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G Brawerman

Publications and source records attributed to G Brawerman.

At least 37 records · Page 2Linked to original sources
PubMed1982

Messenger RNA species partially in a repressed state in mouse sarcoma ascites cells.

Four major mRNA species of mouse sarcoma ascites cells, coding for polypeptides designated P65, P40, P36, and P21, occur predominantly as untranslated messenger ribonucleoprotein particles. Cloned cDNA probes were used to study their distribution in cytoplasmic extracts of these cells. A considerable portion of the mRNA molecules sedimented as small particles, whereas the rest was present in polyribosomes. In contrast, the actin mRNA was present almost exclusively in polyribosomes. Incubation of the ascites cells in culture medium, particularly after a starvation treatment, caused an enhancement in polypeptide chain initiation relative to elongation in these cells, as evidenced by a shift of ribosomes into the polyribosome fraction and by an increase in polyribosome size. Exposure of the cells to a low concentration of cycloheximide, an inhibitor of the elongation step, had a similar effect. The actin mRNA and the active P65, P40, P36, and P21 mRNA molecules were shifted to larger polyribosomes in the treated cells, but no shift of molecules from small particles to polyribosomes was observed. The incubation in culture also led to considerable increases in the proportion of P65 and P40 mRNA molecules in the untranslated state. The results indicate that the untranslated state cannot be attributed to poor initiation efficiency. It is suggested that a portion of the mRNA molecules is maintained in a repressed state and that mRNA repression may represent an important translation control process.

Actins↗
PubMed1981

The Role of the poly(A) sequence in mammalian messenger RNA.

The poly(A) sequence is added to 3' termini of nuclear RNA segments destined to become part of the mRNA, and may play an essential role in the selection of these segments. It appears to be required for at least some of the splicing events involved in mRNA processing. In the cytoplasm, the poly(A) segment is the target of a degradation process which causes its gradual shortening, and leads to a heterogeneous steady-state poly(A)-size distribution. Complete loss of the poly(A) is probably followed by inactivation of the mRNA, since chains depleted of poly(A) do not accumulate in the cells. A role for this sequence in the promotion of mRNA stability is suggested by the behavior of globin mRNA depleted of poly(A) after injection into frog oocytes. The poly(A) shortening process may be part of the mRNA inactivation mechanism, as indicated by the greater sensitivity to degradation of the poly(A) of some short-lived mRNAs. However, the stochastic mRNA decay implies that new and old mRNA chains, with long and short poly(A) segments, respectively are equally susceptible to inactivation. The poly(A)-lacking histone mRNAs are stable only in cells engaged in DNA replication. Present knowledge favors a role for poly(A) in the control of mRNA stability. Loss of this sequence could be controlled through modulation of poly(A)-protein interactions or through masking of a sequence directly adjacent to the poly(A). In the nucleus, the poly(A) sequence could also serve as stabilizing agent, but, in addition, it might interact with the splicing machinery.

Actins↗
PubMed1979

Inactive mRNA-protein complexes from mouse sarcoma-180 ascites cells.

Mouse sarcoma ascites cells do not utilize fully their capacity for protein synthesis. A considerable portion of their ribosomes occur as inactive monomers. Also, a substantial amount of the cellular mRNA is in the form of ribonucleoprotein particles that sediment in the 20-70S range. This is indicated both by measurements of poly(A) content and by translation of the RNA in cell-free systems. The population of polypeptides synthesized under the direction of the RNA from these particles is less heterogeneous than that directed by RNA from polysomes. The mRNAs for some polypeptides are present predominantly in the small particles. Others are distributed to varying degrees between particles and polysomes. Incubation of the cells with cycloheximide drives most of the ribosomal monomers and a portion of the untranslated mRNA into polysomes. Some of the mRNAs that were predominantly in the inactive fraction seem to be refractory to this treatment. Particles released from polysomes in cells subjected to starvation are quite effective in promoting polypeptide synthesis in a reticulocyte cell-free system and cause the synthesis of a population of polypeptides similar to that coded by the polysomal RNA. The particles from cells exposed to cycloheximide are inactive but yield active RNA upon deproteinization. It is suggested that some mRNA species are maintained in an inactive state in the cell by a component of the nucleoprotein complex.

Animals↗
PubMed1977

Differential translation of rat liver albumin messenger RNA in a wheat germ cell-free system.

