Biphasic Cell-Size and Growth-Rate Homeostasis by Single Bacillus subtilis Cells

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The structure of FtsZ filaments in vivo suggests a force-generating role in cell division

In prokaryotes, FtsZ (the filamentous temperature sensitive protein Z) is a nearly ubiquitous GTPase that localizes in a ring at the leading edge of constricting plasma membranes during cell division. Here we report electron cryotomographic reconstructions of dividing Caulobacter crescentus cells wherein individual arc-like filaments were resolved just underneath the inner membrane at constriction sites. The filaments' position, orientation, time of appearance, and resistance to A22 all suggested that they were FtsZ. Predictable changes in the number, length, and distribution of filaments in cells where the expression levels and stability of FtsZ were altered supported that conclusion. In contrast to the thick, closed-ring-like structure suggested by fluorescence light microscopy, throughout the constriction process the Z-ring was seen here to consist of just a few short (approximately 100 nm) filaments spaced erratically near the division site. Additional densities connecting filaments to the cell wall, occasional straight segments, and abrupt kinks were also seen. An 'iterative pinching' model is proposed wherein FtsZ itself generates the force that constricts the membrane in a GTP-hydrolysis-driven cycle of polymerization, membrane attachment, conformational change, depolymerization, and nucleotide exchange.     

Modulation of prey size reveals adaptability and robustness in the cell cycle of an intracellular predator

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Direct interaction of FtsZ and MreB is required for septum synthesis and cell division in Escherichia coli

How bacteria coordinate cell growth with division is not well understood. Bacterial cell elongation is controlled by actin–MreB while cell division is governed by tubulin–FtsZ. A ring‐like structure containing FtsZ (the Z ring) at mid‐cell attracts other cell division proteins to form the divisome, an essential protein assembly required for septum synthesis and cell separation. The Z ring exists at mid‐cell during a major part of the cell cycle without contracting. Here, we show that MreB and FtsZ of Escherichia coli interact directly and that this interaction is required for Z ring contraction. We further show that the MreB–FtsZ interaction is required for transfer of cell‐wall biosynthetic enzymes from the lateral to the mature divisome, allowing cells to synthesise the septum. Our observations show that bacterial cell division is coupled to cell elongation via a direct and essential interaction between FtsZ and MreB.     

Cell cycle regulation and the timing of chromosome replication in a marine synechococcus (cyanobacteria) during light- and nitrogen-limited growth

Cell cycle behavior in the marine Synechococcus strain WH8101 was examined in detail over a wide range of light- and nitrogen-limited growth rates. The presence of bimodal DNA frequency distributions under all conditions confirms that the overlapping rounds of DNA replication that characterize E. coli and other fast-growing prokaryotes are not present in this organism. Although chromosome replication time, C , was constrained to a fairly narrow range of values overall, it nevertheless did vary with growth rate and limiting factor. Light-limited cells growing at moderate rates had higher C values than did N-limited cells growing at comparable rates (by as much as a factor of 2). As these cells became light saturated, however, C decreased sharply to the level observed under N limitation. The post-replication period, D , decreased monotonically with growth rate under both light and N limitation, approaching a constant value at moderate to high growth rates. Average cell volume at the time of initiation of DNA replication was calculated from the values of C and D , combined with directly measured mean cell volume, and was found to be constant at all growth rates above ∼0.7 d⁻¹. This pattern was confirmed by estimates of initiation volume based on flow cytometric light scatter measurements, and suggests that as has been found in other prokaryotic systems, cell mass may play an important role in regulating the timing of chromosome replication in cyanobacteria. Furthermore, because the magnitude of C + D influences average cell mass (given a constant mass at initiation), changes in these parameters (particularly C ) may be responsible for the previously reported nonlinear relationship between light-limited growth rate and both RNA cell⁻¹ and average cell volume.     

A 4-aminofurazan derivative-A189-inhibits assembly of bacterial cell division protein FtsZ in vitro and in vivo

Out of 95,000 commercially available chemical compounds screened by the anucleate cell blue assay, 138 selected hit compounds were further screened. As a result, A189, a 4-aminofurazan derivative was found to inhibit FtsZ GTPase with an IC(50) of 80 mug/ml and to exhibit antibacterial activity against Staphylococcus aureus and Escherichia coli. Light scattering demonstrated that A189 inhibited FtsZ assembly in vitro, and microscopic observation of A189-treated E. coli indicated that A189 perturbed FtsZ ring formation and made bacterial cells filamentous. However, nucleoids staining with DAPI revealed that A189 did not affect DNA replication and chromosome segregation in bacterial filamentous cells. Furthermore, A189 made sulA-deleted E. coli cells filamentous. Taken together, these findings suggest that A189 inhibits FtsZ GTPase activity, resulting in perturbation of FtsZ ring formation, which leads to bacterial cell death.     

