Morphological analysis of the division cycle of two Escherichia coli substrains during slow growth.

Morphological parameters of the cell division cycle have been examined in Escherichia coli B/r A and K. Whereas the shape factor (length of newborn cell/width) of the two strains was the same at rapid growth (doubling time, tau, less than 60 min), with decreasing growth rate the dimensions of the two strains did change so that B/r A cells became more rounded and B/r K cells became more elongated. The process of visible cell constriction (T period) lasted longer in B/r A than in B/r K during slow growth, reaching at tau = 200 min values of 40 and 17 min, respectively. The time between termination of chromosome replication and cell division (D period) was found to be longer in B/r A than in B/r K. As a result, in either strain completion of chromosome replication seemed always to occur before initiation of cell constriction. Nucleoplasmic separation did not coincide with termination as during rapid growth but occurred in both strains within the T period, about 10 min before cell division.     

Morphological analysis of nuclear separation and cell division during the life cycle of Escherichia coli.

Quantitative electron microscope observations were performed on Escherichia coli B/r after balanced growth with doubling times (tau) of 32 and 60 min. The experimental approach allowed the timing of morphological events during the cell cycle by classifying serially sectioned cells according to length. Visible separation of the nucleoplasm was found to coincide with the time of termination of chromosome replication as predicted by the Cooper-Helmstetter model. The duration of the process of constrictive cell division (10 min) appeared to be independent of the growth rate for tau equals 60 min or less but to increase with increase doubling time in more slowly growing cells. Physiological division, i.e., compartmentalization prior to physical separation of the cells, was only observed to occur in the last minute of the cell cycle. The morphological results indicate that cell elongation continues during the division process in cells with tau equals 32 min, but fails to continue in cells with tau equals 60 min.     

Correlation between size and age at different events in the cell division cycle of Escherichia coli.

The variability of (i) the B period between birth and initiation of chromosome replication, (ii) the U period between initiation of chromosome replication and initiation of cell constriction, and (iii) the interdivision period (tau) have been estimated for slowly growing Escherichia coli B/r F. Cultures synchronized by the membrane elution technique were pulse-labeled with [3H]thymidine or continuously labeled with [3H]thymine. After fixation, the pattern of deoxyribonucleic acid replication was analyzed by electron microscopic radioautography. Cell length was found to increase exponentially with age at two different slow growth rates. The coefficient of variation of the B period was estimated to be 60%, that of the U period was 29%, and that of the interdivision period was 12%. From these values and the coefficient of variation of length at different cell cycle events were calculated a negative correlation between the B and U period (r = -0.9) and a positive correlation between length at birth and cell separation (r = 0.6). Initiation of chromosome replication and cell constriction were strictly correlated both with respect to age (r = 0.7) and length (r = 0.8). On the other hand, length at initiation of chromosome replication was distantly correlated with age (r = 0.1) or length at birth (r = 0.3). This low correlation excludes a model in which chromosome initiation is controlled by a random event in the B period. It favors a model in which chromosome initiation occurs at a particular distributed size independent of cell division.     

Cell cycle parameters of slowly growing Escherichia coli B/r studied by flow cytometry

The cell cycle kinetics of Escherichia coli B/r A and B/r K cells were studied by flow cytometry. Three-dimensional histograms of cell cultures show the number of cells as a function of cellular DNA and protein contents and give detailed pictures of the cell cycle distribution with regard to these parameters. Histograms of slowly growing chemostat cultures showed that cell cycle periods B and C + D increase with a decreasing growth rate and that the B period occupies an increasing fraction of the cycle. The DNA replication patterns of B/r A and K were found to be quite similar. At extremely low growth rates (doubling time [T] = 17 h), B/r A cells had a B period of 0.8 T, a C period of 0.1 T, and a D period of 0.1 T, and B/r K cells (T = 16 h) had a B period of 0.6 T, a C period of 0.15 T, and a D period of 0.25 T. Mass increase, i.e., essentially protein synthesis, was seen in all three periods of the cell cycle. For B/r A cells, the average rate of mass increase was 11 times greater in the D period than in the B period, whereas for B/r K cells the rate of mass increase was twice as great in the D period as in the B period. The DNA and cell size distributions of batch cultures in exponential growth were found to vary with time, indicating that such cultures are not suitable for studies of cell cycle kinetics.     

