INHIBITION OF PROTEIN SYNTHESIS IN MAMMALIAN CELL-FREE SYSTEMS BY CHLORAMPHENICOL

Chloramphenicol can inhibit protein synthesis in mammalian cell-free systems as effectively as it inhibits protein synthesis in analogous microbial systems. Significant inhibition in mammalian systems was obtained only when protein synthesis was stimulated by the addition of template RNA to the system, there being comparatively little inhibition of protein synthesis by ribosomes in the absence of stimulatory RNA. It is postulated that chloramphenicol may inhibit the function of messenger or template RNA by successfully competing for ribosomal binding sites, thereby preventing the attachment of RNA to ribosomes. The apparent discrepancy in the sensitivity of mammalian and microbial cells to chloramphenicol may be related to differences in turnover or stability of ribosomal template RNA rather than to intrinsic differences in mechanism of protein synthesis. These observations are in accord with the suggestion that protein synthesis in intact mammalian cells may be susceptible to chloramphenicol inhibition only at the time that new messenger or template RNA is being deposited on ribosomes and that more mature cells may be resistant because informative RNA already deposited on ribosomes is not accessible to chloramphenicol inhibition. The inhibitory effect of chloramphenicol on protein synthesis in proliferating cells may be an important factor in hematologic toxicity attributable to chloramphenicol.

DIFFERENTIAL MUTATION PRODUCTION BY THE DECAY OF INCORPORATED TRITIUM COMPOUNDS IN E. COLI

We have studied the differential mutation production by the decay of incorporated tritium compounds in E. coli (WWU) using DNA-seeking precursors (H(3)-thymidine), RNA-seeking precursors (H(3)-uracil, H(3)-uridine), and protein-seeking precursors (H(3)-histidine, H(3)-proline). In particular we have determined the reversion frequency of an arginine locus. The reversion frequency is measured in units of revertants/surviving bacteria/H(3) decay, and has an average value of 1.84 x 10(-8) for H(3)-uridine and H(3)-uracil, 0.67 x 10(-8) for H(3)-thymidine, and 0.28 x 10(-8) for H(3)-proline and H(3)-histidine. Thus, the revertants are produced most effectively by H(3) decays when the label is introduced in the form of an RNA precursor. The macromolecular distribution of the label shows that 5 to 8 per cent of the H(3)-uridine or H(3)-uracil is incorporated into DNA.

LONG-LIVED MESSENGER RNA: EVIDENCE FROM COTTON SEED GERMINATION

In germinating cotton embryos the partial inhibition of RNA synthesis by actinomycin D does not inhibit the incorporation of leucine-(14)C into soluble protein nor cause a loss of polyribosomes during the first 16 hours of germination. This suggests that the protein synthesis observed during this period is directed by messenger RNA which exists in the mature seed and which is not rapidly degraded and resynthesized.

THE SEQUENTIAL SYNTHESIS OF THE POLYPEPTIDE CHAIN OF SERUM ALBUMIN BY THE MICROSOME FRACTION OF RAT LIVER

1. The isolated microsome fraction of regenerating rat liver was incubated with cell sap, a source of energy and [(35)S]methionine, [(14)C]isoleucine or [(14)C]leucine for different periods of time, and microsomal albumin isolated. 2. The distribution of these isotopes in albumin was determined by separation of tryptic peptides from the protein. Radioactivity was measured in peptides either qualitatively by radioautography or quantitatively by labelling with both (3)H and (14)C. 3. A gradient of radioactivity existed at all times in albumin isolated after incubating microsomes. 4. The shorter the incubation time the fewer the peptides labelled in albumin, but the peptides with highest specific activity after short incubation times corresponded to those with highest specific activities after long incubation times. 5. Leucine released from the C-terminus of albumin had a higher specific activity than the mean specific activity of the remaining leucine residues in albumin. 6. The peptide with the highest specific activity in albumin is probably derived from the C-terminus of the protein. 7. [(14)C]Glutamic acid is incorporated into the N-terminus of albumin after incubating the microsome fraction with this isotopically labelled amino acid, cell sap and a source of energy. The specific activity of the N-terminal glutamic acid under these conditions is less than the mean specific activity of the remaining glutamic acid and glutamine residues in albumin. 8. The results are interpreted as reflecting a sequential synthesis of serum albumin in the isolated microsome fraction of rat liver. The direction of synthesis of albumin is from the N-terminus towards the C-terminus. 9. The bulk of incorporation of radioactive amino acid into albumin in the isolated microsome fraction is due to completion of partially completed, pre-existing peptide and polypeptide chains. A limited synthesis of new chains of albumin does, however, occur.

CHROMOSOMAL LOCATION OF THYMINE AND ARGININE GENES IN ESCHERICHIA COLI AND AN F' INCORPORATING THEM

Ishibashi, Masahide (Osaka University, Osaka, Japan), Yoshinobu Sugino, and Yukinori Hirota. Chromosomal location of thymine and arginine genes in Escherichia coli and an F' incorporating them. J. Bacteriol. 87:554-561. 1964.-The gene responsible for thymine requirement or independence in Escherichia coli K-12 is located on the chromosome, near Arg(2) (arginine) and between Sm (streptomycin) and Ade (adenine). A new F' (called F(15)) carrying the Thy (thymine) gene as well as the Arg(2) gene was discovered. The Thy(+) (thymine independent) gene on F(15) relieves the defect of all Thy(-) (thymine requiring) mutants of E. coli examined, such as E. coli 15 T(-), B3, and Thy(-) mutants obtained by aminopterine treatment. It can also be transferred to Thy(-) mutants of Salmonella typhimurium and Serratia marcescens, converting them to Thy(+).

