Overproduction of FtsZ suppresses sensitivity of lon mutants to division inhibition

Regulation of Cell Division in Bacteria by Monitoring Genome Integrity and DNA Replication Status

All organisms regulate cell cycle progression by coordinating cell division with DNA replication status. In eukaryotes, DNA damage or problems with replication fork progression induce the DNA damage response (DDR), causing cyclin-dependent kinases to remain active, preventing further cell cycle progression until replication and repair are complete. In bacteria, cell division is coordinated with chromosome segregation, preventing cell division ring formation over the nucleoid in a process termed nucleoid occlusion. In addition to nucleoid occlusion, bacteria induce the SOS response after replication forks encounter DNA damage or impediments that slow or block their progression. During SOS induction, Escherichia coli expresses a cytoplasmic protein, SulA, that inhibits cell division by directly binding FtsZ. After the SOS response is turned off, SulA is degraded by Lon protease, allowing for cell division to resume. Recently, it has become clear that SulA is restricted to bacteria closely related to E. coli and that most bacteria enforce the DNA damage checkpoint by expressing a small integral membrane protein. Resumption of cell division is then mediated by membrane-bound proteases that cleave the cell division inhibitor. Further, many bacterial cells have mechanisms to inhibit cell division that are regulated independently from the canonical LexA-mediated SOS response. In this review, we discuss several pathways used by bacteria to prevent cell division from occurring when genome instability is detected or before the chromosome has been fully replicated and segregated.

Protease regulation and capacity during Caulobacter growth

Cell growth requires the removal of proteins that are unwanted or toxic. In bacteria, AAA+ proteases like the Clp family and Lon selectively destroy proteins defined by intrinsic specificity or adaptors. Caulobacter crescentus is a gram-negative bacterium that undergoes an obligate developmental transition every cell division cycle. Here we highlight recent work that reveals how a hierarchy of adaptors targets the degradation of key proteins at specific times during this cell cycle, integrating protein destruction with other cues. We describe recent insight into how Caulobacter manages DNA replication and repair through Lon and Clp proteases. Because proteases must manage a broad substrate repertoire there must be methods to compensate for protease saturation and we discuss these scenarios.

The conserved C-terminal tail of FtsZ is required for the septal localization and division inhibitory activity of MinC(C)/MinD

The Escherichia coli Min system contributes to spatial regulation of cytokinesis by preventing assembly of the Z ring away from midcell. MinC is a cell division inhibitor whose activity is spatially regulated by MinD and MinE. MinC has two functional domains of similar size, both of which have division inhibitory activity in the proper context. However, the molecular mechanism of the inhibitory action of either domain is not very clear. Here, we report that the septal localization and division inhibitory activity of MinC(C)/MinD requires the conserved C-terminal tail of FtsZ. This tail also mediates interaction with two essential division proteins, ZipA and FtsA, to link FtsZ polymers to the membrane. Overproduction of MinC(C)/MinD displaces FtsA from the Z ring and eventually disrupts the Z ring, probably because it also displaces ZipA. These results support a model for the division inhibitory action of MinC/MinD. MinC/MinD binds to ZipA and FtsA decorated FtsZ polymers located at the membrane through the MinC(C)/MinD-FtsZ interaction. This binding displaces FtsA and/or ZipA, and more importantly, positions MinC(N) near the FtsZ polymers making it a more effective inhibitor.

Investigation of regulation of FtsZ assembly by SulA and development of a model for FtsZ polymerization

In Escherichia coli FtsZ organizes into a cytoskeletal ring structure, the Z ring, which effects cell division. FtsZ is a GTPase, but the free energy of GTP hydrolysis does not appear to be used for generation of the constriction force, leaving open the question of the function of the GTPase activity of FtsZ. Here we study the mechanism by which SulA, an inhibitor of FtsZ induced during the SOS response, inhibits FtsZ function. We studied the effects of SulA on the in vitro activities of FtsZ, on Z rings in vivo, and on a kinetic model for FtsZ polymerization in silico. We found that the binding of SulA to FtsZ is necessary but not sufficient for inhibition of polymerization, since the assembly of FtsZ polymers in the absence of the GTPase activity was not inhibited by SulA. We developed a new model for FtsZ polymerization that accounts for the cooperativity of FtsZ and could account for cooperativity observed in other linear polymers. When SulA was included in the kinetic scheme, simulations revealed that SulA with strong affinity for FtsZ delayed, but did not prevent, the assembly of polymers when they were not hydrolyzing GTP. Furthermore, the simulations indicated that SulA controls the assembly of FtsZ by binding to a polymerization-competent form of the FtsZ molecule and preventing it from participating in assembly. In vivo stoichiometry of the disruption of Z rings by SulA suggests that FtsZ may undergo two cooperative transitions in forming the Z ring.