Escherichia coli DNA distributions measured by flow cytometry and compared with theoretical computer simulations.

A computer simulation routine has been made to calculate the DNA distributions of exponentially growing cultures of Escherichia coli. Calculations were based on a previously published model (S. Cooper and C.E. Helmstetter, J. Mol. Biol. 31:519-540, 1968). Simulated distributions were compared with experimental DNA distributions (histograms) recorded by flow cytometry. Cell cycle parameters were determined by varying the parameters to find the best fit of theoretical to experimental histograms. A culture of E. coli B/r A with a doubling time of 27 min was found to have a DNA replication period (C) of 43 min and an average postreplication period (D) of 22 to 23 min. Similar cell cycle parameters were found for a 60-min B/r A culture. Initiations of DNA replication at multiple origins in one and the same cell were shown to be essentially synchronous. A slowly growing B/r A culture (doubling time, 5.5 h) had an average prereplication period (B) of 2.3 h; C = 2.4 h and D = 0.8 h. It was concluded the the C period has a constant duration of 43 min (at 37 degrees C) at fast growth rates (doubling times, less than 1 h) but increases at slow growth rates. Thus, our results obtained with unperturbed exponential cultures in steady state support the model of Cooper and Helmstetter which was based on data obtained with synchronized cells.     

Chromosome replication during the division cycle in slowly growing, steady-state cultures of three Escherichia coli B/r strains.

The period of DNA synthesis C during the cell cycle was determined over a broad range of generation times in slowly growing, steady-state batch cultures in the exponential phase and in chemostat cultures of three strains of Escherichia coli, strains B/r A, B/r K, and B/r TT, utilizing measurements of average amounts of DNA per cell and cell survival after radioactive decay of 125I incorporated into the DNA of synthesizing cells. At each growth rate, values for cell survival and for C periods were the same within experimental errors for the three strains. The length of the DNA synthesis period increased linearly with generation (doubling) time T of the culture and approached a limiting value of C = 0.36T at very long generation times. In very slowly growing cultures, DNA replication was limited almost entirely to the final third of the cell cycle. D periods, between termination of DNA replication and cell division, were found to be relatively short at all growth rates for each strain. Average amounts of DNA per cell measured in slowly growing cultures of strains B/r A and B/r TT were indistinguishable from results for strain B/r K at the same growth rates. Amounts of DNA per cell calculated from the cell survival values alone are completely consistent with the measured DNA per cell.     

Dependency of size of Saccharomyces cerevisiae cells on growth rate.

The mean size and percentage of budded cells of a wild-type haploid strain of Saccharomyces cerevisiae grown in batch culture over a wide range of doubling times (tau) have been measured using microscopic measurements and a particle size analyzer. Mean size increased over a 2.5-fold range with increasing growth rate (from tau = 450 min to tau = 75 min). Mean size is principally a function of growth rate and not of a particular carbon source. The duration of the budded phase increased at slow growth rates according to the empirical equation, budded phase = 0.5 tau + 27 (all in minutes). Using a recent model of the cell cycle in which division is thought to be asymmetric, equations have been derived for mean cell age and mean cell volume. The data are consistent with the notion that initiation of the cell cycle occurs at "start" after attainment of a critical cell size, and this size is dependent on growth rate, being, at slow growth rates, 63% of the volume of fast growth rates. Previous reports are reanalyzed in the light of the unequal division model and associated population equations.     

Escherichia coli contains a DNA replication compartment in the cell center

The active replication forks of E. coli B/r K cells growing with a doubling time of 210 min have been pulse-labeled with [(3)H] thymidine for 10 min. By electron-microscopic autoradiography the silver grains have been localized in the various length classes. From the known pattern of the DNA replication period in the cell cycle at slow growth and from the average position of grains per length class it was deduced that DNA replication starts in the cell center and that it remains there for a substantial part of the DNA replication period. This suggests the occurrence of a centrally located DNA replication compartment.     

