Search PubMed⌕ Search

Biomedical subjects

H Bremer

Publications and source records attributed to H Bremer.

At least 55 records · Page 3Linked to original sources
PubMed1986

Analysis of the physiological control of replication of ColE1-type plasmids.

The physiology of ColE1-type plasmid replication in a growing host has been examined both theoretically, using computer simulation, and experimentally, by observing replication of the plasmid pBR322 after a nutritional shift-up from glycerol minimal medium (doubling time 71 min) to LB medium (doubling time 24 min). The theory was based on a negative control model and uses three rate equations: for the accumulation of cell mass, for the accumulation of the replication inhibitor, and for the rate of plasmid synthesis. The implications of the theory were explored by simulating the effects of changes in the expression of replication control genes. The nutritional shift-up experiment showed that plasmid replication was blocked immediately after the shift for about half a mass doubling time; after that time, replication rapidly increased until plasmid numbers per unit volume of culture parallelled the increase in culture mass. After the establishment of steady-state growth in the post-shift medium, the plasmid concentration (plasmids per cell mass) was reduced in comparison to pre-shift growth in the same proportion as the culture doubling time. The results showed that plasmid replication factors are under metabolic control and that the changes in the control of these factors compensate one another during steady-state growth, but not immediately after the medium shift.

Bacteriocin Plasmids↗
PubMed1986

A stochastic process determines the time at which cell division begins in Escherichia coli.

The theoretical distributions of cell masses in exponential cultures of bacteria were derived for both total cells and cells having formed a constriction in preparation for division. The parameters used for this derivation include the mass doubling time, tau, the T-period, and 3 statistical parameters (h, sigma 1, sigma 2) which describe the variability of the cell cycle. The theoretical distributions were compared with observed distributions from E. coli B/rA growing in glucose minimal medium (45 min doubling time) to determine whether a stochastic process in the division pathway affects the time of initiation of constriction or the duration of the constriction process. The results indicate that the stochastic process determines the onset rather than the completion of constriction and that the timing of this process is coupled (6% variability, = sigma 1) to a given cell mass. The values obtained for the duration of the T-period, T = 9.3 min, and for a half-life parameter associated with the stochastic process, h = 4.3 min, agree with previously reported data.

Cell Division↗
PubMed1986

Effect of the bacterial growth rate on replication control of plasmid pBR322 in Escherichia coli.

The concentration of plasmid pBR322, of its replication inhibitor, RNAI, and preprimer, RNAII, were observed in E. coli as functions of the bacterial growth rate. At growth rates between 0.6 and 2.5 doubling/h, the copy number (number of plasmids per genome equivalent of chromosomal DNA) decreased from 32 to 15, the number of plasmids per cell increased from 39 to 55, and the plasmid concentration decreased from 4.6 to 1.1 X 10(10) plasmids per OD460 unit of cell mass. The concentrations of RNAI and RNAII also decreased with increasing growth rate, but differently, such that their ratio, RNAI/RNAII, increased. In glycerol minimal medium both RNAI and RNAII had the same halflife, 0.55 min, and were synthesized at a ratio of about 3 RNAI transcripts per every RNAII transcript. These results were interpreted on the basis of the negative control model and suggest that the activities of the RNAI and RNAII promoters, and the efficiency with which RNAI inhibits plasmid replication, are controlled by the growth rate.

Chromosomes, Bacterial↗
PubMed1986

Effect of relA function on the replication of plasmid pBR322 in Escherichia coli.

Replication of the plasmid pBR322, and the accumulation and life time of its primer transcript, RNAII, and replication inhibitor, RNAI, were measured in an isogenic relA+/relA pair of E. coli strains during exponential growth, or following amino acid starvation, or during treatment with chloramphenicol. (1) The synthesis rates of RNAI and RNAII decreased during inhibition of protein synthesis in either strain, i.e. their promoters are not under stringent control; (2) during amino acid starvation, RNAI and RNAII lifetimes increased in complex, rel-dependent patterns; (3) the changes in RNAI and RNAII synthesis and accumulation had no immediate effect on the rate of plasmid replication; (4) continued plasmid replication requires a protein which is synthesized during amino acid deprivation or treatment with low concentrations of chloramphenicol in relA+, but not in relA bacteria.

Bacterial Proteins↗
PubMed1986

Increased amplification of plasmids pBR322 and pBR327 by low concentrations of chloramphenicol.

