Evidence for the involvement of unsaturated fatty acids in initiating chromosome replication in Escherichia coli

Fundamental principles in bacterial physiology-history, recent progress, and the future with focus on cell size control: a review

Bacterial physiology is a branch of biology that aims to understand overarching principles of cellular reproduction. Many important issues in bacterial physiology are inherently quantitative, and major contributors to the field have often brought together tools and ways of thinking from multiple disciplines. This article presents a comprehensive overview of major ideas and approaches developed since the early 20th century for anyone who is interested in the fundamental problems in bacterial physiology. This article is divided into two parts. In the first part (sections 1-3), we review the first 'golden era' of bacterial physiology from the 1940s to early 1970s and provide a complete list of major references from that period. In the second part (sections 4-7), we explain how the pioneering work from the first golden era has influenced various rediscoveries of general quantitative principles and significant further development in modern bacterial physiology. Specifically, section 4 presents the history and current progress of the 'adder' principle of cell size homeostasis. Section 5 discusses the implications of coarse-graining the cellular protein composition, and how the coarse-grained proteome 'sectors' re-balance under different growth conditions. Section 6 focuses on physiological invariants, and explains how they are the key to understanding the coordination between growth and the cell cycle underlying cell size control in steady-state growth. Section 7 overviews how the temporal organization of all the internal processes enables balanced growth. In the final section 8, we conclude by discussing the remaining challenges for the future in the field.

The membrane: transertion as an organizing principle in membrane heterogeneity

The bacterial membrane exhibits a significantly heterogeneous distribution of lipids and proteins. This heterogeneity results mainly from lipid-lipid, protein-protein, and lipid-protein associations which are orchestrated by the coupled transcription, translation and insertion of nascent proteins into and through membrane (transertion). Transertion is central not only to the individual assembly and disassembly of large physically linked groups of macromolecules (alias hyperstructures) but also to the interactions between these hyperstructures. We review here these interactions in the context of the processes in Bacillus subtilis and Escherichia coli of nutrient sensing, membrane synthesis, cytoskeletal dynamics, DNA replication, chromosome segregation, and cell division.

Crosstalk between DnaA protein, the initiator of Escherichia coli chromosomal replication, and acidic phospholipids present in bacterial membranes

Anionic (i.e., acidic) phospholipids such as phosphotidylglycerol (PG) and cardiolipin (CL), participate in several cellular functions. Here we review intriguing in vitro and in vivo evidence that suggest emergent roles for acidic phospholipids in regulating DnaA protein-mediated initiation of Escherichia coli chromosomal replication. In vitro acidic phospholipids in a fluid bilayer promote the conversion of inactive ADP-DnaA to replicatively proficient ATP-DnaA, yet both PG and CL also can inhibit the DNA-binding activity of DnaA protein. We discuss how cellular acidic phospholipids may positively and negatively influence the initiation activity of DnaA protein to help assure chromosomal replication occurs once, but only once, per cell-cycle. Fluorescence microscopy has revealed that PG and CL exist in domains located at the cell poles and mid-cell, and several studies link membrane curvature with sub-cellular localization of various integral and peripheral membrane proteins. E. coli DnaA itself is found at the cell membrane and forms helical structures along the longitudinal axis of the cell. We propose that there is cross-talk between acidic phospholipids in the bacterial membrane and DnaA protein as a means to help control the spatial and temporal regulation of chromosomal replication in bacteria.

Chromosome Replication in Escherichia coli: Life on the Scales

At all levels of Life, systems evolve on the 'scales of equilibria'. At the level of bacteria, the individual cell must favor one of two opposing strategies and either take risks to grow or avoid risks to survive. It has been proposed in the Dualism hypothesis that the growth and survival strategies depend on non-equilibrium and equilibrium hyperstructures, respectively. It has been further proposed that the cell cycle itself is the way cells manage to balance the ratios of these types of hyperstructure so as to achieve the compromise solution of living on the two scales. Here, we attempt to re-interpret a major event, the initiation of chromosome replication in Escherichia coli, in the light of scales of equilibria. This entails thinking in terms of hyperstructures as responsible for intensity sensing and quantity sensing and how this sensing might help explain the role of the DnaA protein in initiation of replication. We outline experiments and an automaton approach to the cell cycle that should test and refine the scales concept.