Timing the start of division in E. coli: a single-cell study

We monitor the shape dynamics of individual E. coli cells using time-lapse microscopy together with accurate image analysis. This allows measuring the dynamics of single-cell parameters throughout the cell cycle. In previous work, we have used this approach to characterize the main features of single-cell morphogenesis between successive divisions. Here, we focus on the behavior of the parameters that are related to cell division and study their variation over a population of 30 cells. In particular, we show that the single-cell data for the constriction width dynamics collapse onto a unique curve following appropriate rescaling of the corresponding variables. This suggests the presence of an underlying time scale that determines the rate at which the cell cycle advances in each individual cell. For the case of cell length dynamics a similar rescaling of variables emphasizes the presence of a breakpoint in the growth rate at the time when division starts, tau(c). We also find that the tau(c) of individual cells is correlated with their generation time, tau(g), and inversely correlated with the corresponding length at birth, L(0). Moreover, the extent of the T-period, tau(g) - tau(c), is apparently independent of tau(g). The relations between tau(c), tau(g) and L(0) indicate possible compensation mechanisms that maintain cell length variability at about 10%. Similar behavior was observed for both fast-growing cells in a rich medium (LB) and for slower growth in a minimal medium (M9-glucose). To reveal the molecular mechanisms that lead to the observed organization of the cell cycle, we should further extend our approach to monitor the formation of the divisome.     

Elongation and surface extension of individual cells of Escherichia coli B/r: comparison of theoretical and experimental size distributions

The way individual cells grow and divide uniquely determines the (time-invariant) cell size distribution of populations in steady-state exponential growth. In the preceding article, theoretical distributions were derived for two exponential and six linear models containing a small number of adjustable parameters but no assumptions other than that all cells obey the same growth law. The linear models differ from each other with respect to the timing of the presumptive doubling in their growth rate, the exponential models--according to whether there is or is not a part of the cell that does not contribute to the growth rate. Here we compared the size distributions predicted by each of these models with those of cell length and surface area measured by electron microscopy; the quality of the fit, as determined by the mean-square successive-differences test and the chi 2 goodness-of-fit test, was taken as a measure of the adequacy of the model. The actual data came from two slow-growing E. coli B/r cultures, an A strain (pi = 125 min) and a K strain (pi = 106 min), and a correction was introduced in each to account for the distortion caused by the finite size of the picture frame. The parameter estimates produced by the various models are quite reliable (cv less than 0.1%); we discuss them briefly and compare their values in the two strains. All the length extension models were rejected outright whereas most of the surface growth versions were not. When the same models were tested on A-strain data from a faster growing culture (tau = 21 min), those models that provided an adequate fit to the cell surface area data proved equally satisfactory in the case of cell length. These findings are evaluated and shown to be consistent with cell surface area rather than cell length being the dimension under active control. Three surface area models, all linear, are rejected--those in which doubling of the growth rate occurs with a constant probability from cell birth, at a particular cell age, and precisely at cell division. The evidence in the literature that appears to contradict this last result, rejection of the simple linear surface growth model, is shown to be faulty. The 16 original models are here reduced to five, two involving exponential surface growth and three linear, and possible reasons are presented for our inability to discriminate further at this stage.     

Approximation of the cell cycle in synchronized populations of Streptococcus faecium.

Slowly growing populations (TD = 70 to 80 min) of Streptococcus faecium (S. faecalis ATCC 9790) were synchronized by selection after sucrose gradient fractionation. The cell cycle was approximated by correlating the patterns of DNA accumulation and cell division. More specifically, the beginning of cell cycle was equated with the beginning of a rapid linear increase in DNA accumulation. The DNA content of the culture approximately doubled during the period of accumulation, which lasted about 51 min. The period of rapid DNA accumulation, was followed by a period of reduced accumulation that lasted about 24 min. During synchronized growth, cell numbers increased rapidly in coordination with the period of rapid DNA accumulation and exhibited a plateau during the period of reduced DNA accumulation. In contrast, RNA and protein appeared to accumulate exponentially throughout the cell cycle at the same rate as culture mass.     