The replication and amplification of plasmids pBR322 and pBR327 is maximal during partial inhibition of protein synthesis by low concentrations (10-20 micrograms/ml) of chloramphenicol in rich medium (LB). In this manner, 5- to 10-fold greater yields of plasmid DNA at 2- to 5-fold greater initial purity (less protein and chromosomal DNA content) can be obtained than by using the standard high concentrations of chloramphenicol. These results have been obtained with an improved method of plasmid quantitation in unfractionated bacterial lysates.

Chloramphenicol↗
PubMed1986

[Histological, ultrastructural and topochemical studies on the brood care of Symphysodon aequifasciatus Pellegrin 1903].

In the period of brood care, the epidermis of the genus Symphysodon has a trophical function. Compartment-like filament-containing surface layer cells and secretocytes of different degree of maturity are the first nourishment for the young larvae. Because of the modifications on the outer surface, the larvae are able to take in epidermal nourishment. Epibiotical bacteria on the epidermis of the adults and other epibiotical organisms are found in the intestine of the larvae.

Animals↗
PubMed1985

Initiation of chromosome replication in Escherichia coli after induction of dnaA gene expression from a lac promoter.

Escherichia coli HB282 carries a dnaA46(Ts) allele on the chromosome, a wild-type dnaA allele under the control of the lacUV5 promoter on the multicopy plasmid pBC32, and an overproducing lac repressor allele on an F' factor. When the plasmid dnaA gene is repressed, the strain is thermosensitive. After a temporary deficiency in active dnaA protein at nonpermissive temperature, the addition of isopropyl-beta-D-thiogalactopyranoside to the culture was found to produce a burst of initiations within 5 to 10 min at 30% of the origins in 90% of the cells. Initiations then continued at a rate slightly faster than the mass-doubling time such that after 2 h the origin-to-mass ratio of the control culture was restored.

Bacterial Proteins↗
PubMed1984

Transcription of ribosomal component genes and lac in a relA+/relA pair of Escherichia coli strains.

To determine the stringent response, a repression of gene activity during amino acid starvation assumed to be mediated by the effector necleotide guanosine tetraphosphate (ppGpp), of metabolically regulated constitutive genes, we measured the transcription of ribosomal protein genes, the constitutive lac operon, and stable RNA genes in a variety of growth media and after amino acid starvation in a relA+/relA pair of Escherichia coli B/r strains. For rRNA and tRNA (stable RNA) it has previously been shown that the distinction between stringent control and growth rate control is unfounded, as the function describing the stable RNA gene activities at different concentrations of guanosine tetraphosphate is independent of growth conditions (exponential growth or amino acid starvation) and of the relA allele present. Here, the results indicated that the stringent responses of ribosomal protein genes and lac differ from their metabolic control during exponential growth in different media. This can be explained by polarity and RNA polymerase sink effects during amino acid starvation which are irrelevant for stable RNA genes but which are superimposed on mRNA gene activities.

Amino Acids↗
PubMed1984

[Age-dependence of adenosine deaminase activity in mouse lymphocytes from the spleen and thymus].

The ADA activity was determined in splenocytes and thymocytes of female CBA-mice in dependence on the age of the animals. The investigations were carried out in period of 0 to 120 weeks, the enzyme activities of corresponding cells of animals in the age of 6 weeks served as controls. Whereas the ADA activity of splenocytes did not change during the time of investigations (P = 5%; not significant in analysis of variance), the enzyme activity of thymocytes was diminished significantly in all animals older than 14 weeks. The ratio of the ADA-activities in splenocytes and thymocytes was 1:2 at the beginning of the investigations and 1:1 after 120 weeks.

Adenosine Deaminase↗
PubMed1983

Cell division in Escherichia coli after changes in the velocity of DNA replication.

A method of computer analysis was developed to evaluate the kinetic changes in the rate of cell division in non-synchronous cultures of E. coli resulting from changes in the velocity or initiation of chromosome replication. This method takes into account that the cell division pathway in E. coli includes a reaction of indeterminate length described by a probability function that applies to the cell population. The analysis yields a hypothetical cell number kinetics as it would be observed if the stochastic element in the division pathway were absent. Since this derived cell number curve responds to experimentally induced perturbations of replication at defined times whereas the actual cell number curve reflects these perturbations only in a blurred fashion, replication and division events can be precisely correlated with this method. The method was applied to the evaluation of thymine starvation experiments with two Thy- derivatives of E. coli B/r; one of the strains has a mutationally altered (60% increased) cell mass at initiation of chromosome replication. In both strains, the stochastic phase of the cell cycle had the same half-life value of 10 min and began 18 min after each termination of replication. This suggests that the time of cell division is linked to replication, not to cell mass or length. This interpretation is supported by results of experiments in which the rate of cell growth was altered at the time of thymine starvation.