Speculations on the initiation of chromosome replication in Escherichia coli: the dualism hypothesis

The exact nature of the mechanism that triggers initiation of chromosome replication in the best understood of all organisms, Escherichia coli, remains mysterious. Here, I suggest that this mechanism evolved in response to the problems that arise if chromosome replication does not occur. E. coli is now known to be highly structured. This leads me to propose a mechanism for initiation of replication based on the dynamics of large assemblies of molecules and macromolecules termed hyperstructures. In this proposal, hyperstructures and their constituents are put into two classes, non-equilibrium and equilibrium, that spontaneously separate and that are appropriate for life in either good or bad conditions. Maintaining the right ratio(s) of non-equilibrium to equilibrium hyperstructures is therefore a major challenge for cells. I propose that this maintenance entails a major transfer of material from equilibrium to non-equilibrium hyperstructures once per cell and I further propose that this transfer times the cell cycle. More specifically, I speculate that the dialogue between hyperstructures involves the structuring of water and the condensation of cations and that one of the outcomes of ion condensation on ribosomal hyperstructures and decondensation from the origin hyperstructure is the separation of strands at oriC responsible for triggering initiation of replication. The dualism hypothesis that comes out of these speculations may help integrate models for initiation of replication, chromosome segregation and cell division with the 'prebiotic ecology' scenario of the origins of life.

Compartmentalization of prokaryotic DNA replication

It becomes now apparent that prokaryotic DNA replication takes place at specific intracellular locations. Early studies indicated that chromosomal DNA replication, as well as plasmid and viral DNA replication, occurs in close association with the bacterial membrane. Moreover, over the last several years, it has been shown that some replication proteins and specific DNA sequences are localized to particular subcellular regions in bacteria, supporting the existence of replication compartments. Although the mechanisms underlying compartmentalization of prokaryotic DNA replication are largely unknown, the docking of replication factors to large organizing structures may be important for the assembly of active replication complexes. In this article, we review the current state of this subject in two bacterial species, Escherichia coli and Bacillus subtilis, focusing our attention in both chromosomal and extrachromosomal DNA replication. A comparison with eukaryotic systems is also presented.

Hypothesis: hyperstructures regulate initiation in Escherichia coli and other bacteria

Hyperstructures or modules have been proposed to constitute a level of organisation intermediate between macromolecules and whole cells. In this model of intracellular organisation, hyperstructures compete and collaborate for existence within the membrane and cytoplasm. Those directly involved in the cell cycle include initiation, replication and division hyperstructures based on DnaA, SeqA and the 2-minute cluster, respectively. During the run-up to initiation, the mass to DNA ratio increases and, we contend, differential gene expression leads to some hyperstructures becoming more active and stable than others. This results in a drop in the diversity of hyperstructures, some of which release DnaA as they dissociate, and a DnaA-initiation hyperstructure forms. Subsequent DNA replication and cell division generate different daughter cells containing different hyperstructures. This has the advantage of increasing the phenotypic diversity of the population. In developing this model, we also invoke hyperstructures in the partitioning of origins of replication.

Pressure effects on in vivo microbial processes

Pressures between 10 and 100 MPa can exert powerful effects on the growth and viability of organisms. Here I describe the effects of elevated pressure in this range on mesophilic (atmospheric pressure adapted) and piezophilic (high-pressure adapted) microorganisms. Examination of pressure effects on mesophiles makes use of this unique physical parameter to aid in the characterization of fundamental cellular processes, while in the case of piezophiles it provides information on the essence of the adaptation of life to high-pressure environments, which comprise the bulk of our biosphere. Research is presented on the isolation of pressure-resistant mutants, high-pressure regulation of gene expression, the role of membrane lipids and proteins in determining growth ability at high pressure, pressure effects on DNA replication and topology as well as on cell division, and the role of extrinsic factors in modulating enzyme activity at high pressure.

