the isolation procedure. 4. The polyribosome patterns obtained by density-gradient centrifugation were the same for normal and dystrophic muscle, and comparable polyribosome fractions of different sizes obtained from such gradients had similar capacities for the incorporation of radioactive amino acids in a standard protein-synthesizing system. 5. By using a standard protein-synthesizing system with normal polyribosomes similar extents of incorporation were found with normal- or dystrophic-muscle pH5 fraction or partially purified transfer RNA preparation. 6. It is concluded that there is no absolute difference between the protein-synthesizing systems of normal and dystrophic mouse muscle and that the observed apparent differences result from concentration differences caused by changes in muscle volume. 7. A possible cause of the failure of dystrophic muscle to resynthesize myofibrils is also suggested.     

The effect of pancreatic ribonuclease on rabbit reticulocyte ribosomes and its interpretation in terms of ribosome structure

1. Parts of the 16s and 30s RNA species of reticulocytes are readily hydrolysed by pancreatic ribonuclease. The biological activity of the ribosomes is diminished after treatment with low concentrations of the enzyme (e.g. 1ng. of ribonuclease/2.5mg. of polyribosome fraction/ml.). A high proportion of the chain scissions are ;hidden' owing to the secondary structure of the RNA moiety. 2. As the concentration of ribonuclease is increased RNA is lost from the ribosome. About 20-30% of the RNA may be removed from the ribosome without altering appreciably its sedimentation coefficient or its appearance in the electron microscope. 3. The amount of RNA removed from the ribosome is not increased by raising the concentration of enzyme from about 1mug. to 2.5mg. of ribonuclease/2.5mg. of polyribosome fraction/ml., or by increasing the temperature from 0 degrees to 30 degrees , or by first converting the RNA moiety into a single-stranded form before exposure to ribonuclease. 4. Untreated polyribosomes aggregate at about 75 degrees , whereas ribosomes treated with ribonuclease aggregate at about 45 degrees . The aggregates that are found on heating ribosomes after enzymic hydrolysis contain about 40-50% of the complement of RNA of intact ribosomes. 5. From the size of the fragments of RNA isolated from RNA-depleted ribosomes it is inferred that there is one site/60-100 nucleotides that is sensitive to ribonuclease. 6. The RNA moiety of RNA-depleted ribosomes has some double-helical character as shown by the optical properties and X-ray-diffraction pattern of ribonuclease-treated ribosomes and by the ;melting' properties of the isolated RNA. 7. Subparticles prepared by titration with an excess of EDTA are readily hydrolysed by ribonuclease to fragments of S(20,w) less than 4s, in contrast with the intact particle.     

Changes in ribosome population and in nucleic acids during breaking of dormancy and development of apple flower buds

Changes in ribosome population, RNA species and DNA composition in flower buds of apples ( Malus pumila Mill. cvs Ralls and White winter pearmain) were investigated during breaking of dormancy and development. After bursting of flower buds, total ribosomes increased approximately 4-fold, and the polyribosomal fraction increased from 66% to 94% of total ribosomes. The newly synthesized ribosomes were identified by incorporation of radioactive precursor. The observed decrease in specific radioactivity of the monoribosomes is caused by the recruitment of monoribo-somes into polyribosomes after breaking of dormancy.
In both cultivars, the 25S and 18S rRNA peaks increased to a high level on April 8. The peaks of low molecular weight RNA were apparently increased after initial swelling of the flower buds. The DNA of flower buds was separated into three bands by electrophoresis. The median band is the main band of nuclear DNAs. The ahead band and the slow-moving band are satellite components of nuclear DNAs, and they obviously rose after initial swelling of the flower buds. On April 8, when the flower buds had opened, two other small DNA bands could be detected. These results suggest that the changes in level of different ribosome populations, RNA species and DNA composition are related to dormancy breaking development of apple flower buds.     

Phosphorylation of ribosomal proteins in rat cerebral cortex in vitro.