Bacillus subtilis Cell Cycle as Studied by Fluorescence Microscopy: Constancy of Cell Length at Initiation of DNA Replication and Evidence for Active Nucleoid Partitioning

Fluorescence microscopic methods have been used to characterize the cell cycle of Bacillus subtilis at four different growth rates. The data obtained have been used to derive models for cell cycle progression. Like that of Escherichia coli, the period required by B. subtilis for chromosome replication at 37°C was found to be fairly constant (although a little longer, at about 55 min), as was the cell mass at initiation of DNA replication. The cell cycle of B. subtilis differed from that of E. coli in that changes in growth rate affected the average cell length but not the width and also in the relative variability of period between termination of DNA replication and septation. Overall movement of the nucleoid was found to occur smoothly, as in E. coli, but other aspects of nucleoid behavior were consistent with an underlying active partitioning machinery. The models for cell cycle progression in B. subtilis should facilitate the interpretation of data obtained from the recently introduced cytological methods for imaging the assembly and movement of proteins involved in cell cycle dynamics.     

Asymmetrical division of Saccharomyces cerevisiae.

The unequal division model proposed for budding yeast (L. H. Hartwell and M. W. Unger, J. Cell Biol. 75:422-435, 1977) was tested by bud scar analyses of steady-state exponential batch cultures of Saccharomyces cerevisiae growing at 30 degrees C at 19 different rates, which were obtained by altering the carbon source. The analyses involved counting the number of bud scars, determining the presence or absence of buds on at least 1,000 cells, and independently measuring the doubling times (gamma) by cell number increase. A number of assumptions in the model were tested and found to be in good agreement with the model. Maximum likelihood estimates of daughter cycle time (D), parent cycle time (P), and the budded phase (B) were obtained, and we concluded that asymmetrical division occurred at all growth rates tested (gamma, 75 to 250 min). D, P, and B are all linearly related to gamma, and D, P, and gamma converge to equality (symmetrical division) at gamma = 65 min. Expressions for the genealogical age distribution for asymmetrically dividing yeast cells were derived. The fraction of daughter cells in steady-state populations is e-alpha P, and the fraction of parent cells of age n (where n is the number of buds that a cell has produced) is (e-alpha P)n-1(1-e-alpha P)2, where alpha = IN2/gamma; thus, the distribution changes with growth rate. The frequency of cells with different numbers of bud scars (i.e., different genealogical ages) was determined for all growth rates, and the observed distribution changed with the growth rate in the manner predicted. In this haploid strain new buds formed adjacent to the previous buds in a regular pattern, but at slower growth rates the pattern was more irregular. The median volume of the cells and the volume at start in the cell cycle both increased at faster growth rates. The implications of these findings for the control of the cell cycle are discussed.     

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.     

Circadian periodicity in cockroaches altered by high frequency light-dark cycles

Summary The circadian period of the freerunning activity rhythm in the cockroach is systematically altered by high frequency light-dark cycles (HF-LD) according to the ratio of light to dark within each cycle. With a standard 10 min cycle time, brief (e.g., 0.5 min) exposure to light each cycle causes the free-running period to shorten significantly in comparison to its steady-state value in constant darkness. As the ratio of light to dark in HF-LD is increased, the period of the rhythm is progressively lengthened. These findings are discussed in terms of the general proposition that light, applied throughout the circadian cycle, predictably modifies periodicity according to the asymmetrical shape of the circadian phase response curve.Abbreviations LD light-dark cycles in which cycle length is in hours - HF-LD light-dark cycles in which cycle length is in min; period of the activity rhythm; change in period of the activity rhythm - PRC phase response curve - LL constant light     

Estimation of the D period from residual division after exposure of exponential phase bacteria to chloramphenicol

Summary Values of the D period, between termination of chromosome replication and cell division, were determined from measurements of residual cell division after exposure of exponential phase cultures of Escherichia coli B/r and K12 and of Salmonella typhimurium to chloramphenicol. The results obtained by this method were compared with earlier results for E. coli B/r obtained from measurements of DNA content per cell and were found to be almost identical. For each, values of the D period were independent of growth rate, and the average value of D=26.1±1.2 min obtained by residual division is in good agreement with the value of 25 min obtained earlier. These results indicate that the method of residual division provides a good measure of the duration of the D period. Values of D were also independent of growth rate for each of the other strains.This work supported by the U.S. Atomic Energy Commission.     