Escherichia coli DnaA protein--phospholipid interactions: in vitro and in vivo

DNA replication in Escherichia coli is controlled at the initiation stage, possibly by regulation of the essential activity of DnaA protein. The cellular membrane has long been hypothesized to be involved in chromosomal replication. Accumulating evidence, both in vitro and in vivo, that supports the importance of membrane phospholipids influencing the initiation activity of DnaA is reviewed.

A SeqA hyperstructure and its interactions direct the replication and sequestration of DNA

A level of explanation in biology intermediate between macromolecules and cells has recently been proposed. This level is that of hyperstructures. One class of hyperstructures comprises the genes, mRNA, proteins and lipids that assemble to fulfil a particular function and disassemble when no longer required. To reason in terms of hyperstructures, it is essential to understand the factors responsible for their formation. These include the local concentration of sites on DNA and their cognate DNA-binding proteins. In Escherichia coli, the formation of a SeqA hyperstructure via the phenomenon of local concentration may explain how the binding of SeqA to hemimethylated GATC sequences leads to the sequestration of newly replicated origins of replication.

Hypothesis: hyperstructures regulate bacterial structure and the cell cycle

A myriad different constituents or elements (genes, proteins, lipids, ions, small molecules etc.) participate in numerous physico-chemical processes to create bacteria that can adapt to their environments to survive, grow and, via the cell cycle, reproduce. We explore the possibility that it is too difficult to explain cell cycle progression in terms of these elements and that an intermediate level of explanation is needed. This level is that of hyperstructures. A hyperstructure is large, has usually one particular function, and contains many elements. Non-equilibrium, or even dissipative, hyperstructures that, for example, assemble to transport and metabolize nutrients may comprise membrane domains of transporters plus cytoplasmic metabolons plus the genes that encode the hyperstructure's enzymes. The processes involved in the putative formation of hyperstructures include: metabolite-induced changes to protein affinities that result in metabolon formation, lipid-organizing forces that result in lateral and transverse asymmetries, post-translational modifications, equilibration of water structures that may alter distributions of other molecules, transertion, ion currents, emission of electromagnetic radiation and long range mechanical vibrations. Equilibrium hyperstructures may also exist such as topological arrays of DNA in the form of cholesteric liquid crystals. We present here the beginning of a picture of the bacterial cell in which hyperstructures form to maximize efficiency and in which the properties of hyperstructures drive the cell cycle.

Visualization of membrane domains in Escherichia coli

Bacterial membrane and nucleoids were stained concurrently by the lipophilic styryl dye FM 4-64 [N-(3-triethylammoniumpropyl)-4-(6-(4-(diethylamino)phenyl) hexatrienyl)pyridinium dibromide] and 4',6-diamidino-2-phenylindole (DAPI), respectively, and studied using fluorescence microscopy imaging. Observation of plasmolysed cells indicated that FM 4-64 stained the inner membrane preferentially. In live Escherichia coli pbpB cells and filaments, prepared on wet agar slabs, an FM 4-64 staining pattern developed in the form of dark bands. In dividing cells, the bands occurred mainly at the constriction sites and, in filaments, between partitioning nucleoids. The FM 4-64 pattern of dark bands in filaments was abolished after inhibiting protein synthesis with chloramphenicol. It is proposed that the staining patterns reflect putative membrane domains formed by DNA-membrane interactions and have functional implications in cell division.

Differential reactivities of hypochlorous and hypobromous acids with purified Escherichia coli phospholipid: formation of haloamines and halohydrins

Hypochlorous (HOCl) and hypobromous (HOBr) acids are strong oxidants derived from myeloperoxidase and eosinophil peroxidase, the major antimicrobial enzymes of neutrophils and eosinophils, respectively. These oxidants are highly reactive with a wide range of biomolecules. At physiological pH, both HOCl and HOBr react readily with amines to form haloamines and with the unsaturated bonds of fatty acids to form halohydrins. We have investigated which of these reactions occur with phosphatidylethanolamine (PE), the predominant phospholipid of Escherichia coli. The formation of haloamines was determined by TLC and colorimetrically and the formation of halohydrins was determined by TLC and GC-MS. With HOCl, chloramines were much the preferred product and chlorohydrins were formed in substantial amounts only when HOCl was in excess of the amount required to convert the amine to the dichloramine. With HOBr at all concentrations, bromamines and bromohydrins were formed concurrently, indicating a greater relative reactivity with unsaturated fatty acids than with HOCl. The bromamine derivatives of PE, and other primary amines, were found to be more reactive than the equivalent chloramines, and were able to brominate the unsaturated bonds of fatty acids. Bromohydrins (formed directly or through the action of bromamines) may, therefore, be suitable biomarkers for the production of HOBr in vivo.