Incubation of cerebral cortical tissue from immature rats in the presence of [32P]orthophosphate resulted in similar rates of incorporation of radioactivity into the proteins of free and membrane-bound ribosomes. Incorporation of label into ribosomal proteins of both species continued actively for at least 3 hours. Since recovery of membrane-bound ribosomes from rat cerebral cortex is quite low, further analyses of the radioactive phosphoproteins were restricted to the free ribosome population. A significant fraction of the radioactivity which was precipitated with trichloroacetic acid was not removed by heating in trichloroacetic acid at 90 degrees or extracted with organic solvents and therefore was presumed to be covalently bound to protein. The radioactive phosphoryl groups present in the ribosomal proteins were mainly in ester linkages since they were readily removed by exposure to 1 N NaOH, relatively unaltered by 1N HCl, and unaffected by hydroxylamine. This conclusion was supported by the isolation of labeled o-phosphoserine and o-phosphothreonine residues from hydrolysates of ribosomal proteins. A significant fraction of the labeled phosphoproteins in the purified ribosomes appeared to be bound tightly to the ribosome structure since only 40% of the radioactivity could be removed by extraction of these ribosomes with 1 M KCl. Phosphorylation of proteins of cerebral monoribosomes was more rapid than the same process in polyribosomes from the same source. Eight radioactive phosphoprotein bands could be detected by electrophoresis of proteins obtained from unfractionated cerebral ribosomes on unidimensional polyacrylamide gels containing sodium dodecyl sulfate. The protein nature of these materials was confirmed by pronase digestion. Proteins of subribosomal particles isolated from the total free ribosomal population were labeled differentially. When dissociation was carried out in the presence of EDTA, the small subunit contained four radioactive phosphoprotein bands, whereas the large subunit contained five. Three of the radioactive phosphoprotein components of the small subunit were removed when dissociation of cerebral ribosomes which were previously washed with high salt media was carried out in the presence of puromycin and high salt. However, only the largest labeled phosphoprotein band of the large subunit was removed by this procedure. This component exhibited the same electrophoretic mobility as one of the radioactive phosphoprotein bands which was removed from the small subunit by high salt treatment..     

Ribosome degradation in Escherichia coli induced by colicin E2

The mode of action of colicin E₂ on ribosomes in $\text{Escherichia coli}$ cells was investigated by zonal centrifugation analysis. Ribosome particles, both 50S and 30S, were degraded to smaller contents with the lapse of time by the action of colicin E₂. Gradual reduction of S values of each particles could not be observed and degradative intermediates of possible RNA-protein complex were detected only at the position between 30S and 4S in the zonal centrifugation profile, which indicated the destruction of ribosome in burst-out attitude. 50S ribosome fraction influenced by colicin E₂ contained both 23S and half-sized RNA. From these data, the mode of action of colicin E₂ on ribosomes in $\text{E. coli}$ was discussed.     

Characterization of cytoplasmic and nuclear genomes in the colorless alga Polytoma. II. General characterization of organelle nucleic acids.

Polytoma obtusum has a main band DNA (alpha) with a buoyant density in CsC1 of rho = 1.711 g/ml and a light DNA satellite (beta) with rho = 1.682 g/ml. beta-DNA was substantially enriched in a fraction containing small leucoplast fragments and some mitochondria, which was obtained in a pellet sedimenting between 3,000 g and 5,000 g. A crude mitochondrial pellet was also obtained by sedimenting at 12,000 g to recover particulates remaining in the supernate after 10 min at 5,000 g. This fraction contained a third DNA component (gamma) with rho = 1.714 g/ml. We have concluded that the leucoplasts of P. obtusum contain the beta-DNA (1.6882) and the mitochondria possess the gamma-component (1.714). Two distinct classess of ribosomes were isolated and separated by sucrose density gradients, a major 79S species and a minor species at 75S. The major species possessed the 25S and 18S ribosomal RNA (rRNA), characteristic of cytoplasmic ribosomes, and these particles co-sedimented in sucrose gradients with the 79S cytoplasmic ribosomes of Chlamydomonas reinhardtii. The minor species was present in about 2% of the total ribosomal population but showed an eight-to-ninefold enrichment in the leucoplast pellet, suggesting that it was of organelle origin. These 73S particles had RNA components migrating very closely with the 18S and 25S species of the 79S ribosomes, but the base composition of the rRNA from these two classes of ribosomes was significantly different; the rRNA from the 79S ribosomes had a G+C mole ratio of 50.0%, while the rRNA from the 73S class had a ratio of 47.5%. By comparison, chloroplast ribosomes of C. reinhardtii were found to sediment at 70S and contain rRNA molecules of 23S and 16S, with a G + C content of 51.0%. These findings support the concept that the Polytoma leucoplast possesses characteristic genetic and protein-forming systems.     

Unusual pattern of ribonucleic acid components in the ribosome of Crithidia fasciculata, a trypanosomatid protozoan.

