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

ELECTRON MICROSCOPY OF CELLULAR DIVISION IN ESCHERICHIA COLI

Conti, S. F. (Brookhaven National Laboratory, Upton, N. Y.) and M. E. Gettner. Electron microscopy of cellular division in Escherichia coli. J. Bacteriol. 83:544-550. 1962.-Exponentially growing cells of Escherichia coli were fixed in formalin, exposed to uranyl nitrate, dehydrated at low temperatures with ethanol, and embedded in methacrylate. Polymerization was carried out at -70 C, by exposure of specimens to radiation from a cobalt(60) source. Electron micrographs revealed that cellular division occurs by the centripetal growth of the cell wall. The fine structure of the cytoplasm, nuclear apparatus, cell wall, and cytoplasmic membrane was also studied.

Quantitative radioautographic studies on exponentially growing cultures of Escherichia coli. The distribution of parental DNA, RNA, protein, and cell wall among progeny cells

Exponentially growing cultures of E. coli were examined by quantitative radioautographic techniques to determine the distribution of labeled DNA, RNA, protein, and cell wall among the progeny cells of successive generations. It was found that DNA is in large structures, non-randomly distributed in the progeny. About one-half of the cells have four such structures and approximately one-half contain these four structures plus four smaller ones. These structures show remarkable stability. Fewer than 3.5 per cent of the large structures break in one division time. Protein, RNA, and cell wall are all distributed randomly among progeny cells. The number of units of each component that show random segregation must be 200 or more.

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

The distribution of RNA in cells of E. coli 15 T^-U^- labeled with uridine-H³ was studied by methods involving the analysis of radioautographic grain counts over random thin cross-sections and serial sections of the cells. The results were correlated with electron microscope morphological data. Fractionation and enzyme digestion studies showed that a large proportion of the label was found in RNA uracil and cytosine, the rest being incorporated as DNA cytosine. In fully labeled cells the distribution of label was found to be uniform throughout the cell. The situation remained unchanged when labeled cells were subsequently treated with chloramphenicol. When short pulses of label were employed a localization of a large proportion of the radioactivity became apparent. The nuclear region was identified as the site of concentration. Similar results were obtained when cells were exposed to much longer pulses of uridine-H³ in the presence of chloramphenicol. If cells were subjected to a short pulse of cytidine-H³, then allowed to grow for a while in unlabeled medium, the label, originally concentrated to some extent in the nuclear region, was found dispersed throughout the cell. The simplest hypothesis which accounts for these results is that a large fraction of the cell RNA is synthesized in a region in or near the nucleus and subsequently transferred to the cytoplasm.

The uptake and development of vaccinia virus in strain L cells followed with labeled viral deoxyribonucleic acid

Vaccinia virus which had its DNA labeled with thymidine-H³ was purified and used as inoculum for L cells growing in suspension. Samples taken over an 8-hour period after infection were studied by light and electron microscopic autoradiography. Within 20 minutes of its being taken up at the cell membrane in phagocytic vesicles, the outer coat of vaccinia becomes disrupted and the virus core containing the labeled DNA passes into the cytoplasmic matrix. Within 1 hour after inoculation the labeled material passes out of the cores into zones of viroplasm, where cores or remnants of cores are gathered and the label becomes more concentrated by 3 hours after inoculation. Most of the label is conserved in the viroplasm areas during the remainder of the experiment. However, 6 hours after inoculation a very small proportion of progeny virus in the cytoplasm, morphologically distinct from the cores of the inoculum, has associated with it labeled material, perhaps derived from the DNA of the inoculum.

A CYTOCHEMICAL LOCALIZATION OF REDUCTIVE SITES IN A GRAM-NEGATIVE BACTERIUM. TELLURITE REDUCTION IN PROTEUS VULGARIS

In order to obtain information on the exact location of the respiratory enzyme chain in Gram-negative bacteria, an electron microscopic study was made of the sites of reducing activity of cells that had, in the living state, incorporated tellurite. In the test object Proteus vulgaris, the reduced tellurite was found to be deposited in bodies contiguous with the plasma membrane but different in structure from those described in the Gram-positive Bacillus subtilis (2). In fact, the bodies proved to consist of a conglomerate of elements which contained the strongly electron-scattering reduced tellurite and a delicately granular "matrix." A limiting membrane was not observed around these complexes. In serial sections details of the complexes are illustrated. Reduced tellurite was not deposited in the plasma membrane to any important degree. Since no other sites of deposition of the reduced product were revealed, it is assumed that the complexes represent the mitochondrial equivalents in the investigated organism. In addition, the bodies might function as the basal granules of the flagella.

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.

Autoradiographic studies of chromosome replication during the cell cycle of Streptococcus faecium

Analysis of the distribution of autoradiographic grains around cells of Streptococcus faecium which had been either continuously or pulse-labeled with tritiated thymidine (mass doubling time, 90 min) showed a non-Poisson distribution even when the distribution of cell sizes in the populations studied was taken into account. These non-Poisson distributions of grains were assumed to reflect the discontinuous nature of chromosome replication. To study this discontinuous process further, we fitted an equation to the grain distribution observed for the pulse-labeled cells that assumed that in any population of cells there were subpopulations in which there were zero, one, or two replicating chromosomes. This analysis predicted an average time for chromosome replication and for the period between completion of rounds of chromosome replication and division of 55 and 43 min, respectively, which were in excellent agreement with estimates made by other techniques. The present investigation extended past studies in indicating that the initiation and completion of rounds of chromosome replication are poorly phased with increases in cell volume and that the amount of chromosome replication may be different in different cell halves.

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.

Localization of Active Microorganisms in Cheese by Autoradiography

By using an autoradiographic technique, one can follow, during ripening of a cheese, the distribution, size, and metabolic activity of microcolonies. Fragments of cheese were labeled with [ 3 H]leucine, fixed, and mounted in epoxy resin. After exposure and development, sections were examined by optical microscopy. In Camembert cheese, bacterial microcolonies synthesized protein rapidly during the beginning of the ripening process. At the end of the ripening process, active bacterial clusters were scarcer and of two types: (i) large microcolonies with reduced labeling, and (ii) microcolonies having the same size as those observed at the beginning of the ripening process, but with slight or no labeling.

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.

Calculated energy deposits from the decay of tritium and other radioisotopes incorporated into bacteria

Transmutation of the radioisotope tritium occurs with the production of a low energy electron, having a range in biological material similar to the dimensions of a bacterium. A computer program was written to determine the radiation dose distributions which may be expected within a bacterium as a result of tritium decay, when the isotope has been incorporated into specific regions of the bacterium. A nonspherical model bacterium was used, represented by a cylinder with hemispherical ends. The energy distributions resulting from a wide variety of simulated labeled regions were determined; the results suggested that the nuclear region of a bacterium receives on the average significantly different per decay doses, if the labeled regions were those conceivably produced by the incorporation of thymidine-(3)H, uracil-(3)H, or (3)H-amino acids. Energy distributions in the model bacterium were also calculated for the decay of incorporated (14)carbon, (35)sulfur, and (32)phosphorous.