Of the morphogenes that make a ring, a rod and a sphere in Escherichia coli

In 1993, William Donachie wrote "The success of molecular genetics in the study of bacterial cell division has been so great that we find ourselves, armed with much greater knowledge of detail, confronted once again with the same naive questions that we set to answer in the first place". Indeed, attempts to answer the apparently simple question of how a bacterial cell divides have led to a wealth of new knowledge, in particular over the past decade and a half. And while some questions have been answered to a great extent since the early reports of isolation of division mutants of Escherichia coli, some key pieces of the puzzle remain elusive. In addition to it being a fundamental process in bacteria that merits investigation in its own right, studying the process of cell division offers an abundance of new targets for the development of new antibacterial compounds that act directly against key division proteins and other components of the cytoskeleton, which are encoded by the morphogenes of E. coli. This review aims to present the reader with a snapshot summary of the key players in E. coli morphogenesis with emphasis on cell division and the rod to sphere transition.

Cell division inhibitors SulA and MinC/MinD block septum formation at different steps in the assembly of the Escherichia coli division machinery

SulA and MinCD are specific inhibitors of cell division in Escherichia coli. In this paper, size exclusion chromatography was used to study the effect of the SulA and MinCD division inhibitors on the oligomerization state of endogenous FtsZ in cytoplasmic extracts, and immunofluorescence microscopy was used to determine the effect of SulA and MinCD on the formation of FtsZ, FtsA and ZipA rings at potential division sites. SulA prevented the formation of high-molecular-weight FtsZ polymers by interfering with FtsZ dimerization and subsequent oligomerization. In contrast, the MinCD division inhibitor did not prevent the oligomerization of FtsZ in the cell extracts or the formation of FtsZ and ZipA ring structures in vivo. However, MinCD did prevent the formation of FtsA rings. Increased expression of ftsA suppressed MinCD-induced division inhibition, but had no effect on SulA-induced division inhibition. These results indicate that MinCD blocks the assembly of the septation machinery at a later step than SulA, at the stage at which FtsA is added to the FtsZ ring.

Redundant in vivo proteolytic activities of Escherichia coli Lon and the ClpYQ (HslUV) protease

The ClpYQ (HslUV) ATP-dependent protease of Escherichia coli consists of an ATPase subunit closely related to the Clp ATPases and a protease component related to those found in the eukaryotic proteasome. We found that this protease has a substrate specificity overlapping that of the Lon protease, another ATP-dependent protease in which a single subunit contains both the proteolytic active site and the ATPase. Lon is responsible for the degradation of the cell division inhibitor SulA; lon mutants are UV sensitive, due to the stabilization of SulA. lon mutants are also mucoid, due to the stabilization of another Lon substrate, the positive regulator of capsule transcription, RcsA. The overproduction of ClpYQ suppresses both of these phenotypes, and the suppression of UV sensitivity is accompanied by a restoration of the rapid degradation of SulA. Inactivation of the chromosomal copy of clpY or clpQ leads to further stabilization of SulA in a lon mutant but not in lon+ cells. While either lon, lon clpY, or lon clpQ mutants are UV sensitive at low temperatures, at elevated temperatures the lon mutant loses its UV sensitivity, while the double mutants do not. Therefore, the degradation of SulA by ClpYQ at elevated temperatures is sufficient to lead to UV resistance. Thus, a protease with a structure and an active site different from those of Lon is capable of recognizing and degrading two different Lon substrates and appears to act as a backup for Lon under certain conditions.

A conserved domain in Escherichia coli Lon protease is involved in substrate discriminator activity

Lon protease of Escherichia coli regulates a diverse set of physiological responses including cell division, capsule production, plasmid stability, and phage replication. Little is known about the mechanism of substrate recognition by Lon. To examine the interaction of Lon with two of its substrates, RcsA and SulA, we generated point mutations in lon which affected its substrate specificity. The most informative lon mutant overproduced capsular polysaccharide (RcsA stabilized) yet was resistant to DNA-damaging agents (SulA degraded). Immunoblots revealed that RcsA protein persisted in this mutant whereas SulA protein was rapidly degraded. The mutant contains a single-base change within lon leading to a single amino acid change of glutamate 240 to lysine. E240 is conserved among all Lon isolates and resides in a charged domain that has a high probability of adopting a coiled-coil conformation. This conformation, implicated in mediating protein-protein interactions, appears to confer substrate discriminator activity on Lon. We propose a model suggesting that this coiled-coil domain represents the discriminator site of Lon.