In a previous study from this laboratory, presumptive ribosomal ribonucleic acid (RNA) species were identified in the total cellular RNA directly extracted from intact cells of the trypanosomatid protozoan Crithidia fasciculata (M. W. Gray, Can. J. Biochem. 57:914-926, 1979). The results suggested that the C. fasciculata ribosome might be unusual in containing three novel, low-molecular-weight ribosomal RNA components, designated e, f, and g (apparent chain lengths 240, 195, and 135 nucleotides, respectively), in addition to analogs of eucaryotic 5S (species h) and 5.8S (species i) ribosomal RNAs. In the present study, all of the presumptive ribosomal RNAs were indeed found to be associated with purified C. fasciculata ribosomes, and their localization was investigated in subunits produced under different conditions of ribosome dissociation. When ribosomes were dissociated in a high-potassium (880 mM K+, 12.5 mM Mg2+) medium, species e to i were all found in the large ribosomal subunit, which also contained an additional, transfer RNA-sized component (species j). However, when subunits were prepared in a low-magnesium (60 mM K+, 0.1 mM Mg2+) medium, two of the novel species (e and g) did not remain with the large subunit, but were released, apparently as free RNAs. Control experiments have eliminated the possibility that the small RNAs are generated by quantitative and highly specific (albeit artifactual) ribonuclease cleavage of large ribosomal RNAs during isolation. In terms of RNA composition and dissociation properties, therefore, the ribosome of C. fasciculata is the most "atypical" eucaryotic ribosome yet described. These observations raise interesting questions about the function and evolutionary origin of C. fasciculata ribosomes and about the organization and expression of ribosomal RNA genes in this organism.     

Isolation of stable ribosomal subunits from spores of Bacillus cereus.

Analyses of ribosomes extracted from spores of Bacillus cereus T by a dryspore disruption technique indicated that previously reported defects in ribosomes from spores may arise during the ribosome extraction process. The population of ribosomes from spores is shown to cotain a variable quantity of free 50S subunits which are unstable, giving rise to slowly sedimenting particles in low-Mg2+ sucrose gradients and showing extremely low activity in in vitro protein synthesis. The majority of the ribosomal subunits in spores, obtained by dissociation of 70S ribosomes and polysomes, are shown to be as stable as subunits from vegetative cells, though the activity of spore polysomes was lower than that of vegetative ribosomes. In spite of the instability and inactivity of a fraction of the spore's ribosomal subunits, the activity of the total population obtained from spores by the dry disruption technique was 32% of vegetative ribosome activity, fivefold higher than previously obtained with this species. The improvement in activity and the observed variability of subunit destabilization are taken as evidence for partial degradation of spore ribosomes during extraction.     

Study of the type of RNA, degraded by polynucleotide phosphorylase in polyribosomal fraction of rat liver]

The type of RNA is studied, which is degraded by polynucleotide phosphorylase (PNPase) in the fraction of free ribosomes and ribosomes released from endoplasmic reticulum membranes with Triton X-100. Beta-32P labelled ADP, UDP, GDP and CDP are found among the degradation products of endogenous RNA of free and bound ribosomes in vitro in the presence of 32P-ortophosphate. An analysis of molar ratio of beta-32P-NDP isolated revealed that PNPase degrades RNA of GC type in both ribosome fractions. The amount of PNPase-degraded RNA in bound ribosimes is 4-fold as high as that in free ribosomes under the same conditions. Analysis of stable 32P-RNA and rapidly labelled 32-P-dRNA, isolated from bound ribosomes after the incubation with and without inorganic phosphate, revealed that PNPase attacks the 28S fragment of RNA, which consists of about 370 nucleotides, and dRNA having a sedimentation coefficient less than 12S. The rate of dRNA degradation is considerably higher than that of rRNA. 5'-RNAase, hydrolysing synthetic homopolyribonucleotides to oligonucleotides with free 3'-OH terminal group, apparently participates, together with PNPase, in dRNA and rRNA degradation.     

