Two pathways of division inhibition in UV-irradiated E. coli

Cationic oligo-p-phenylene ethynylenes form complexes with surfactants for long-term light-activated biocidal applications

Cationic oligo-p-phenylene ethynylenes are highly effective light-activated biocides that deal broad-spectrum damage to a variety of pathogens, including bacteria. A potential problem arising in the long-term usage of these compounds is photochemical breakdown, which nullifies their biocidal activity. Recent work has shown that these molecules complex with oppositely-charged surfactants, and that the resulting complexes are protected from photodegradation. In this manuscript, we determine the biocidal activity of an oligomer and a complex formed between it and sodium dodecyl sulfate. The complexes are able to withstand prolonged periods of irradiation, continuing to effectively kill both Gram-negative and Gram-positive bacteria, while the oligomer by itself loses its biocidal effectiveness quickly in the presence of light. In addition, damage and stress responses induced by these biocides in both E. coli and S. aureus are discussed. This work shows that complexation with surfactants is a viable method for long-term light-activated biocidal applications.

A replication-inhibited unsegregated nucleoid at mid-cell blocks Z-ring formation and cell division independently of SOS and the SlmA nucleoid occlusion protein in Escherichia coli

Chromosome replication and cell division of Escherichia coli are coordinated with growth such that wild-type cells divide once and only once after each replication cycle. To investigate the nature of this coordination, the effects of inhibiting replication on Z-ring formation and cell division were tested in both synchronized and exponentially growing cells with only one replicating chromosome. When replication elongation was blocked by hydroxyurea or nalidixic acid, arrested cells contained one partially replicated, compact nucleoid located mid-cell. Cell division was strongly inhibited at or before the level of Z-ring formation. DNA cross-linking by mitomycin C delayed segregation, and the accumulation of about two chromosome equivalents at mid-cell also blocked Z-ring formation and cell division. Z-ring inhibition occurred independently of SOS, SlmA-mediated nucleoid occlusion, and MinCDE proteins and did not result from a decreased FtsZ protein concentration. We propose that the presence of a compact, incompletely replicated nucleoid or unsegregated chromosome masses at the normal mid-cell division site inhibits Z-ring formation and that the SOS system, SlmA, and MinC are not required for this inhibition.

Contribution of cell elongation to the biofilm formation of Pseudomonas aeruginosa during anaerobic respiration

Pseudomonas aeruginosa, a gram-negative bacterium of clinical importance, forms more robust biofilm during anaerobic respiration, a mode of growth presumed to occur in abnormally thickened mucus layer lining the cystic fibrosis (CF) patient airway. However, molecular basis behind this anaerobiosis-triggered robust biofilm formation is not clearly defined yet. Here, we identified a morphological change naturally accompanied by anaerobic respiration in P. aeruginosa and investigated its effect on the biofilm formation in vitro. A standard laboratory strain, PAO1 was highly elongated during anaerobic respiration compared with bacteria grown aerobically. Microscopic analysis demonstrated that cell elongation likely occurred as a consequence of defective cell division. Cell elongation was dependent on the presence of nitrite reductase (NIR) that reduces nitrite (NO₂⁻) to nitric oxide (NO) and was repressed in PAO1 in the presence of carboxy-PTIO, a NO antagonist, demonstrating that cell elongation involves a process to respond to NO, a spontaneous byproduct of the anaerobic respiration. Importantly, the non-elongated NIR-deficient mutant failed to form biofilm, while a mutant of nitrate reductase (NAR) and wild type PAO1, both of which were highly elongated, formed robust biofilm. Taken together, our data reveal a role of previously undescribed cell biological event in P. aeruginosa biofilm formation and suggest NIR as a key player involved in such process.

Transcription of essential cell division genes is linked to chromosome replication in Escherichia coli

Cell division normally follows the completion of each round of chromosome replication in Escherichia coli. Transcription of the essential cell division genes clustered at the mra region is shown here to depend on continuing chromosomal DNA replication. After chromosome replication was blocked by either nalidixic acid treatment or thymine starvation, the transcription of these cell division genes was repressed significantly. This suggests a way in which cell division is controlled by chromosome replication.

Principles of macromolecular organization and cell function in bacteria and archaea

Structural organization of the cytoplasm by compartmentation is a well established fact for the eukaryotic cell. In prokaryotes, compartmentation is less obvious. Most prokaryotes do not need intracytoplasmic membranes to maintain their vital functions. This review, especially dealing with prokaryotes, will point out that compartmentation in prokaryotes is present, but not only achieved by membranes. Besides membranes, the nucleoid, multienzyme complexes and metabolons, storage granules, and cytoskeletal elements are involved in compartmentation. In this respect, the organization of the cytoplasm of prokaryotes is similar to that in the eukaryotic cell. Compartmentation influences properties of water in cells.