Division competence in Tetrahymena: determination of minimum cell volume and rate of nutrient uptake

Cell volume and doubling time have been determined for exponentially growing Tetrahymena pyriformis cells in broth medium with and without glucose and in media made from these media by dilution with water. The cells tolerate media with dry weights from 105 down to 0.06 g/L. In the diluted media the cells have small volumes and the doubling time is increased. When the cell volume increase per time per cell in a given medium is expressed as a function of the cell volume in this same medium, a direct proportionality is found. From this equation the minimum cell volume of division competence (MVDC) can be found. It is 2,100 microns 3 for T. pyriformis at 28 degrees C. The lag period resulting from an upshift of exponentially growing cells from diluted media to more concentrated media is a function of the initial and resulting cell volumes and MVDC. The increase in cell volume per unit of time for a given cell depends on the dry weight of the medium. This parameter can be transformed to mass increase per cell surface area per time, which represents rate of nutrient uptake. When plotted against the dry weight of the media, a Michaelis-Menten-like curve is obtained with two Km values of 3.8 and 0.08 g/L with corresponding Vmax values of 20 and 4 ng/cm2.s. The low Km value (0.08 g/L) indicates that Tetrahymena is able to take up nutrients from highly diluted media. The high value of Vmax (20 ng/cm2.s) increases the ability of growth in more concentrated media.(ABSTRACT TRUNCATED AT 250 WORDS)     

细菌毒素-抗毒素系统的研究进展

毒素-抗毒素系统(toxin-antitoxin system,TA)由两个共表达的基因组成,其中一个基因编码不稳定的抗毒素蛋白(antitoxin),另一个基因编码稳定的毒素蛋白(toxin).毒素-抗毒素系统最早发现于一些低拷贝的质粒,用来维持低拷贝质粒在菌群中的稳定存在.随后的研究表明,毒素-抗毒素系统广泛存在于细菌,包括一些致病菌的染色体上.在营养缺乏等不良生长条件下,由于基因表达的抑制和蛋白酶的降解作用,不稳定的抗毒素蛋白减少,从而产生游离的毒素蛋白,导致细菌的生长抑制和死亡.毒素-抗毒素系统的生理功能目前还存在争议,有学者认为细茼染色体上的毒素-抗毒素系统可以在不良生长状况下介导细菌的死亡,即细茼程序性细胞死亡(baeterial programmedcell death).但也有证据显示,毒素-抗毒素系统的功能更偏向于应激状态下的生理调节方面,即只起应激状态下的抑菌作用而不是杀菌作用.对细菌生长调控中毒素-抗毒素系统的作用机理进行综述,并探讨毒素-抗毒素系统研究的理论和应用价值.     

Growth patterns and alkaloid accumulation in hairy root and untransformed root cultures of Datura stramonium

The aim of this work was to study the possible relationship between alkaloid production and growth measured as: biomass increase and cellular division frequency, in Datura stramonium in vitro root cultures (hairy root and normal cultures). A comparison of growth values on a fresh and dry weight basis showed that there were differences between transformed and non-transformed lines. The differential growth between lines occurred due to a real biomass increase and not because of water accumulation. On the other hand, the rate of cell division showed a similar pattern for all lines studied. Therefore, the differences in growth are not due to different cell division rates, nor to the presence of larger meristems, but to the development and growth of lateral roots and the presence of active intercalary meristematic zones in each line. The maximum alkaloid production occurred when the cultures were not growing. This suggests an inverse relationship. Finally, the data support a specific model of growth at the level of cell division in root cultures which has not been described before in the literature. This revised version was published online in June 2006 with corrections to the Cover Date.     

The Arg Fingers of Key DnaA Protomers Are Oriented Inward within the Replication Origin oriC and Stimulate DnaA Subcomplexes in the Initiation Complex

ATP-DnaA binds to multiple DnaA boxes in the Escherichia coli replication origin (oriC) and forms left-half and right-half subcomplexes that promote DNA unwinding and DnaB helicase loading. DnaA forms homo-oligomers in a head-to-tail manner via interactions between the bound ATP and Arg-285 of the adjacent protomer. DnaA boxes R1 and R4 reside at the outer edges of the DnaA-binding region and have opposite orientations. In this study, roles for the protomers bound at R1 and R4 were elucidated using chimeric DnaA molecules that had alternative DNA binding sequence specificity and chimeric oriC molecules bearing the alternative DnaA binding sequence at R1 or R4. In vitro, protomers at R1 and R4 promoted initiation regardless of whether the bound nucleotide was ADP or ATP. Arg-285 was shown to play an important role in the formation of subcomplexes that were active in oriC unwinding and DnaB loading. The results of in vivo analysis using the chimeric molecules were consistent with the in vitro data. Taken together, the data suggest a model in which DnaA subcomplexes form in symmetrically opposed orientations and in which the Arg-285 fingers face inward to mediate interactions with adjacent protomers. This mode is consistent with initiation regulation by ATP-DnaA and bidirectional loading of DnaB helicases.     

