Localization of macromolecules in Escherichia coli. II. RNA and its site of synthesis

Comparative Killing Efficiencies for Decays of Tritiated Compounds Incorporated into E. coli

The killing efficiencies due to the decay of incorporated H(3)-thymidine, H(3)-uridine, and H(3)-histidine in E. coli 15(T-L-) have been determined. Decays from H(3)-thymidine are 2.0 times as effective in producing lethality as those from H(3)-uridine and 2.5 times as effective as those from H(3)-histidine. Therefore, it seems that the greater part of damage from H(3)-thymidine decays is due to chemical changes associated with nuclear transmutation.

Localization of macromolecules in Escherichia coli. I. DNA and proteins

If thin sections of Escherichia coli, labeled uniformly with tritium, are radioautographed calculations, based on the distribution of section sizes show that the number of H³ decays per section should be very close to a Poisson distribution. We might, therefore, expect that the distribution of radioautographic grain counts among random cross-sections should follow a Poisson distribution. It can then be inferred that a deviation from a Poisson indicates a high concentration of label in a preferred region. This region can then be identified by analysis of serial section and comparison with electron micrographs. Sections of cells labeled with leucine-H3 gave a Poisson distribution of grain counts, and it was concluded that proteins were distributed fairly uniformly throughout the cell. The situation was not changed if labeled cells were placed in chloramphenicol or if very short pulses of label were used. When Escherichia coli is grown in presence of chloramphenicol a major morphological change concerns the nuclear region: it becomes more regular in outline, nearly spherical, and occupies a smaller proportion of the cell length. The previously described association between DNA labeled with thymidine-H3 and the nuclear region was confirmed by showing that the distribution of the label in the cell followed exactly the morphological changes of the nuclear region. It was also shown that the concentration of DNA in the nuclear region was at least 45 times higher than that of the cytoplasm. Several morphological features of cells grown in chloramphenicol and examined in the electron microscope are discussed.

The synthesis of ribosomes in E. coli. IV. The synthesis of ribosomal protein and the assembly of ribosomes

The incorporation of C(14) leucine into the protein moiety of ribosomes has been studied as a sequel to the studies of ribosomal RNA synthesis. In contrast to the latter studies, labeled leucine is incorporated directly into 50S and 30S ribosomes without measurable delay by precursor stages. There is, however, evidence of some transfer of radioactivity from the 43S group of particles to the 50S. The inhibition of protein synthesis by chloramphenicol results in the accumulation of material similar to the eosome-the primary precursor in ribosome synthesis. There is also evidence for the synthesis of some neosome. The results of the studies of ribosomal RNA and protein synthesis are combined into a model of ribosome synthesis. Finally, consideration is made of the significance of these studies of ribosome synthesis for general problems of protein synthesis and information transfer.

High-resolution autoradiography. I. Methods

Methods used in obtaining high resolution in autoradiography, with special emphasis on the technique of electron microscopic autoradiography, are described, together with control experiments designed to establish the optimum conditions or procedures. On the basis of these experiments the emulsion selected was Ilford L-4, with a crystal size slightly larger than 0.1 micron. It is applied to the specimen in the form of a gelled film consisting of a monolayer of silver halide crystals. Background, when present, can be eradicated by a simple method. The preparations can be stored, in presence of a drying agent, at room temperature or in a refrigerator. Photographic development is done in Microdol, or in a special fine grain "physical" developer. For examination in the electron microscope the sections are stained with uranyl or lead stains. These methods give a good localization of the label, at the subcellular level, and good reproducibility in relative grain counts.

The relationship between nuclear and cytoplasmic ribonucleic acid

The relationship between nuclear and cytoplasmic ribonucleic acid ( RNA ) was examined in HeLa cells growing exponentially in suspension culture. The cells were exposed to radioactive precursors of RNA and then transferred to non-radioactive medium. Observations were made directly after transfer to non-radioactive medium, when the concentration of radioactive precursors in the intracellular pool was still high, and 12 h later when the concentration of radioactive precursors in the pool was much reduced. The labelled nuclear RNA was separated from the labelled cytoplasmic RNA and the specific activity of the individual bases in both determined. It could be shown (1) that the labelled cytoplasmic RNA was synthesized from acid-soluble precursors by pathways which did not necessarily implicate the labelled nuclear RNA , and (2) that, within the limits of measurement, the labelled nuclear RNA was not transferred to the cytoplasm in a stable form but was rapidly broken down within the cell. The behaviour of the nuclear RNA in multiplying cells is thus seen to be essentially similar to that previously described in non-multiplying cells. In the light of the present findings the available evidence suggests that at least some of the nuclear RNA will prove to be labile in many, if not all, animal and plant cells.