Protein Synthesis by Free and Bound Paramecium Ribosomes in Vivo and in Vitro

SYNOPSIS. Cytoplasmic extracts of Paramecium aurelia were fractionated by density gradient centrifugation. The gradient fractions were characterized by chemical analysis and electron microscopy. Membrane-bound ribosomes were separated from free polyribosomes and the ability of each of these forms to incorporate C¹⁴-leucine into protein was tested. Incorporation was measured in both in vivo and in vitro systems, and similar results were obtained in both types of experiment except that there was little release of soluble labelled protein in the in vitro system. Paramecium appears to synthesize most of its protein on free polyribosomes but membrane-bound ribosomes constitute an important protein synthetic fraction, perhaps accounting for as much as 30% of the total synthesis. When isolated in the in vitro system, increasing concentration of the ribosome fraction gave increased incorporation, but increasing concentration of the membrane fraction gave decreased incorporation after a critical value. This inhibitory effect can be removed by adding excess cytoplasmic-supernatant to the system. The nature of the association of ribosomes with membranes is discussed.     

ISOLATION OF TWO DISTINCT CLASSES OF POLYSOMES FROM A NUCLEAR FRACTION OF RAT LIVER

Isolated rat liver nuclei were washed with Triton-X-100 in the presence of liver cell sap. This treatment liberated a fraction of polysomes which were isolated by differential centrifugation and were designated "outer membrane polysomes." The outer membrane polysomes synthesized protein in vivo. Shortly after injection of orotic acid-¹⁴C, the RNA of outer membrane polysomes had a higher specific activity than that of cytoplasmic polysomes. It was postulated that outer membrane polysomes may be an intermediate in the transfer of newly synthesized RNA from the nucleus to the cytoplasm. In other experiments, Triton-washed rat liver nuclei were lysed in the presence of deoxycholate and deoxyribonuclease. A ribonucleoprotein fraction was isolated from the lysate by differential centrifugation. This fraction contained "intranuclear ribosomes," which sedimented like partially degraded polysomes in sucrose gradients. This degradation could be partially prevented if intranuclear ribosomes were purified by sedimentation through heavy sucrose. The resulting pellets were termed "intranuclear polysomes" because they contained some undergraded polysomes. Intranuclear polysomes were highly radioactive after a brief pulse with orotic acid-¹⁴C, but did not appear to synthesize protein rapidly in vivo. Intranuclear polysomes may represent the initial stage of assembly of polyribosomes in the nucleus.     

Inhibition of bacterial growth by metal salts. The accumulation of ribonucleic acid during inhibition of Escherichia coli by cobalt chloride

During inhibition of the growth of Escherichia coli by cobalt chloride protein synthesis was decreased more than the synthesis of RNA. Three species of particle accumulated during the incubation. These had sedimentation coefficients of about 44s, 33s and 23s in tris buffer containing 10 mm-magnesium acetate and 100 mm-potassium chloride, but their sedimentation properties were susceptible to changes in buffer composition. The particles contained RNA but were more readily degraded by ribonuclease than were the ribosomes. RNA isolated from the particles differed slightly in sedimentation properties from the major species of ribosomal RNA. The particles are likely to be closely related to ribosome precursors that have been detected in other circumstances. Changes in the polyribosome fraction during inhibition by cobalt chloride, nickel chloride and chloramphenicol provided further evidence that inhibition by Co(2+) involves specific effects on the protein-synthesizing machinery.     

The Kinetics of the Synthesis of Ribosomal RNA in E. coli

The kinetics of the synthesis of ribosomal RNA in E. coli has been studied using C¹⁴-uracil as tracer. Two fractions of RNA having sedimentation constants between 4 and 8S have kinetic behavior consistent with roles of precursors. The first consists of a very small proportion of the RNA found in the 100,000 g supernatant after ribosomes have been removed. It has been separated from the soluble RNA present in much larger quantities by chromatography on DEAE-cellulose columns. The size and magnitude of flow through this fraction are consistent with it being precursor to a large part of the ribosomal RNA.

A fraction of ribosomal RNA of similar size is also found in the ribosomes. This fraction is 5 to 10 per cent of the total ribosomal RNA and a much higher proportion of the RNA of the 20S and 30S ribosomes present in the cell extract. The rate of incorporation of label into this fraction and into the main fractions of ribosomal RNA of 18S and 28S suggests that the small molecules are the precursors of the large molecules. Measurements of the rate of labeling of the 20, 30, and 50S ribosomes made at corresponding times indicate that ribosome synthesis occurs by concurrent conversion of small to large molecules of RNA and small to large ribosomes.