Chromosome segregation and cell division defects in recBC sbcBC ruvC mutants of Escherichia coli

The RuvC protein is important for DNA recombination and repair in Escherichia coli. The present work shows that a ruvC null mutation introduced into a recBC sbcBC background causes severe defects in chromosome segregation and cell division. Both defects were found to result from abortive recombination initiated by the RecA protein.

Inhibition of FtsZ polymerization by SulA, an inhibitor of septation in Escherichia coli

The bacterial cell division protein FtsZ assembles into the cytokinetic Z ring that directs cytokinesis in prokaryotes. In Escherichia coli the formation of the Z ring is prevented by induction of the cell division inhibitor SulA (SfiA), a component of the SOS response. Here we show that a MalE-SulA fusion that retains this inhibitory function in vivo inhibits the GTPase activity and polymerization of FtsZ in vitro. MalE-SulA10, which does not block Z ring formation in vivo, is unable to inhibit the GTPase activity and polymerization in vitro. Furthermore, FtsZ114, which is refractory to SulA in vivo, is not inhibited by MalE-SulA. These results indicate that SulA blocks Z ring formation by blocking FtsZ polymerization.

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 degrees 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.

Bacterial cell division and the Z ring

Bacterial cell division occurs through the formation of an FtsZ ring (Z ring) at the site of division. The ring is composed of the tubulin-like FtsZ protein that has GTPase activity and the ability to polymerize in vitro. The Z ring is thought to function in vivo as a cytoskeletal element that is analogous to the contractile ring in many eukaryotic cells. Evidence suggests that the Z ring is utilized by all prokaryotic organisms for division and may also be used by some eukaryotic organelles. This review summarizes our present knowledge about the formation, function, and evolution of the Z ring in prokaryotic cell division.

Are there DNA damage checkpoints in E. coli?

The concept of regulatory 'checkpoints' in the eukaryotic cycle has proved to be a fruitful one. Here, its applicability to the bacterial cell cycle is examined. A primitive DNA damage checkpoint operates in E. coli such that, after exposure to ultraviolet light, while excision repair occurs, chromosome replication continues very slowly with the production of discontinuous daughter strands. The slower the rate of excision of photoproducts, the greater the delay before the normal rate of DNA replication is restored, the additional time for repair ensuring that normal survival is maintained. A model is proposed in which replication rate is controlled by the ratio of RecA-coated to uncoated single stranded regions of DNA in the replication fork. There are also two cell division inhibitors SulA (= SfiA) and SfiC under the control of the SOS system and sensitive to DNA damage, but they are irrelevant to the survival of wild-type bacteria under normal conditions. In strains where SulA and SfiC do not operate, inhibition is not influenced by the rate of excision repair and so fails one of the criteria for a DNA damage checkpoint, namely the monitoring of the DNA for the level of residual damage.

holE, the gene coding for the theta subunit of DNA polymerase III of Escherichia coli: characterization of a holE mutant and comparison with a dnaQ (epsilon-subunit) mutant

DNA polymerase III holoenzyme is a multiprotein complex responsible for the bulk of chromosomal replication in Escherichia coli and Salmonella typhimurium. The catalytic core of the holoenzyme is an alpha epsilon theta heterotrimer that incorporates both a polymerase subunit (alpha; dnaE) and a proofreading subunit (epsilon; dnaQ). The role of theta is unknown. Here, we describe a null mutation of holE, the gene for theta. A strain carrying this mutation was fully viable and displayed no mutant phenotype. In contrast, a dnaQ null mutant exhibited poor growth, chronic SOS induction, and an elevated spontaneous mutation rate, like dnaQ null mutants of S. typhimurium described previously. The poor growth was suppressible by a mutation affecting alpha which was identical to a suppressor mutation identified in S. typhimurium. A double mutant null for both holE and dnaQ was indistinguishable from the dnaQ single mutant. These results show that the theta subunit is dispensable in both dnaQ+ and mutant dnaQ backgrounds, and that the phenotype of epsilon mutants cannot be explained on the basis of interference with theta function.

FtsZ ring in bacterial cytokinesis

FtsZ is localized to a cytokinetic ring at the cell division site in bacteria. In this review a model is discussed that suggests that FtsZ self assembles into a ring at a nucleation site formed on the cytoplasmic membrane under cell-cycle control. This model suggests that formation of the cytokinetic FtsZ ring initiates and coordinates the circumferential invagination of the cytoplasmic membrane and cell wall, leading to formation of the septum. It is also suggested that this process may be conserved among the peptidoglycan-containing eubacteria. In addition, similarities between FtsZ and tubulin are discussed.