Molecular basis for membrane phospholipid diversity: why are there so many lipids?

Phospholipids play multiple roles in cells by establishing the permeability barrier for cells and cell organelles, by providing the matrix for the assembly and function of a wide variety of catalytic processes, by acting as donors in the synthesis of macromolecules, and by actively influencing the functional properties of membrane-associated processes. The function, at the molecular level, of phosphatidylethanolamine, phosphatidylglycerol, and cardiolipin in specific cellular processes is reviewed, with a focus on the results of combined molecular genetic and biochemical studies in Escherichia coli. These results are compared with primarily biochemical data supporting similar functions for these phospholipids in eukaryotic organisms. The wide range of processes in which specific involvement of phospholipids has been documented explains the need for diversity in phospholipid structure and why there are so many membrane lipids.

Membrane fluidity of Escherichia coli during heat-shock

The excimer-forming fluorophore dipyrenylpropane has been used to measure the relative fluidity of total membranes isolated from Escherichia coli grown at 30 or 45 degrees C, or exposed to a heat-shock from 30 to 45 degrees C for various periods of time. Parallel experiments were performed using [35S]methionine pulse-labeling of cells, to study the induction of heat-shock proteins (HSPs) at different times after the sudden change in E. coli growth-temperature from 30 to 45 degrees C. Results suggest that upon an abrupt temperature upshift from 30 to 45 degrees C, membrane fluidity adjustment to the steady-state level at the high temperature, takes place during the E. coli heat-shock response.

In vivo evidence for the involvement of anionic phospholipids in initiation of DNA replication in Escherichia coli

In vitro, anionic phospholipids can reactivate inactivated DnaA protein, which is essential for initiation of DNA replication at the oriC site of Escherichia coli [Sekimizu, K. & Kornberg, A. (1988) J. Biol. Chem. 263, 7131-7135]. Mutations in the pgsA gene (encoding phosphatidylglycerophosphate synthase) limit the synthesis of the major anionic phospholipids and lead to arrest of cell growth. We report herein that a mutation in the rnhA gene (encoding RNase H) that bypasses the need for the DnaA protein through induction of constitutive stable DNA replication [Kogoma, T. & von Meyenburg, K. (1983) EMBO J. 2, 463-468] also suppressed the growth arrest phenotype of a pgsA mutant. The maintenance of plasmids dependent on an oriC site for replication, and therefore DnaA protein, was also compromised under conditions of limiting anionic phospholipid synthesis. These results provide support for the involvement of anionic phospholipids in normal initiation of DNA replication at oriC in vivo by the DnaA protein.

Participation of the bacterial membrane in DNA replication and chromosome partition

The concept that the bacterial membrane plays an active role in the regulation of DNA replication and in segregation, or 'partition', of the bacterial chromosome at cell division was proposed in 1963. Membrane participation offered a relatively simple way to coordinate replication and partition. Some of the details of this model have been confirmed, while others have been changed. In fact, it appears that the membrane may play several distinct roles in these processes, and recent experiments have begun to identify the complexity of membrane involvement.

Reversal by phosphatidylglycerol and cardiolipin of inhibition of transcription and replication by histones in vitro

We examined the effects of phospholipids on transcription and replication in vitro in the presence of histones. Phosphatidylglycerol and cardiolipin were shown to reverse the inhibitory effects of histones in both random RNA synthesis with purified RNA polymerase II and accurate transcription initiated from the adenovirus 2 major late promoter in a nuclear extract. Phosphatidylserine, phosphatidic acid, and phosphatidylcholine did not activate RNA synthesis although they bound as strongly as phosphatidylglycerol and cardiolipin to histones. Phosphatidylglycerol and cardiolipin also reversed the in vitro inhibition of SV40 DNA replication by histones. Unsaturation of the fatty acid residues was shown to be necessary for the restoration of transcription and replication by phosphatidylglycerol.