     

The concentrations of stable RNA and ribosomes in Rickettsia prowazekii

The obligate Intracellular parasite, Rickettsia prowazekii, is a slow-growing bacterium with a doubling time of about 10h. In the present study, DNA and RNA were obtained from the rickettsiae by two independent methods, i.e. simultaneous isolation of DNA and RNA from the same sample by phenol:chloroform extraction and CsCI gradient centrifugation. In addition, ribosomal RNA was obtained by sedimentation of partially purified ribosomes from the rickettsiae. The results demonstrated that, after correction for the cell volumes, the concentrations of stable RNA and ribosomes in R prowazekii, a slow-growing organism, were about 62fg μm⁻³ and 17000 per μm³, respectively, which were very simitar (66fg μm⁻³ and 21 000 per μm³) to those in Escherichia coli with a generation time of 40min. However, on a per cell basis, R. prowazekii had 5.6 fg of RNA and 1500 ribosomes per cell, which was only about 8% of the amount of both stable RNA (71.2 fg) and ribosomes (24000) per cell as was found in E. coli. These results indicated that R. prowazekii possesses a ribosome concentration greater than might have been predicted from its slow growth rate. This high concentration of ribosomes could be due to a large population of non-functioning ribosomes, a low efficiency of amino acid production, or a high rate of protein turnover. However, this study also demonstrated that the rickettsiae have very limited protein turnover. Knowledge of the kinetics and control mechanisms for protein synthesis in R. prowazekii remains to be established to determine the logic of the extra rickettsial ribosomes.     

Bound Ribosomes of Pea Chloroplast Thylakoid Membranes: Location and Release in Vitro by High Salt, Puromycin, and RNase

The mode of attachment of 70S ribosomes to thylakoid membranes from pea leaves was studied by determining the proportion of the bound RNA which was released by various incubation conditions. The results supported a model in which several classes of bound ribosomes could be distinguished: (a) very tightly bound, not released by any conditions yet tested (20% of the total); (b) monomeric ribosomes attached by electrostatic interaction with the membranes (30 to 40% of the total) and released by high salt; and (c) polysomes, with some of the ribosomes attached by a combination of electrostatic interactions and insertion of the nascent polypeptide chain into the membrane. These required a combination of puromycin and high salt for release. Other ("hanging") ribosomes of the polysomes were inferred to be attached through mRNA but not actually attached to the membranes directly; they could be released by RNase under low salt conditions, as well as by puromycin plus high salt.To obtain these results, chloroplasts had to be prepared in media containing 0.2 molar Tris at pH 8.5. Using Tricine buffers at pH 7.5 yielded thylakoid membranes whose ribosomes were removed almost completely by high salt alone; these showed no response to puromycin. However, pH 7.5 had to be used in all cases for ribosome dissociation in high salt media, as the ribosome structure appeared to be degraded by high salt at pH 8.5, and release then occurred without the need for puromycin.The kinetics of ribosome release by high salt showed a rapid initial phase with a half-life of 20 seconds. The extent of release by high salt was very dependent on the temperature of the incubation. Plotting the data according to the Arrhenius interpretation shows a significant break at about 15 C, with apparent activation energy of 20 kilocalories per mole below that temperature and 5 kilocalories per mole above that temperature. This result suggests that membrane fluidity might be an important factor permitting release of ribosomes under high salt conditions.Electron microscope pictures of the washed thylakoids showed polysomes closely associated with the outer membranes of grana stacks, and with the stroma lamellae. Following digitonin treatment of the membranes and centrifugation, fractions enriched in Photosystem I and presumed stroma lamellae were also enriched in bound RNA.     

Studies on the protein-synthesizing activity of the ribosomes of rat liver. The activity of free polysomes