Analysis of the effect of ppGpp on the ftsQAZ operon in Escherichia coli

Escherichia coli loses its rod shape by inactivation of PBP2 (penicillin-binding protein 2), target of the beta-lactam mecillinam. Under these conditions, cell division is blocked in rich medium. Division in the absence of PBP2 activity is restored (and resistance to mecillinam is conferred) when the three cell division proteins FtsQ, FtsA and FtsZ are overproduced, but not when only one or two of them are overproduced. Division in the absence of PBP2 activity is also restored by a doubling in the ppGpp pool, as in the argS201 mutant. However, the nucleotide ppGpp, a transcriptional regulator of many operons, does not govern any of the five promoters of the ftsQAZoperon, as shown by S1 mapping of ftsQAZ mRNA 5' ends in exponentially growing wild-type cells in the mecillinam-resistant argS201 mutant (intermediate ppGpp level) or during the stringent response elicited by isoleucine starvation (high ppGpp level). Furthermore, the concentration of FtsZ protein is not increased in exponentially growing mecillinam-resistant argS201 cells. These results show that the ftsQAZ operon is not the ppGpp target responsible for mecillinam resistance. We are currently trying to identify those targets that, at intermediate ppGpp levels, allow cells to divide as spheres in the absence of PBP2.

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.

Analysis of the interaction of FtsZ with itself, GTP, and FtsA

The interaction of FtsZ with itself, GTP, and FtsA was examined by analyzing the sensitivity of FtsZ to proteolysis and by using the yeast two-hybrid system. The N-terminal conserved domain consisting of 320 amino acids bound GTP, and a central region of FtsZ, encompassing slightly more than half of the protein, was cross-linked to GTP. Site-directed mutagenesis revealed that none of six highly conserved aspartic acid and asparagine residues were required for GTP binding. These results indicate that the specificity determinants for GTP binding are different than those for the GTPase superfamily. The N-terminal conserved domain of FtsZ contained a site for self-interaction that is conserved between FtsZ proteins from distantly related bacterial species. FtsZ320, which was truncated at the end of the conserved domain, was a potent inhibitor of division although it expressed normal GTPase activity and could polymerize. FtsZ was also found to interact directly with FtsA, and this interaction could also be observed between these proteins from distantly related bacterial species.

Interaction between FtsZ and inhibitors of cell division

The interaction between inhibitors of cell division and FtsZ were assessed by using the yeast two-hybrid system. An interaction was observed between FtsZ and SulA, a component of the SOS response, and the interacting regions were mapped to their conserved domains. This interaction was reduced by mutations in sulA and by most mutations in ftsZ that make cell refractory to sulA. No interaction was detected between FtsZ and MinCD, an inhibitory component of the site selection system. However, interactions were observed among various members of the Min system, and MinE was found to reduce the interaction between MinC and MinD. The implications of these findings for cell division are discussed.

Characterization of the ftsZ gene from Mycoplasma pulmonis, an organism lacking a cell wall

The ftsZ gene is required for cell division in Escherichia coli and Bacillus subtilis. In these organisms, FtsZ is located in a ring at the leading edge of the septum. This ring is thought to be responsible for invagination of the septum, either causing invagination of the cytoplasmic membrane or activating septum-specific peptidoglycan biosynthesis. In this paper, we report that the cell division gene ftsZ is present in two mycoplasma species, Mycoplasma pulmonis and Acholeplasma laidlawii, which are eubacterial organisms lacking a cell wall. Sequencing of the ftsZ homolog from M. pulmonis revealed that it was highly homologous to other known FtsZ proteins. The M. pulmonis ftsZ gene was overexpressed, and the purified M. pulmonis FtsZ bound GTP. Using antisera raised against this purified protein, we could demonstrate that it was expressed in M. pulmonis. Expression of the M. pulmonis ftsZ gene in E. coli inhibited cell division, leading to filamentation, which could be suppressed by increasing expression of the E. coli ftsZ gene. The implications of these results for the role of ftsZ in cell division are discussed.