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.

The bacterial nucleoid revisited

This review compares the results of different methods of investigating the morphology of nucleoids of bacteria grown under conditions favoring short generation times. We consider the evidence from fixed and stained specimens, from phase-contrast and fluorescence microscopy of growing bacteria, and from electron microscopy of whole as well as thinly sectioned ones. It is concluded that the nucleoid of growing cells is in a dynamic state: part of the chromatin is "pulled out" of the bulk of the nucleoid in order to be transcribed. This activity is performed by excrescences which extend far into the cytoplasm so as to reach the maximum of available ribosomes. Different means of fixation provide markedly different views of the texture of the DNA-containing plasm of the bulk of the nucleoid. Conventional chemical fixatives stabilize the cytoplasm of bacteria but not their protein-low chromatin. Uranyl acetate does cross-link the latter well but only if the cytoplasm has first been fixed conventionally. In the interval between the two fixations, the DNA arranges itself in liquid-crystalline form, supposedly because of loss of supercoiling. In stark contrast, cryofixation preserves bacterial chromatin in a finely granular form, believed to reflect its native strongly negatively supercoiled state. In dinoflagellates the DNA of their permanently visible chromosomes (also low in histone-like protein) is natively present as a liquid crystal. The arrangement of chromatin in Epulocystis fishelsoni, one of the largest known prokaryotes, is briefly described.

Autoradiographic studies of the synthesis of RNA and protein as a function of cell volume in Streptococcus faecium

Mid-exponential-phase cultures were either labeled continuously with tritiated leucine and uracil or pulse-labeled with tritiated leucine. The amount of leucine and uracil incorporated into protein or RNA per cell was determined by grain counts of autoradiographs of cells seen in electron micrographs; the volume of each cell was determined by three-dimensional reconstruction. The average number of autoradiographic grains around cells continuously labeled with uracil and leucine increased linearly with cell volume. In contrast, while the average grain count around cells pulse-labeled with leucine increased in a near-linear fashion over most of the volume classes, less than the expected number of grains were seen around cells in large- and small-size classes. The distribution of grains around cells from both the continuously and pulse-labeled populations could be fit at the 5% confidence level with a Poisson distribution modified to take into consideration the volume distribution of each population of cells analyzed. These findings suggested that large changes in the density of RNA and protein do not occur in most cells as they increase in size; however, there may be decreases in the rate of protein synthesis in some large and small cells. The decrease in the rate of protein synthesis appears consistent with the hypothesis that new sites of envelope growth must be introduced into cells that are close to the division event to restore rapid growth.

Quantitative immunoautoradiography at the cellular level. II. Absolute measurements using labeled standard cells as a source of reference

A quantitative autoradiographic method is presented for determining absolute amounts of 125I-labeled compounds on the surface of individual cells. Autoradiographic evaluation of single cell radioactivity is accomplished by comparing the silver grain densities over the specimen and a radioactive standard being exposed simultaneously. In order to obtain a reference source of comparable physical properties, surface-radioiodinated human erythrocytes are used, the radioactivity of which is determined in a crystal counter. A simple enzymatic method for preparing such standard erythrocytes of very uniform label density is described. Numerous experimental advantages derived from the use of the standard are discussed and demonstrated by examples employing various exposure times and different radioactive standards. Hereby, very similar results were obtained when the number of A-antigenic sites was quantified on single erythrocytes in different experiments. The quantification of membrane-bound IgM on single human lymphocytes is shown as another application of this scheme.

Nuclear envelope-associated resumption of RNA synthesis in late mitosis of HeLa cells

The restitution of RNA synthesis in cultures progressing from metaphase into interphase (G(1)) has been investigated in synchronized HeLa S(3) cells by using inhibitors of macro-molecular synthesis and the technique of electron microscope autoradiography. The rate of incorporation of radioactive uridine into RNA approached interphase levels in the absence of renewed protein synthesis. In contrast, maintenance of this rate in G(1) was dependent upon renewed protein synthesis. Restoration of synthesis of heterogeneous nuclear RNA occurred under conditions that inhibited production of ribosomal precursor RNA. In autoradiographs of individual cells exposed to radioactive uridine, silver grains were first detected after nuclear envelope reformation at the periphery of the chromosome mass but before chromosomal decondensation. These data are consistent with the following interpretation. Multiple RNA polymerase activities persist through mitosis and are involved in the initiation of RNA synthesis in early telophase at sites on the nuclear envelope.