Rat liver polysomal RNA directs the synthesis of albumin in a wheat germ cell-free system. The in vitro product has the same mobility in polyacrylamide-sodium dodecyl sulfate gels as purified rat liver albumin and can be immune precipitated with rabbit anti-rat liver albumin serum. The incorporation of [35S] methionine into total protein in an in vitro system increased linearly with increasing messenger RNA concentration until a saturation plateau was reached. The synthesis of albumin compared to total protein increased continually with increasing RNA concentration. Addition of the polynucleotide poly(A) to the cell-free system inhibited incorporation, presumably through interference with the initiation process, and increased the relative albumin synthesis. Translation of albumin messenger RNA was also resistant to inhibition by high levels of potassium acetate where total incorporation was cut in half. The results suggest that the messenger RNA for albumin, in addition to the mRNAs for the beta chain of globin and for the immunoglobulin polypeptides, is particularly efficient in initiating protein synthesis.

Albumins↗
PubMed1977

Entry of mRNA into polyribosomes during recovery from starvation in mouse sarcoma 180 cells.

Starvation in mouse sarcoma 180 cells leads to a block in initiation of protein synthesis which results in the conversion of polysomes to 80-S ribosomes. The mRNA is released as nucleoprotein particles. Addition of nutrients to the starved cell causes the rapid uptake of the mRNA into large polysomal structures. The ribosomes enter these structures more gradually, without much effect on the sedimentation profile of the labeled mRNA. The results indicate that the stored mRNA in starved cells is readily available for uptake into polysomes. They also suggest that the mRNA may be binding to components other than ribosomes during the initial stages of polysome formation.

Animals↗
PubMed1977

Control of breakdown of the polyadenylate sequence in mammalian polyribosomes: role of poly(adenylic acid)-protein interactions.

The poly(adenylic acid) [poly (A)] segment in mouse sarcoma polysomes in not hydrolyzed by snake venom exonuclease under conditions which cause extensive degradation of the poly(A) in deproteinized polysomal RNA. The protecting effect of polysomes is presumably caused by the interaction between the poly(A) sequence and the protein known to be associated with it. This protection is reduced at low KCl concentration, but addition of exogenous RNA restores the protecting effect. The poly(A) segment also becomes susceptible to exonuclease after fragmentation of the polysomes by mild ribonuclease treatment. The latter treatment releases the poly(A) in association with protein. The poly(A) sequence in polysomes in readily degraded by a cytoplasmic extract of S-180 cells. Partial purification leads to a preparation active against the poly(A) in polysomes under conditions where no fragmentation of the messenger RNA is observed. Snake venom exonuclease increases the activity of the cytoplasmic preparation against poly(A) in polysomes. The active cytoplasmic factor appears to interfere with the poly(A)-protein interaction, thus rendering the polynucleotide susceptible to degradation by exonuclease. The poly(A) sequences in polysomes and in free cytoplasmic nucleoprotein particles are hydrolyzed to the same extent. The results suggest that the poly(A) sequence is normally protected from nucleases by virtue of its association with protein. The slow reduction in poly(A) size in cytoplasmic mRNA can be accounted for by a factor capable of interfering with the poly(A)-protein interaction. The latter interaction seems also dependent on the structural integrity of the polysomes or messenger ribonucleoproteins. It is suggested that a polynucleotide segment adjacent to the poly(A) can modulate the affinity of the protein for the latter sequence, thus permitting control of poly(A) stability in individual messenger RNAs.

Animals↗
PubMed1976

Differential translation of mouse myeloma messenger RNAs in a wheat germ cell-free system.

Translation of the polysomal mRNA of mouse myeloma cells in a wheat germ cell-free system leads to the immunoglobulin (Ig) light-chain precursor as the major product. Excess polysomal RNA causes strong inhibition of polypeptide synthesis, but has little effect of light-chain precursor synthesis. The inhibitory effect of excess RNA is avoided when the poly(A)-containing RNA fraction is used. With nearly saturating amounts of the latter RNA, light-chain recursor synthesis becomes more predominant, possibly as a result of competition between different mRNA species. High levels of potassium acetate cause strong inhibition of overall translation, but do not inhibit light-chain precursor synthesis. Addition of poly(A) to the cell-free system also causes inhibition, presumably through interference with the intiation process. Again, light-chain precursor synthesis is relatively resistant. Ig heavy-chain synthesis is relatively inefficent, but its resistance to the inhibitors tends to be nearly as great as that of the light-chain precursor. The results indicate that the Ig mRNAs are particulary efficient in initiating translation. This characteristic may account for certain features of the regulation of Ig synthesis in intact myeloma cells.