Identification of a developmentally regulated septin and involvement of the septins in spore formation in Saccharomyces cerevisiae

The Saccharomyces cerevisiae CDC3, CDC10, CDC11, and CDC12 genes encode a family of related proteins, the septins, which are involved in cell division and the organization of the cell surface during vegetative growth. A search for additional S. cerevisiae septin genes using the polymerase chain reaction identified SPR3, a gene that had been identified previously on the basis of its sporulation-specific expression. The predicted SPR3 product shows 25-40% identity in amino acid sequence to the previously known septins from S. cerevisiae and other organisms. Immunoblots confirmed the sporulation-specific expression of Spr3p and showed that other septins are also present at substantial levels in sporulating cells. Consistent with the expression data, deletion of SPR3 in either of two genetic backgrounds had no detectable effect on exponentially growing cells. In one genetic background, deletion of SPR3 produced a threefold reduction in sporulation efficiency, although meiosis appeared to be completed normally. In this background, deletion of CDC10 had no detectable effect on sporulation. In the other genetic background tested, the consequences of the two deletions were reversed. Immunofluorescence observations suggest that Spr3p, Cdc3p, and Cdc11p are localized to the leading edges of the membrane sacs that form near the spindle-pole bodies and gradually extend to engulf the nuclear lobes that contain the haploid chromosome sets, thus forming the spores. Deletion of SPR3 does not prevent the localization of Cdc3p and Cdc11p, but these proteins appear to be less well organized, and the intensity of their staining is reduced. Taken together, the results suggest that the septins play important but partially redundant roles during the process of spore formation.

Identification of FtsW and characterization of a new ftsW division mutant of Escherichia coli

The product of the ftsW gene has been identified as a polypeptide that, like the related RodA protein, shows anomalous mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. FtsW is produced at low levels that can be increased by altering the translation initiation region of the mRNA. Overproduction of FtsW strongly inhibits cell growth. A new mutant allele, ftsW201, causes a temperature-dependent block in the initiation stage of cell division which is similar to the division block in ftsZ mutants. The block in initiation of division in the ftsW201 allele is shown to be independent of FtsZ or the FtsZ inhibitor, SulA. In addition, the ftsW201 mutant is hypersensitive to overproduction of the division initiation protein FtsZ at the permissive temperature. Our results suggest a role for FtsW in an early stage of division which may involve an interaction with FtsZ.

Cell division inhibitors SulA and MinCD prevent formation of the FtsZ ring

Immunoelectron microscopy was used to assess the effects of inhibitors of cell division on formation of the FtsZ ring in Escherichia coli. Induction of the cell division inhibitor SulA, a component of the SOS response, or the inhibitor MinCD, a component of the min system, blocked formation of the FtsZ ring and led to filamentation. Reversal of SulA inhibition by blocking protein synthesis in SulA-induced filaments led to a resumption of FtsZ ring formation and division. These results suggested that these inhibitors block cell division by preventing FtsZ localization into the ring structure. In addition, analysis of min mutants demonstrated that FtsZ ring formation was also associated with minicell formation, indicating that all septation events in E. coli involve the FtsZ ring.

Regulation by proteolysis: energy-dependent proteases and their targets

A number of critical regulatory proteins in both prokaryotic and eukaryotic cells are subject to rapid, energy-dependent proteolysis. Rapid degradation combined with control over biosynthesis provides a mechanism by which the availability of a protein can be limited both temporally and spatially. Highly unstable regulatory proteins are involved in numerous biological functions, particularly at the commitment steps in developmental pathways and in emergency responses. The proteases involved in energy-dependent proteolysis are large proteins with the ability to use ATP to scan for appropriate targets and degrade complete proteins in a processive manner. These cytoplasmic proteases are also able to degrade many abnormal proteins in the cell.

The proper ratio of FtsZ to FtsA is required for cell division to occur in Escherichia coli

Interactions among cell division genes in Escherichia coli were investigated by examining the effect on cell division of increasing the expression of the ftsZ, ftsA, or ftsQ genes. We determined that cell division was quite sensitive to the levels of FtsZ and FtsA but much less so to FtsQ. Inhibition of cell division due to an increase in FtsZ could be suppressed by an increase in FtsA. Inhibition of cell division due to increased FtsA could be suppressed by an increase in FtsZ. In addition, although wild-type strains were relatively insensitive to overexpression of ftsQ, we observed that cell division was sensitized to ftsQ overexpression in ftsI, ftsA, and ftsZ mutants. Among these, the ftsI mutant was the most sensitive. These results suggest that these gene products may interact and that the proper ratio of FtsZ to FtsA is critical for cell division to occur.

ftsZ is an essential cell division gene in Escherichia coli

The ftsZ gene is thought to be an essential cell division gene in Escherichia coli. We constructed a null allele of ftsZ in a strain carrying additional copies of ftsZ on a plasmid with a temperature-sensitive replication defect. This strain was temperature sensitive for cell division and viability, confirming that ftsZ is an essential cell division gene. Further analysis revealed that after a shift to the nonpermissive temperature, cell division ceased when the level of FtsZ started to decrease, indicating that septation is very sensitive to the level of FtsZ. Subsequent studies showed that nucleoid segregation was normal while FtsZ was decreasing and that ftsZ expression was not autoregulated. The null allele could not be complemented by lambda 16-2, even though this bacteriophage can complement the thermosensitive ftsZ84 mutation and carries 6 kb of DNA upstream of the ftsZ gene.