Inhibition of cell division in hupA hupB mutant bacteria lacking HU protein

Escherichia coli hupA hypB double mutants that lack HU protein have severe cellular defects in cell division, DNA folding, and DNA partitioning. Here we show that the sfiA11 mutation, which alters the SfiA cell division inhibitor, reduces filamentation and production of anucleate cells in AB1157 hupA hupB strains. However, lexA3(Ind-) and sfiB(ftsZ)114 mutations, which normally counteract the effect of the SfiA inhibitor, could not restore a normal morphology to hupA hupB mutant bacteria. The LexA repressor, which controls the expression of the sfiA gene, was present in hupA hupB mutant bacteria in concentrations half of those of the parent bacteria, but this decrease was independent of the specific cleavage of the LexA repressor by activated RecA protein. One possibility to account for the filamentous morphology of hupA hupB mutant bacteria is that the lack of HU protein alters the expression of specific genes, such as lexA and fts cell division genes.

Escherichia coli parA is an allele of the gyrB gene

A thermosensitive (ts) parA mutant, MFT110, of Escherichia coli carried at least two ts mutations. The major ts defect, resulting from a mutation mapped originally at 95 min and complemented by pLC8-47, was most probably due to psd. A plasmid carrying the 1.6 kb BamHI-PvuII fragment recloned from pLC8-47 complemented the major ts mutation in MFT110 and psd(ts) in two mutants, but did not correct the Par phenotype of MFT110. The second ts mutation was salt-repairable and mapped at 83 min close to recF and tnaA. This mutation was linked with the Par phenotype as shown unambiguously by 4',6-diamidino-2-phenylindole stained nucleoids in parA mutant cells with the W3110 genetic background. Both salt-repairable ts and Par traits were corrected concomitantly by a plasmid carrying the chromosomal region solely for the gyrB gene. This strongly suggests that parA is an allele of gyrB.

Effects of overproduction of single-stranded DNA-binding protein on RecA protein-dependent processes in Escherichia coli

Overproduction of single-stranded DNA-binding protein (SSB) in Escherichia coli led to a decrease in the basal level of repressor LexA. Expression of the LexA-controlled genes was increased differentially, depending on the affinity of the LexA repressor for each promoter: expression of the recA and sfiA genes was increased 5-fold and 1.5-fold, respectively. Despite only a slight effect on expression of sfiA, which codes for an inhibitor of cell division, bacteria overproducing SSB produced elongated cells. In fact, the effect on cell shape appeared to be essentially independent of the expression of the sfiA and recA genes. Bacteria overproducing SSB were therefore phenotypically similar to bacteria partially starved of thymine, in which filamentation results from both sfiA-dependent and sfiA-recA-independent pathways. These data indicate that excess SSB acts primarily by perturbing DNA replication, thereby favoring gratuitous activation of RecA protein to promote cleavage of LexA protein. When bacteria overproducing SSB were exposed to a DNA-damaging agent such as ultraviolet light or mitomycin C, the recA and sfiA genes were fully induced. Induction of the sfiA gene occurred, however, at higher doses in bacteria overproducing SSB protein than in bacteria with normal levels of SSB. Whereas the efficiency of excision repair was apparently increased by excess SSB, the efficiency of post-replication recombinational repair was reduced as judged by a decrease in the recombination proficiency between a prophage and ultraviolet-irradiated heteroimmune infecting phage. Following induction of ssb+ bacteria with mitomycin C, the cellular content of SSB was slightly increased. These results provide evidence that SSB modulates RecA protein-dependent activities in vivo. It is proposed that SSB favors the formation of short complexes of RecA protein and single-stranded DNA that mediate cleavage of the LexA and lambda repressors, while it delays the formation of long nucleoprotein filaments, thereby slowing down RecA-promoted recombinational events in uninduced as well as in induced bacteria.

Structural inhibition and reactivation of Escherichia coli septation by elements of the SOS and TER pathways

The inhibition of cell division caused by induction of the SOS pathway in Escherichia coli structurally blocks septation, as deduced from two sets of results. Potential septation sites active at the time of SOS induction became inactivated, while those initiated during the following doubling time were active. Penicillin resistance increased in wild-type UV light-irradiated cells, a behavior similar to that observed in mutants in which structural blocks were introduced by inactivation of FtsA. Potential septation sites that have been structurally blocked by either the SOS division inhibitor, furazlocillin inhibition of PBP3, or inactivation of a TER pathway component, FtsA3, could be reactivated one doubling time after removal of the inhibitory agent in the presence of an active lon gene product. Reactivation of potential septation sites blocked by the presence of an inactivated FtsA3 was significantly lower when the lon protease was not active, suggesting that Lon plays a role in the removal of inactivated TER pathway products from the blocked potential septation sites.