Metabolic regulations and biological functions of phospholipids in Escherichia coli

Extensive genetic and biochemical studies in the last two decades have elucidated almost completely the framework of synthesis and turnover of quantitatively major phospholipids in E. coli. The knowledge thus accumulated has allowed to formulate a novel working model that assumes sophisticated regulatory mechanisms in E. coli to achieve the optimal phospholipid composition and content in the membranes. E. coli also appears to possess the ability to adapt phospholipid synthesis to various cellular conditions. Understanding of the functional aspects of E. coli phospholipids is now advancing significantly and it will soon be able to explain many of the hitherto unclear cell's activities on the molecular basis. Phosphatidylglycerol is believed to play the central role both in metabolism and functions of phospholipids in E. coli. The results obtained with E. coli should undoubtedly be helpful in the study of more complicated phospholipid metabolism and functions in higher organisms.

A protein kinase C-like activity in Escherichia coli

The protein kinase C (PKC) family comprises calcium- and phospholipid-dependent kinases whose activity is stimulated by diacylglycerol and tumour-promoting phorbol esters such as 12-tetradecanoyl phorbol-13-acetate (TPA). In the Gram-negative bacterium Escherichia coli, functional similarity to PKC was demonstrated in crude extracts by calcium and phospholipid-dependent, TPA-stimulated phosphorylation of a small number of endogenous substrates. Activity was reduced by sphingosine, a known inhibitor of eukaryotic PKC. Structural similarity to PKC was demonstrated in crude and partially purified bacterial extracts by cross-reactivity with several monoclonal antibodies. This revealed isozyme-specific homology between a protein(s) of relative molecular mass 80-85,000 in E. coli and the alpha- and gamma-isozymes, but probably not the beta-isozyme, of eukaryotic PKC.

Studies on the alteration of chromosome copy number and cell division potential in a dnaA mutant of Escherichia coli

The dnaA167 mutant of Escherichia coli, N167, maintains, on the average, two replicating chromosomes per cell at the permissive growth temperature of 30 degrees C and only one per cell at the higher permissive growth temperature of 38 degrees C. When the growth temperature of this mutant is changed from 30 degrees to 38 degrees C the cells rapidly readjust their chromosome copy number from two to one. I have examined the kinetics of this transition with reference to DNA replication and cell division. My results indicate that this mutant uncouples cell division from chromosome duplication to achieve the appropriate copy number, suggesting that the dnaA gene product may be involved in the coordination between these two cellular events.

DNA replication in Escherichia coli is initiated by membrane detachment of oriC. A model

An adequate model for the initiation of chromosome replication in Escherichia coli should explain why the introduction of multiple copies of the chromosomal origin of replication, oriC, does not perturb cells seriously and why such multiple origins are replicated synchronously; it should explain why the key initiator protein, DnaA, is activated in vitro by binding specifically to acidic phospholipids and why the Dam methyltransferase is essential for the correct timing of initiation; it should explain why phospholipid synthesis and fluidity are necessary for initiation. In the detachment model, presented here, cyclical changes in the phospholipid composition of the cytoplasmic membrane activate initiator proteins such as DnaA protein and cause origins to detach; this detachment allows torsional stresses to open 13mer sequences in oriC; DnaA assists in the serial opening of these sequences and guides the entry of the helicase to form a pre-priming complex and trigger initiation; the greater affinity of hemi-methylated origin for membrane is re-interpreted as a mechanism for preventing re-initiation.

Membrane attachment activates dnaA protein, the initiation protein of chromosome replication in Escherichia coli

ADP and ATP are tightly bound to dnaA protein and are crucial to its function in DNA replication; the exchange of these nucleotides is effected specifically by the acidic phospholipids (cardiolipin and phosphatidylglycerol) present in Escherichia coli membranes [Sekimizu, K. & Kornberg, A. (1988) J. Biol. Chem. 263, 7131-7135]. We now find that phospholipids derived from membranes lacking an unsaturated fatty acid (e.g., oleic acid) are unable to promote the exchange. This observation correlates strikingly with the long-known effect of 3-decynoyl-N-acetylcysteamine, a "suicide analog" that prevents initiation of a cycle of replication in E. coli by inhibiting the synthesis of oleic acid, an inhibition that can be overcome by providing the cells with oleic acid. Profound influences on the specific binding of dnaA protein to phospholipids by temperature, the content of unsaturated fatty acids, and the inclusion of cholesterol can be explained by the need for the phospholipids to be in fluid-phase vesicles. These findings suggest that membrane attachment of dnaA protein is vital for its function in the initiation of chromosome replication in E. coli.