METHODS FOR THE PURIFICATION OF THYMUS NUCLEI AND THEIR APPLICATION TO STUDIES OF NUCLEAR PROTEIN SYNTHESIS

Procedures are described for the purification of calf thymus nuclei using mild hypotonit shock to break intact cells, and layering techniques to remove cytoplasmic debris. Ficolc (a high polymer of sucrose) was dissolved in isotonic sucrose to give dense solutions suitable for gradient centrifugation. The method yields nuclei which can incorporate amino acids in vitro. Thymus nuclei isolated under isotonic conditions were incubated with C¹⁴-amino acids and later purified by centrifugation through dense sucrose solutions. The distribution of radioactivity in different nuclear proteins was measured and it was found that isotopic amino acids are actively incorporated into characteristically chromosomal proteins, such as the arginine-rich and lysine-rich histones. Protein synthesis in the nucleus is markedly inhibited by puromycin and by agents, such as 2,4-dinitrophenol, which inhibit ATP synthesis. The synthesis of histones is also inhibited by puromycin, but the uptake of several amino acids into the lysine-rich histone fraction seems less sensitive to puromycin inhibition than is uptake into the arginine-rich histones or other proteins of the nucleus. High resolution autoradiography using tritiated leucine and observing grain distribution over thin sections of isolated nuclei and whole cells shows that amino acid incorporation occurs within the nucleus and is not due to cytoplasmic contamination.

EFFECT OF HORMONES ON THE TURNOVER OF POLYSACCHARIDES IN CONNECTIVE TISSUES

A number of hormones somehow modify the turnover of the polysaccharides in a variety of connective tissues. In hypophysectomized animals the turnover of chondroitin sulfate and hyaluronic acid is decreased; when such animals are given growth hormone the turnover of chondroitin sulfate is enhanced but that of hyaluronic acid is unaltered. The effect of parathyroid extracts may be of a dual nature: in some connective tissues there may be an increase in the rate at which chondroitin sulfate is catabolized, in other tissues its synthesis may be stimulated. Thyroxine effectively restores toward normal the depressed synthesis and breakdown of polysaccharides in hypothyroid animals. Estradiol, in addition to inhibiting the resorption of the metaphyses in rats, inhibits the synthesis of chondroitin sulfate in cartilage and aorta. Cortisone too inhibits the synthesis of chondroitin sulfates and hyaluronic acid; its effect on their catabolism is not as striking.

PROTEIN SYNTHESIS, STORAGE, AND DISCHARGE IN THE PANCREATIC EXOCRINE CELL. AN AUTORADIOGRAPHIC STUDY

The synthesis, intracellular transport, storage, and discharge of secretory proteins in and from the pancreatic exocrine cell of the guinea pig were studied by light- and electron microscopical autoradiography using DL-leucine-4,5-H³ as label. Control experiments were carried out to determine: (a) the length of the label pulse in the blood and tissue after intravenous injections of leucine-H³; (b) the amount and nature of label lost during tissue fixation, dehydration, and embedding. The results indicate that leucine-H³ can be used as a label for newly synthesized secretory proteins and as a tracer for their intracellular movements. The autoradiographic observations show that, at ∼5 minutes after injection, the label is localized mostly in cell regions occupied by rough surfaced elements of the endoplasmic reticulum; at ∼20 minutes, it appears in elements of the Golgi complex; and after 1 hour, in zymogen granules. The evidence conclusively shows that the zymogen granules are formed in the Golgi region by a progressive concentration of secretory products within large condensing vacuoles. The findings are compatible with an early transfer of label from the rough surfaced endoplasmic reticulum to the Golgi complex, and suggest the existence of two distinct steps in the transit of secretory proteins through the latter. The first is connected with small, smooth surfaced vesicles situated at the periphery of the complex, and the second with centrally located condensing vacuoles.

THE INFLUENCE OF PRECURSOR POOL SIZE ON MITOCHONDRIAL COMPOSITION IN NEUROSPORA CRASSA

The chemical composition of mitochondria obtained from exponentially growing Neurospora can be varied by addition of choline or amino acids to the culture medium. The variation affects the phospholipid to protein ratio, and the density of mitochondria as determined by isopycnic centrifugation in sucrose gradients. These variations have been observed in biochemical mutant strains as well as wild type cultures. In a choline-requiring strain, two levels of choline supplementation to the medium have been defined: a low choline concentration just adequate to support maximal logarithmic growth, and a high choline concentration which permits maximal incorporation of radioactive choline into cellular lipids. Mitochondria isolated from cultures growing at the low choline concentration have one-half the phospholipid to protein ratio of those from high choline cultures, and their density is significantly higher. Artificial mixtures of the two types of mitochondria can be resolved into two populations by isopycnic centrifugation. The concentration of cytochromes (measured by mitochondrial difference spectra) and of malate and succinate dehydrogenases (measured by enzyme activity) were the same in both types of mitochondria, on a protein basis. The results suggest that during growth of the mitochondrial mass, the incorporation of phospholipid and protein components can vary independently. Direct kinetic measurements did indeed show that choline, added to a culture growing at low choline concentration, was incorporated into mitochondrial lipids at a rate faster than the incorporation of protein. The mitochondrial phospholipid to protein ratio can also be influenced by the level of leucine supplementation to a leucine-requiring mutant, so that with leucine concentrations above those required for maximal exponential growth, mitochondria of increasing density and decreasing phospholipid to protein ratio are produced. Additions of choline or amino acids to the minimal medium of wild type cultures influence mitochondrial composition in a manner directly comparable to that observed in biochemical mutant strains. The results suggest that mitochondrial composition, in general, is determined by rates of incorporation of the two major components, phospholipid and protein; that these rates can vary independently in response to precursor concentration in the culture medium; and that they normally operate at a precursor (substrate) concentration below saturation level.