Genetic and morphological characterization of ftsB and nrdB mutants of Escherichia coli

The ftsB gene of Escherichia coli is believed to be involved in cell division. In this report, we show that plasmids containing the nrdB gene could complement the ftsB mutation, suggesting that ftsB is an allele of nrdB. We compared changes in the cell shape of isogenic nrdA, nrdB, ftsB, and pbpB strains at permissive and restrictive temperatures. Although in rich medium all strains produced filaments at the restrictive temperature, in minimal medium only a 50 to 100% increase in mean cell mass occurred in the nrdA, nrdB, and ftsB strains. The typical pbpB cell division mutant also formed long filaments at low growth rates. Visualization of nucleoid structure by fluorescence microscopy demonstrated that nucleoid segregation was affected by nrdA, nrdB, and ftsB mutations at the restrictive temperature. Measurements of beta-galactosidase activity in lambda p(sfiA::lac) lysogenic nrdA, nrdB, and ftsB mutants in rich medium at the restrictive temperature showed that filamentation in the nrdA mutant was caused by sfiA (sulA) induction, while filamentation in nrdB and ftsB mutants was sfiA independent, suggesting an SOS-independent inhibition of cell division.

Overproduction of FtsZ suppresses sensitivity of lon mutants to division inhibition

Escherichia coli lon mutants are sensitive to UV light and other DNA-damaging agents. This sensitivity is due to the loss of the lon-encoded ATP-dependent proteolytic activity which results in increased stability of the cell division inhibitor SulA. Introduction of the multicopy plasmid pZAQ containing the ftsZ gene, which is known to increase the level of FtsZ, suppressed the sensitivity of lon mutants to the DNA-damaging agents UV and nitrofurantoin. Alterations of pZAQ which reduced the expression of ftsZ reduced the ability of this plasmid to suppress the UV sensitivity. Examination of the kinetics of cell division revealed that pZAQ did not suppress the transient filamentation seen after exposure to UV, but did suppress the long-term inhibition that is normally observed. lon strains carrying pZAQ could stably maintain a multicopy plasmid carrying sulA (pBS2), which cannot otherwise be introduced into lon mutants. In addition, the increased temperature sensitivity of lexA(Ts) strains containing pBS2 was suppressed by pZAQ. These results suggest that SulA inhibits cell division by inhibiting FtsZ and that this interaction is stoichiometric.

Coupling between DNA replication and cell division mediated by the FtsA protein in Escherichia coli: a pathway independent of the SOS response, the "TER" pathway

Inhibition of DNA synthesis prevented the recovery of cell division in filaments of D-3R [ftsA3(Ts) recA56] returned to the permissive temperature. The FtsA protein may be a signal involved in the "TER" pathway, a series of events that coordinate cell division with DNA replication, that is independent of the SOS pathway.

Cell-division control in Escherichia coli: specific induction of the SOS function SfiA protein is sufficient to block septation

Blocks in DNA replication cause a rapid arrest of cell division in Escherichia coli. We have previously established that the function SfiA (SulA), induced under these conditions as part of the SOS response, is involved in this inhibition of division. To separate the effects of SfiA from those of other SOS functions, we have constructed a plac-sfiA operon fusion, permitting specific induction of SfiA protein by addition of the lac operon inducer isopropyl beta-D-thiogalactopyranoside (IPTG). In lon mutants, in which the unstable SfiA protein has a longer half-life, IPTG caused a rapid arrest of cell division. Under these conditions, there is no concomitant induction of the SOS response. IPTG also caused a rapid arrest of cell division in lon+ strains. These results demonstrate that induction of the SfiA protein is sufficient to cause inhibition of division. Mutations in the sfiB gene can suppress IPTG-induced SfiA-mediated inhibition of division. At higher SfiA concentrations, however, even sfiB mutants cease division; an additional mutation genetically inseparable from sfiB restores normal division. These observations reinforce the hypothesis that the SfiB protein, probably required for cell septation, is the target of action of the SfiA division inhibitor.

Novel mechanism of cell division inhibition associated with the SOS response in Escherichia coli

Certain Escherichia coli strains were shown to possess a novel system of cell division inhibition, called the SfiC+ phenotype. SfiC+ filamentation had a number of properties similar to those of sfiA-dependent division inhibition previously described: (i) both are associated with the SOS response induced by expression of the recA(Tif) mutation, (ii) both are associated with cell death, (iii) both are amplified in mutants lacking the Lon protease, and (iv) both are suppressed by sfiB mutations. SfiC+ filamentation and sfiA-dependent division inhibition differed in (i) the physiological conditions under which loss of viability is observed, (ii) the extent of amplification in lon mutants, (iii) their genetic regulation (SfiC+ filamentation is not under direct negative control of the LexA repressor), and (iv) their genetic determinants (SfiC+ filamentation depends on a locus, sfiC+, near 28 min on the E. coli map and distinct from sfiA).