Biochemical and morphological properties of membranes of unsaturated fatty acid auxotrophs of Salmonella typhimurium: effects of fluorinated myristic acids

In order to investigate the utility of the fluorine-19 nucleus as a spectroscopic probe, a fluorinated analog of myristic acid has been incorporated into the membrane lipids of an unsaturated fatty acid auxotroph of Salmonella typhimurium. It is capable of supporting limited growth at temperatures above 37 degrees C. Freeze-fracture electron microscopic examinations of the membrane ultrastructure show a temperature and fatty acid supplement-dependent segregation of intramembranous protein particles into distinct patches in the auxotrophic membrane leaving intramembranous protein-denuded areas. The occurrence of these patches seems to be related to the phase separation of membrane lipids. Corresponding changes in the transport and accumulation of methyl thio-beta-D-galactopyranoside and tetracycline are observed. However, transport of histidine does not appear to be dependent on the physical state of the membrane lipids. The auxotroph shows differences in growth and morphological characteristics from those of the wild type. Functions of both inner and outer membranes are shown to be affected as a response to the fatty acid chain composition of the lipids.

Duplication of Escherichia coli during inhibition of net phospholipid synthesis

In Escherichia coli BB26-36, the inhibition of net phospholipid synthesis during glycerol starvation affected cell duplication in a manner that was similar in some respects to that observed during the inhibition of protein synthesis. Ongoing rounds of chromosome replication continued, and cells in the D period divided. The initiation of new rounds of chromosome replication and division of cells in the C period were inhibited. Unlike the inhibition of protein synthesis, however, the accumulation of initiation potential in dnaA and dnaC mutants at the nonpermissive temperature was not affected by the inhibition of phospholipid synthesis. Furthermore, proteins synthesized during the inhibition of phospholipid synthesis can be utilized later for division. The results are consistent with a dual requirement for protein and phospholipid synthesis for both the inauguration of new rounds of chromosome replication and the initiation of septum formation. Once initiated, both processes progress to completion independent of continuous phospholipid and protein synthesis.

[The Escherichia coli cell cycle]

This review summarizes present knowledge of the bacterial cell cycle with particular emphasis on Escherichia coli. We discuss data coming from three different types of approaches to the study of cell extension and division: The search for discrete events occurring once per division cycle. It is generally agreed that the initiation and termination of DNA replication and cell septation are discrete events; there is less agreement on the sudden doubling in rate of cell surface extension, murein biosynthesis and the synthesis of membrane proteins and phospholipids. We discuss what is known about the temporal relationship amongst the various cyclic events studied. The search for discrete growth zones in the cell envelope layers. We discuss conflicting reports on the existence of murein growth zones and protein insertion sites in the inner and outer membranes. Elucidation of the mechanism regulating the initiation of DNA replication. The concept of "critical initiation mass" is examined. We review data suggesting that the DNA is attached to the envelope and discuss the role of the latter in the initiation of DNA replication.

Acute effects of ethanol on biosynthesis and glycosylation of IgGl(kappa) antibody molecules in cultured P3/X63-Ag8 myeloma cells

Protein synthesis in cultured P3/X63-Ag8 mouse myeloma cells was inhibited by acute exposure to ethanol. However, the synthesis of IgGl antibody, as a percentage of total protein synthesis, increased slightly. Experiments using actinomycin D suggest that the overall inhibition of protein synthesis by ethanol occurs at the translational level. Following an L-[35S]methionine pulse, cultured P3/X63-Ag8 cells contained one light antibody polypeptide and two heavy antibody polypeptides. One of these heavy chains was shown to be the unglycosylated precursor of the other, mature molecule. Only the glycosylated polypeptide is a normal constituent of secreted IgGl antibody. The glycosylation of the immature heavy chains occurred more rapidly during a 1-hr isotopic pulse in cells exposed to ethanol (0.1 v/v % and above), than in unexposed control cells. The observed effects of ethanol on antibody glycosylation may be related to the increased susceptibility of alcoholic patients to infections. Ethanol may also affect the synthesis of other glycoproteins in myeloma cells and other tissues.