Effect of actinomycin D on the transfer of ribonucleic acid from nucleus to cytoplasm in Lactobacillus acidophilus

After starvation for deoxyribosides, the deoxyribonucleic acid (DNA) of Lactobacillus acidophilus is restricted to a localized region of the cell. (3)H-uracil is first incorporated into such a restricted region but subsequently is found throughout the cell. This spread occurs despite the absence of protein synthesis and a major reduction in the rate of ribonucleic acid (RNA) synthesis. However, blocking RNA synthesis with actinomycin D restricts incorporation to a localized region of the cell. It is concluded that uracil is first incorporated into RNA in the bacterial nucleus from which it subsequently spreads through the cell. Actinomycin D could prevent this spread by preventing the completion of RNA molecules, which therefore do not dissociate from the DNA template.

Effect of bacteriophage R17 infection on host-directed synthesis of ribosomal ribonucleates

Studies were performed on the synthesis of ribosomal ribonucleates in cells of Escherichia coli K-12 infected by the ribonucleic acid (RNA) bacteriophage R17. Host-specific RNA was measured in the presence of phage RNA by in vitro hybridization of the purified ribonucleates with E. coli deoxyribonucleic acid. The results showed that, although the overall rate of RNA synthesis was only slightly affected by phage infection, the level of host RNA synthesis was decreased by 70 to 80%. Fractionation of the purified ribonucleates by sucrose gradient sedimentation, followed by hybridization of fractions sedimenting in the 23S and 16S regions, revealed that the level of ribosomal RNA synthesis was also decreased by 70 to 80%, and that this inhibition occurred during the first 15 to 20 min after infection. These findings are discussed in light of what is known about the inhibition of host RNA synthesis by other virus systems.

Autoradiographic studies of bacterial chromosome replication in amino-acid deficient Escherichia coli 15T-

Autoradiographic experiments on amino-acid requiring strains of Escherichia coli T(-) have been performed with fully-labeled cells harvested in log-phase and after periods of amino-acid starvation. The simplest segregation of incorporated label among progeny grown on non-radioactive medium is into two packets. The result corroborates the two-unit model of E. coli DNA inferred from previous studies with partially labeled cells. Following amino-acid starvation, the distribution of label among clones derived from labeled cells indicates cells are grouped into classes having DNA contents in the ratios 1:2:4. The segregation of label among progeny isolated by micromanipulation from such starved cells supports the view that the chromosomes are brought to a state of completed synthesis with different cell classes containing different integral numbers of chromosomes. Infrequent clones interpretable as arising from cells with three chromosomes suggest that the control of replication of chromosomes lying in the same cytoplasm is on an individual basis. The block in DNA synthesis resulting from amino-acid starvation is not perfect. Nevertheless, such starvation permits characterization of the dispersive replication events for more homogeneously labeled and definable DNA units than otherwise possible. The size-frequency distribution of label among progeny following six rounds of chromosome replication is close to that expected from a model of random breakage of linear polymers. The frequency of dispersive events is estimated as 0.5 to 0.7 per chromosome per generation and is only slightly influenced by tritium decays occurring during bacterial growth.

ACCUMULATION OF RIBONUCLEIC ACID IN BACTERIAL NUCLEAR PREPARATIONS DURING TREATMENT OF WHOLE CELLS WITH 8-AZAGUANINE, TETRACYCLINES, AND OTHER INHIBITORS

Ezekiel, David H. (Albert Einstein Medical Center, Philadelphia, Pa.). Accumulation of ribonucleic acid in bacterial nuclear preparations during treatment of whole cells with 8-azaguanine, tetracyclines, and other inhibitors. J. Bacteriol. 87: 755–760. 1964—Ribonucleic acid (RNA), synthesized in Bacillus megaterium KM during chloramphenicol treatment, accumulates in the chromatin-containing cell fraction obtained by lipase treatment of protoplasts. To determine whether this phenomenon is the cause or an effect of the inhibition of protein synthesis, or neither, other inhibitors of protein synthesis were studied. Chloramphenicol, tetracyclines, azaguanine, and, to a lesser extent, 7-azatryptophan permitted RNA synthesis while inhibiting protein synthesis. In each case, RNA accumulated in the chromatin body fraction. Azaguanine at 5 μg/ml causes more RNA accumulation than at 15 μg/ml, but allows some protein synthesis. Other inhibitors of protein synthesis inhibit RNA synthesis as well, and no accumulation is seen. The evidence favors the hypothesis that inhibition of protein synthesis causes the accumulation in the nuclear fraction. The possible nature and intracellular locus of the RNA accumulation are discussed briefly.

Ribonucleic acid and protein metabolism in the gut. I. Observations in gastro-intestinal cells with rapid turnover

This paper reports the findings of an investigation of ribonucleic acid (R.N.A.) and protein metabolism in those epithelial cells of the gastrointestinal tract which have a rapid turnover, and correlates the results with the rate of production of new cells.