Multiple Myeloma↗
PubMed1976

A major species of mammalian messenger RNA lacking a polyadenylate segment.

Translation of total polysomal RNA from sarcoma 180 ascites cells in a wheat germ cell-free system produces two major polypeptides, A and B, with molecular weights of 50,000 and 45,000, respectively. Fractionation on Millipore filters or on oligo(dT)-cellulose leads to retention of the mRNA specific for protein A in the poly(A)-containing fraction and to accumulation of the B mRNA in the unadsorbed poly(A)-deficient fraction. The mRNA for B sediments at approximately 18 S; it is released as a 50S ribonucleorprotein upon EDTA treatment of polysomes. Its translation is particularly sensitive to an inhibitor present in the polysomal RNA. The poly(A)-deficient mRNA for the 45,000 dalton polypeptide is also present in mouse myeloma MPC-11 cells, where it seems to be localized in membrane-bound polysomes.

Animals↗
PubMed1976

A model for the control of transcription during development.

In order to account for the coordinate expression of large numbers of genes that must occur during development and differentiation, a mechanism of transcription control distinct from those operating in bacteria is proposed. Transcription would be initiated by small RNA chains that would function as primers for elongation. A given species of primer RNA generated by a primary induction event would bind by complementary base pairing to a variety of sites in the genome and thus trigger the transcription of adjacent structural genes. The primer RNA region would be excised from the transcription products in the nucleus, and possibly be reutilized as primers for transcription. This model can account for the occurrence of tissue-specific nuclear RNA complementary to repetitive DNA and does not require that the major RNA polymerases be capable of initiation. A minor RNA polymerase capable of initiating primer RNA chains and subject to conventional transcription controls would be required. Carcinogenesis, which is accompanied in many cases by the appearance of a variety of embryonic antigens, could involve the induction of RNA primers normally programmed to function early in development.

Animals↗
PubMed1975

Association of the polyadenylate segment of messenger RNA with other polynucleotide sequences in mouse sarcoma 180 polyribosomes.

Limited digestion of polysomal RNA with pancreatic ribonuclease releases a structure consisting of poly(A) associated with other polyribonucleotide sequences. This complex can be purified by oligo(dT)-cellulose chromatography. Heating for formamide treatment causes the dissociation of fragments free of poly(A) from the poly(A)-containing components. The two types of fragments tend to reassociate under annealing conditions, and this association is prevented by poly(U). Control experiments indicate that this structure is not an artifact generated during the manipulations. The same structure can be obtained by limited RNase digestion of polyribosomes, followed by deproteinization. The results suggest that the mRNA in polyribosomes may have a defined configuration caused by the interaction of the poly(A) sequence with another segment of the RNA.

Adenine↗
PubMed1975

Metabolism of the polyadenylate sequence of nuclear RNA and messenger RNA in mammalian cells.

The poly(A) sequences at the 3' end of mRNA and nuclear RNA molecules of mouse sarcoma and Chinese hamster cells are subject to an elongation process distinct from de novo synthesis. This process continues in cells treated with a high level of actinomycin D to block transcription. This results in the labeling of the steady-state poly(A) population in the cytoplasm and of unusually long poly(A) segments in the nucleus. In cells incubated with 3H-adenosine in the absence of drug treatment, cytoplasmic steady-state poly(A) segments with short labeled sequences at the 3' end can be detected by their heterogenous size distribution and by measurements of adenosine and AMP released by alkaline hydrolysis. These measurements indicate an average size of 8 residues for the labeled sequences. In the nucleus, a slow elongation of preexisting poly(A) chains can also be detected through measurements of AMP and adenosine. The cytoplasmic elongation process leads to turnover of the 3' end of the poly(A) sequence on mRNA, because of concomitant removal of AMP residues. It is apparently not linked to mRNA translation. The Chinese hamster and mouse sarcoma cells appear to differ markedly with respect to relative extents of poly(A) chain extension and de novo synthesis.

Adenine Nucleotides↗