Effects of light on circadian pacemaker development

1. The effects of raising cockroaches, Leucophaea maderae, in non-24 h light cycles on circadian rhythms in adults were examined. The average period (tau) of freerunning rhythms of locomotor activity of animals exposed to LD 11:11 (T22) during post-embryonic development was significantly shorter (tau = 22.8 +/- 0.47 SD, n = 85) than that of animals raised in LD 12:12 (T24) (tau = 23.7 +/- 0.20 h, n = 142), while animals raised in LD 13:13 (T26) had significantly longer periods (tau = 24.3 +/- 0.21 h, n = 65). Animals raised in constant darkness (DD) had a significantly shorter period (tau = 23.5 +/- 0.21 h, n = 13) than siblings raised in constant light (LL) (tau = 24.0 +/- 0.15 h, n = 10). 2. The differences in tau between animals raised in T22 and T24 were found to be stable in DD for at least 7 months and could not be reversed by exposing animals to LD 12:12 or LD 6:18. 3. Animals raised in either T24 or DD and then exposed as adults to T22 exhibited average freerunning periods that were not different from animals not exposed to T22. 4. Measurement of freerunning periods at different temperatures of animals raised in T22, T24, or T26 showed that the temperature compensation of tau was not affected by the developmental light cycle. These results indicate that the lighting conditions during post-embryonic development can permanently alter the freerunning period of the circadian system in the cockroach, but do not affect its temperature compensation.     

Flagellar Development During the Cell Cycle in Chlamydomonas reinhardtii

Flagellar development during the asexual synchronous cell cycle of Chlamydomonas reinhardtii (11.32 aM) was studied by light microscopy. Cell walls of sporangia of different developmental status were dissolved using gamete lysin (g-lysin) enabling direct observation of flagellar development. Flagellar growth in progeny cells exhibits a linear kinetic with a growth rate of 28 nm/min at 30°C leading to a flagellar length of 7–7.5 μm in 4–4.5 h. After this time the flagellar growth rate drops to 2.8 nm/min (as in interphase). Both flagella of a single cell and all flagella within a sporangium grow out at the same time and with the same rate. Cycloheximide (10 μg/ml) completely blocks flagellar development. If cycloheximide is removed flagellar growth resumes at the normal rate with no lag-phase. Flagellar development during the cell cycle in C. reinhardtii differs considerably from the well-studied model system of flagellar regeneration following amputation in the same species.     

Initiation and Velocity of Chromosome Replication in Escherichia coli B/r and K-12

The macromolecular composition and a number of parameters affecting chromosome replication were examined over a range of exponential growth rates in two common Escherichia coli strains, B/r and K-12 AB1157. Based on improved measurements of DNA after treatment of exponential cultures with rifampin, the cell mass per chromosomal replication origin (initiation mass) and the time required to replicate the chromosome from origin to terminus (C period) were determined. For these two strains, the initiation mass approached values of 8 × 10⁻¹⁰ and 10 × 10⁻¹⁰ units of optical density (at 460 nm) of culture mass per oriC, respectively, at growth rates above 1 doubling/h (at 37°C). The amount of protein per oriC decreased with increasing growth rate for AB1157 and remained nearly constant for the B/r strain. The C period decreased for both strains in an essentially identical manner from about 70 min at 0.6 doublings/h to about 33 min at 3 doublings/h. From the initiation mass and C period, relative or absolute copy numbers for genes with known map locations can be accurately determined at different growth rates. At growth rates above 2 doublings/h, when chromosomes are highly branched, genes near the origin are about threefold more prevalent than genes near the terminus. At a growth rate of 0.6 doubling/h, this ratio is only about 1.7, which reflects the lower degree of chromosome branching.     

Bacteriophage T4 development in Escherichia coli is growth rate dependent

Three independent parameters (eclipse and latent periods, and rate of ripening during the rise period) are essential and sufficient to describe bacteriophage development in its bacterial host. A general model to describe the classical "one-step growth" experiment [Rabinovitch et al. (1999a) J. Bacteriol.181, 1687-1683] allowed their calculations from experimental results obtained with T4 in Escherichia coli B/r under different growth conditions [Hadas et al. (1997) Microbiology143, 179-185]. It is found that all three parameters could be described by their dependence solely on the culture doubling time tau before infection. Their functional dependence on tau, derived by a best-fit analysis, was used to calculate burst size values. The latter agree well with the experimental results. The dependence of the derived parameters on growth conditions can be used to predict phage development under other experimental manipulations.