Regulation of DNA synthesis and capacity for initiation in DNA temperature sensitive mutants of Escherichia coli I. Reinitiation and chain elongation

The capacity for initiation and subsequent chain elongation was examined in several DNA temperature sensitive mutants of Escherichia coli after the mutants had been held at nonpermissive temperature for approximately 1.5 generation equivalents and then returned to permissive temperature in the presence of chloramphenicol. The results obtained indicate that 4-5 sets of replication forks can be initiated after return to permissive temperature in the presence of chloramphenicol but the forks apparently become stalled and fail to complete chromosomal replication in the presence of chloramphenicol. In temperature reversible dnaA mutants, once the chloramphenicol is removed the forks appear to be able to resume replication at the nonpermissive temperature. The relationship between premature initiation and premature chain termination is discussed.

Initiation of chromosome replication in dnaA and dnaC mutants of Escherichia coli B/r F

Regulatory aspects of chromosome replication were investigated in dnaA5 and dnaC2 mutants of the Escherichia coli B/r F. When cultures growing at 25 degrees C were shifted to 41 degrees C for extended periods and then returned to 25 degrees C, the subsequent synchronous initiations of chromosome replication were spaced at fixed intervals. When chloramphenicol was added coincident with the temperature downshift, the extend of chromosome replication in the dnaA mutant was greater than that in the dnaC mutant, but the time intervals between initiations were the same in both mutants. Furthermore, the time interval between the first two initiation events was unaffected by alterations in the rate of rifampin-sensitive RNA synthesis or cell mass increase. In the dnaC2 mutant, the capacities for both initiations were achieved in the absence of extensive DNA replication at 25 degrees C as long as protein synthesis was permitted, but the cells did not progress toward the second initiation at 25 degrees C when both protein synthesis and DNA replication were prevented. Cells of the dnaA5 mutant did not achieve the capacity for the second initiation event in the absence of extensive chromosome replication, although delayed initiation may have taken place. A plausible hypothesis to explain the data is that the minimum interval is determined by the time required for formation of a supercoiled, membrane-attached structure in the vicinity of oriC which is required for initiation of DNA synthesis.

Temperature dependent release of beta-beta' subunits of DNA dependent RNA polymerase from the folded chromosome of a dnaAts mutant of Escherichia coli

DNA-dependent RNA polymerase has been found to be preferentially released at 43 degrees C from the folded nucleoids of an E. coli dnaAts mutant when compared with the same nucleoids at 30 degrees C or with nucleoids of a dnaA+ strain at either 30 degrees or 43 degrees C. The polypeptides released are identical in molecular weight with those of the beta and beta' constituent polypeptides of the core enzyme of a known E. coli RNA polymerase. In addition, these polypeptides are precipitated by specific anti-RNA polymerase rabbit IgG. The implications of the interactions of RNA polymerase with the dnaA gene product are discussed.

The requirement of DNA synthesis for the induction of alkaline phosphatase by bromodeoxyuridine in a derivative of the HeLa cell line

Non-lethal concentrations of bromodeoxyuridine induce a 2- to 5-fold increase in the specific activity of alkaline phosphatase in a HeLa subclone, S3G. Experiments employing 10-hour pulses of BRdU showed that 48 hours were required before induction commenced, and that maximal induction was attained by 96 hours. Under conditions in which DNA synthesis was prevented with hydroxyurea induction did not occur. Upon removal of hydroxyurea both DNA synthesis and induction were rapidly reestablished. Furthermore, experiments employing radiolabelled BRdU demonstrated that the kinetics of the induction process paralleled the incorporation of the analogue into cellular DNA. These results indicate that DNA synthesis, or some process intimately linked to DNA synthesis, is required for the induction of alkaline phosphatase, and suggest that the mode of the induction may be through the incorporation of the analogue into cellular DNA.