Cell Division↗
PubMed1983

Increased expression of the dnaA gene has no effect on DNA replication in a dnaA+ strain of Escherichia coli.

We have constructed a pBR322 plasmid derivative which expresses dnaA protein under the control of the E. coli lac UV5 promotor. Expression of the dnaA protein from the plasmid is inducible by isopropyl-beta-D-thiogalactoside. In a dnaA+ strain induction has no effect on the accumulation of DNA. In contrast, in a thermosensitive dnaA46 strain, induction, at either the permissive or the nonpermissive temperature, results in an immediate stimulation of DNA accumulation. We conclude that, while in a dnaA46 strain dnaA protein limits DNA replication, in a dnaA+ strain dnaA protein activity does not control the timing of replication initiation.

Bacterial Proteins↗
PubMed1983

rpoB mutation in Escherichia coli alters control of ribosome synthesis by guanosine tetraphosphate.

An isogenic pair of relA+ and relA strains of Escherichia coli B/r with a mutation in the RNA polymerase subunit gene rpoB (Rifr) was isolated in which the relationship between guanosine tetraphosphate (ppGpp) concentration and stable RNA (rRNA, tRNA) gene activity was altered. The RNA polymerase in the rpoB strains was found to be about 20-fold more sensitive to ppGpp with respect to its stable RNA promoter activity than was the wild-type enzyme. The existence of such mutants is consistent with the idea that ppGpp interacts with the RNA polymerase enzyme and thereby alters its promoter selectivity, i.e., reduces its affinity for the stable RNA promoters. Under most conditions, the rpoB mutants had a reduced rate of growth and about a 10-fold-reduced intracellular concentration of ppGpp compared with the rpoB wild-type strains. The reduction of the level of ppGpp in the rpoB mutants during exponential growth was presumably a reflection of an indirect effect of the rpoB mutation on the control of relA-independent ppGpp metabolism.

DNA-Directed RNA Polymerases↗
PubMed1983

Physiological characterization of Escherichia coli rpoB mutants with abnormal control of ribosome synthesis.

We have previously reported the isolation of Escherichia coli rpoB mutants in which the control of ribosome synthesis by the nucleotide effector guanosine tetraphosphate (ppGpp) is altered, owing to a 20-fold increased sensitivity of the mutant RNA polymerases to ppGpp. In these mutants, the level of ppGpp during exponential growth is decreased about 10-fold, relative to that of rpoB+ wild-type strains, such that a near normal partitioning of RNA polymerase occurs with respect to stable RNA (rRNA and tRNA) gene activity. Here, the physiological effects of two different rpoB alleles in a relA+ and relA background were analyzed in greater detail by comparison with their isogenic rpoB+ wild-type parents. For a given growth medium, the rpoB mutations were found to affect four parameters which resulted in a reduction of growth rate. The results reinforce a previous conclusion that a key element in control of the bacterial growth rate is a mutual relationship between control of ribosome synthesis by ppGpp and control of relA-independent ppGpp metabolism by the concentration and function of ribosomes.

DNA-Directed RNA Polymerases↗
PubMed1983

Isolation of rel mutants of Escherichia coli B/r.

A method that relies on the biological effect of near-UV (340-nm) irradiation is described by which large numbers of independent rel mutants of Escherichia coli B/r may be rapidly isolated.

Escherichia coli↗
PubMed1982

Variation of generation times in Escherichia coli populations: its cause and implications.

The cell cycle of Escherichia coli contains a period of indeterminate length that reflects a stochastic reaction, beginning at some time after a round of chromosome replication, and ending before the cell divides. Although the chemical nature of this reaction is not known, the time of its onset and the single statistical 'half-life parameter' required for its quantitative description have been measured previously. Here it is shown that this parameter implies the distribution of generation times and the age distribution, as well as the distributions of replication initiation and termination ages; these distributions are derived from this half-life parameter for exponentially growing populations of E. coli. It is also shown that the stochastic reaction affects the results and interpretation of any experiments involving synchronous growth of bacteria.

Cell Cycle↗