A matter of size: developmental control of organ size in plants

The intrinsic size of plant organs is determined by developmental signals, yet the molecular and genetic mechanisms that control organ size are largely unknown. Ongoing functional analysis of Arabidopsis genes is defining important regulators involved in these mechanisms. Key features of this control are the coordinated activation of growth and cell division by growth regulators and the maintenance of meristematic competence by the ANT gene, which acts as an organ-size checkpoint. Alterations of genome size by polyploidization and endoreduplication can reset this checkpoint by ploidy-dependent, epigenetically regulated differential gene expression. In addition, the regulation of polarized growth and phytohormone signaling also affect final organ size. These findings reveal unique aspects of plant organ-size control that are distinct from animal organ-size control.     

A Single Light-Responsive Sizer Can Control Multiple-Fission Cycles in Chlamydomonas

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The cell wall and cell division gene cluster in the Mra operon of Pseudomonas aeruginosa: cloning, production, and purification of active enzymes

We have cloned the Pseudomonas aeruginosa cell wall biosynthesis and cell division gene cluster that corresponds to the mra operon in the 2-min region of the Escherichia coli chromosome. The organization of the two chromosomal regions in P. aeruginosa and E. coli is remarkably similar with the following gene order: pbp3/pbpB, murE, murF, mraY, murD, ftsW, murG, murC, ddlB, ftsQ, ftsA, ftsZ, and envA/LpxC. All of the above P. aeruginosa genes are transcribed from the same strand of DNA with very small, if any, intragenic regions, indicating that these genes may constitute a single operon. All five amino acid ligases, MurC, MurD, MurE, MurF, and DdlB, in addition to MurG and MraY were cloned in expression vectors. The four recombinant P. aeruginosa Mur ligases, MurC, MurD, MurE, and MurF were overproduced in E. coli and purified as active enzymes.     

Pom1 and cell size homeostasis in fission yeast

Cells sense their size and use this information to coordinate cell division with cell growth to maintain a constant cell size within a given population. A model has been proposed for cell size control in the rod-shaped cells of the fission yeast, Schizosaccharomyces pombe. This involves a protein localized to the cell ends, which inhibits mitotic activators in the middle of the cell in a cell size-dependent manner. This protein, Pom1, along with another tip-localized protein, Nif1, have been implicated as direct sensors of cell size controlling the onset of mitosis. Here we have investigated cell size variability and size homeostasis at the G₂/M transition, focusing on the role of pom1 and nif1. Cells deleted for either of these 2 genes show wild-type size homeostasis both in size variability analyses and size homeostasis experiments. This indicates that these genes do not have a critical role as direct cell size sensors in the control mechanism. Cell size homeostasis also seems to be independent of Cdc2–Tyr15 phosphorylation, suggesting that the size sensing mechanism in fission yeast may act through an unidentified pathway regulating CDK activity by an unknown mechanism.     

Expression of the plant cyclin-dependent kinase inhibitor ICK1 affects cell division, plant growth and morphology

The plant CDK inhibitor ICK1 was identified previously from Arabidopis thaliana with its inhibitory activity characterized in vitro. ICK1 displayed several structural and functional features that are distinct from known animal CDK inhibitors. Despite the initial characterization, there is no information on the functions of any plant CDK inhibitor in plants. To gain insight into ICK1 functions in vivo and the role of cell division during plant growth and development, transgenic plants were generated expressing ICK1 driven by the cauliflower mosaic virus 35S promoter. In comparison to control plants, growth was significantly inhibited in transgenic 35S-ICK1 plants, with some plants weighing <10% of wild-type plants at the 3 week stage. Most organs of 35S-ICK1 plants were smaller. There were also modifications in plant morphology such as shape and serration of leaves and petals. The changes were so drastic that 35S-ICK1 plants with strong phenotype no longer resembled wild-type plants morphologically. Analyses showed that increased ICK1 expression resulted in reduced CDK activity and reduced the number of cells in these plants. Cells in 35S-ICK1 plants were larger than corresponding cells in control plants. These results demonstrate that ICK1 acts as a CDK inhibitor in the plant, and the inhibition of cell division by ICK1 expression has profound effects on plant growth and development. They also suggest that alterations of plant organ shape can be achieved by restriction of cell division.