dnaT, dominant conditional-lethal mutation affecting DNA replication in Escherichia coli

Normally, bacteria cease DNA replication in the absence of protein synthesis. A variety of treatments, such as thymine starvation or a shift-up to rich medium, lead to continued DNA replication in the absence of protein synthesis. Mutants are described which always terminate replication under these conditions. These conditional lethal mutants, dnaT1 and dnaT2, contransduce with serB and dnaC. The mutation also affects cell division. All aspects of the mutant phenotype (obligatory termination of replication, temperature sensitivity of DNA replication and growth, and aberrant cell division at permissive growth temperatures) were transdominant to the wild-type phenotype. Episomes carrying the dnaT mutation appeared to be unstable. The existence of such a dominant mutation was predicted by a model of chromosome termination proposed by Kogoma and Lark (J. Mol. Biol. 94:243-256, 1975).

A shift from phospholipid to triglyceride synthesis when cell division is inhibited by trans-fatty acids

The yeast mutant Saccharomyces cerevisiae (KD 46) requires added unsaturated fatty acid for growth. When cell growth was inhibited by the presence of trans-acids there was a marked inhibition of oleate esterification into phospholipids accompanying continued incorporation into triglycerides. Apparently some control point in phospholipid synthesis associated with the cell cycle occurs after the stage of phosphatidate biosynthesis.

Independence of deoxyribonucleic acid replication and initiation from membrane fluidity and the supply of unsaturated fatty acids in Escherichia coli

Mutant derivatives of the unsaturated fatty acid auxotroph K1062 were employed to investigate whether the supposedly membrane-bound bacterial replication machinery requires for its replicatory functions a fluid membrane environment as is known for several membrane-associated protein functions. Temperatures Tt for fluid reversible nonfluid phase transitions of membrane phospholipids are raised from below 18 to 38 degrees C when mutant cells are supplemented with elaidate instead of with oleate. In this experimental system current or synchroneously initiated new rounds of DNA replication are shown in vivo to continue 8 degrees below Tt, provided appropriate corrections for the concurrent cellular metabolic breakdown are considered. Temperature rate profiles for in vitro deoxyribonucleic acid replication rates measured in lysates of either oleate- or elaidate-supplemented cells yield congruent Arrhenius plots without discontinuities at corresponding Tt positions. We conclude that neither the start nor the propagation of replication forks depends on a fluid membrane. The capacity for the assembly of new replication complexes was studied in replication-aligned cells either shifted from oleate to elaidate (at temperatures below Tt for newly synthesized phospholipids) or starved for oleate. Regardless of whether unsaturated fatty acids are exchanged or completely withheld, new replication complexes can be normally assembled and initiated. These results do not support the conclusions reached by Fralick and Lark (1973) that the availability of unsaturated fatty acids is a prerequisite for the assembly of a functional replication complex.

Viral DNA synthesis in vitro with the inclusions isolated from adenovirus 12-infected cells

A fraction defined as the inclusions was isolated by banding in CsC1 gradients from nuclei of adenovirus 12-infected KB cells. When examined by electron microscopy, the isolated inclusions were relatively homogeneous, finely granular materials of moderate electron density, possibly representing the disintegrated type II or IV inclusions. The conditions of endogenous DNA synthesis in vitro with the inclusions were determined. The product of DNA synthesis in vitro with the inclusions was mainly viral and scarcely cellular, as revealed by DNA-DNA hybridization and methylated albumin kieselgur column chromatography. However, viral DNA synthesized in vitro was smaller (18S, 22S) than viral DNA in virions (31 S, 34 S) in neutral and alkaline sucrose gradients. Effects of various treatment of the inclusions on the DNA-synthesizing activity showed that phospholipase C inhibited the activity efficiently. The in vitro DNA synthesis was stimulated by addition of the cytoplasmic extract from adenovirus 12-infected cells and not that from unifected cells. The analysis of the composition of the inclusions showed that the inclusions contained DNA, protein, phospholipid and a small amount of RNA and carbohydrate.