1. The activities of microsome fractions from the liver of adult and 5-day-old rats for the incorporation of [(14)C]phenylalanine into protein were similar in the presence and absence of polyuridylic acid. 2. The activity of a light-microsome fraction from adult liver was greater than that of a heavy-microsome fraction, and the light-microsome fraction was also more markedly stimulated by the presence of polyuridylic acid. 3. The light-microsome fraction, when analysed by density-gradient centrifugation, contained a higher ratio of free ribosomes to bound ribosomes, whereas the reverse was true for the heavy-microsome fraction. Similar results were obtained for liver from adult and 5-day-old rats. 4. When the light-microsome fraction was incubated under conditions in which amino acid was incorporated into protein there was only a small increase in the ratio of free to bound ribosomes. When such a fraction was incubated with [(14)C]leucine and was then subjected to density-gradient centrifugation the fraction with the highest specific activity based on RNA had a density between that of the bound and free ribosomes. Treatment of the incubated fraction with ribonuclease shifted the radioactivity towards the free ribosome peak. These properties are consistent with the presence of active free polysomes. Such a component appeared also to be present when the heavy-microsome fraction was incubated under similar conditions. 5. The effect of the presence of polyuridylic acid on the incorporation of [(14)C]phenylalanine by the light-microsome fractions from liver of adult and 5-day-old rats was greatest in the region of the free ribosomes, but it is probable that some small polysomes containing polyuridylic acid are formed. 6. Polyuridylic acid also stimulated the bound ribosomes to a small extent when the heavy-microsome fraction from the liver of young rats was incubated with [(14)C]phenylalanine. 7. The results are discussed in terms of the various morphological constituents in liver now known to play a role in the synthesis of protein for export and for the internal activity of the cell.     

Regulation of Protein Synthesis in Zoospores of Blastocladiella

The factors responsible for the regulation of protein synthesis in the zoospores of Blastocladiella emersonii were studied by means of cell fractionation and in vitro assays. Charged transfer ribonucleic acid (tRNA) and aminoacyl-tRNA synthetases were found both inside the membrane-bound, ribosomal nuclear cap, and in the extracap cytoplasm. Ribosomes isolated from zoospore nuclear caps in low salt buffer failed to support polyuridylic acid-dependent phenylalanine incorporation. After washing with high salt buffer, the cap ribosomes were equivalent in activity to similarly prepared plant ribosomes. Both the high-salt wash from cap ribosomes and the extracap supernatant fraction contained an unidentified material which inhibited aminoacyl-tRNA binding and peptide bond formation by ribosomes. Ribosomal binding of polyuridylic acid was not inhibited. Washed cap ribosomes supported very low incorporation rates without added messenger RNA, and were highly dependent upon added poly U for phenylalanine incorporation, indicating a low level of messenger in nuclear caps. It is concluded that enclosure of the ribosomes in the nuclear cap does not in itself prevent protein synthesis, and that the lack of activity may be due to the presence of a ribosome inhibitor.     

Altered ribosomes after inhibition of Escherichia coli by rifampicin

Addition of rifampicin to growing cells of Escherichia coli affected the ribosomes. The polyribosomes first decayed to 70S ribosomes. These later dissociated to particles distinct from ribosomal subunits. The altered ribosomes sedimented more slowly than the corresponding subunits and had lost some protein; their ribosomal RNA was intact, but they were more susceptible to degradation by ribonuclease than normal ribosomes. The addition of rifampicin to preparations of lysed cells caused no detectable changes in the ribosome fraction.     

Action of red and far red light on ribosome formation in cabbage seedlings

The action of light on ribosome formation was examined in the cabbage seedlings, a system extensively used in the studies of anthocyanin synthesis. Ribosomes were extracted 18 h after the beginning of the irradiation and separated by sucrose gradient centrifugation. In the cotyledons of dark-grown cabbage seedlings, a brief red light induces an increase both in total ribosomes and in the fraction present as polysomes; the effect of red light is reversed by far red light, indicating the involvement of phytochrome in polysome formation in cabbage seedlings. Continuous red and continuous far red light are about equally effective in bringing about an increase of total ribosomes and of the polysome fraction. Streptomycin, which inhibits chlorophyll synthesis and chloroplast development, and enhances anthocyanin synthesis in cabbage seedlings, causes a decrease of total ribosomes and of the fraction present as polysomes. In hypocotyls, the red-far red reversibility is evident only for the polysome content and streptomycin does not decrease the polysome/monosomo ratio as it does in cotyledons.     

The effect of ribonuclease on polysomes and ribosomes of bacteria and animal cells

The mildest treatment with ribonuclease that causes any disaggregation of the polysomes of Escherichia coli or HeLa cells simultaneously attacks the RNA of the constituent ribosomes. It is concluded that the susceptibility to ribonuclease of polysomes does not suggest that they are held together by a strand of messenger RNA. The RNA of the larger sub-unit of bacterial ribosomes has particularly sensitive regions resulting in a non-random degradation. The RNA of the smaller sub-unit of E. coli ribosomes is relatively resistant to ribonuclease attack. The same may be true of the respective sub-units of the intact HeLa-cell ribosome, but both sub-units become very sensitive to ribonuclease on dissociation from each other.