A GTP-binding protein (Era) has an essential role in growth rate and cell cycle control in Escherichia coli

Era is a membrane-associated GTP-binding protein which is essential for cell growth in Escherichia coli. In order to examine the physiological role of Era, strains in which Era was expressed at 40 degrees C but completely repressed at 27 degrees C were constructed. The growth of these strains was inhibited at the nonpermissive temperature, and cells became elongated. Under such conditions, no constrictions or septum formation could be detected by phase-contrast microscopy, and DNA segregation was apparently normal as revealed by fluorescence staining. These data demonstrate that Era has an essential function in cell growth rate control in liquid media and that depletion of Era blocks cell division either directly or indirectly. Thus, the role of GTP-binding proteins as important regulators of cell growth and division may be ubiquitous in nature.

RcsA, an unstable positive regulator of capsular polysaccharide synthesis

RcsA is an unstable positive regulator required for the synthesis of colanic acid capsular polysaccharide in Escherichia coli. Degradation of the RcsA protein in vivo depends on the ATP-dependent Lon protease. DNA sequence analysis of the rcsA gene reveals a single open reading frame for a 23,500-Da highly basic protein (pI = 9.9), consistent with the observed size of the purified subunit of RcsA. The DNA and protein sequences are highly homologous to the rcsA gene and protein from Klebsiella pneumoniae and other species. The carboxy-terminal region of RcsA contains a possible helix-turn-helix DNA-binding motif that resembles sequences found at the carboxy terminus of RcsB, another positive regulator of capsule synthesis, and in several other transcriptional regulators including members of the LuxR family. rcsA62, a mutation in rcsA that leads to increased capsule synthesis, encodes a protein designated RcsA*, which differs from wild-type RcsA only in the replacement of Met-145 by valine. The RcsA* protein is subject to Lon-dependent degradation. The stability of wild-type RcsA in vivo is increased by multicopy RcsB. Conversely, RcsA is degraded more rapidly in rcsB mutant hosts than in wild-type hosts. These results suggest that RcsA and RcsB interact in vivo and are consistent with genetic experiments that indicate an interaction between RcsA and RcsB. Based on these experiments, we propose a model for capsule regulation in which RcsA interacts directly with RcsB to promote transcription of the genes for capsule synthesis.

FtsZ and cell division

The ftsZ gene in Escherichia coli is thought to be an essential gene and to play a pivotal role in cell division. Gene disruption experiments confirmed that ftsZ is an essential gene. Examination of cellular responses to FtsZ depletion indicated that FtsZ was required for division but not for nucleoid segregation. Analysis of mutations within the ftsZ, gene, selected for resistance to the cell division inhibitor SulA, revealed that they also conferred resistance to MinCD. This raises the possibility that ftsZ is the target of these two cell division inhibitors. Analysis of the ftsZ gene from Bacillus subtilis revealed that the gene was required for both septation during vegetative growth and asymmetric septation during sporulation.

Interaction between the min locus and ftsZ

In Escherichia coli, distinct but similar minicell phenotypes resulting from mutation at the minB locus and increased expression of ftsZ suggested a possible interaction between these genes. A four- to fivefold increase in FtsZ resulting from increased gene dosage was found to suppress the lethality of minCD expressed from the lac promoter. Since increased MinCD did not affect the level of FtsZ, this suggested that MinCD may antagonize FtsZ to inhibit its cell division activity. This possibility was supported by the finding that alleles of ftsZ isolated as resistant to the cell division inhibitor SulA were also resistant to MinCD. Among the ftsZ(Rsa) alleles, two appeared to be completely resistant to MinCD as demonstrated by the lack of an effect of MinCD on cell length and a minicell phenotype observed in the absence of a significant increase in FtsZ. It was shown that SulA inhibits cell division independently of MinCD.

Analysis of ftsZ mutations that confer resistance to the cell division inhibitor SulA (SfiA)

In Escherichia coli, the ftsZ gene is thought to be an essential cell division gene. Several dominant mutations that make lon mutant cells refractory to the cell division inhibitor SulA, sulB9, sulB25, and sfiB114, have been mapped to the ftsZ gene. DNA sequence analysis of these mutations and the sfiB103 mutation confirmed that all of these mutations mapped within the ftsZ gene and revealed that the two sulB mutations were identical and by selection for resistance to higher levels of SulA, contained a second mutation within the ftsZ gene. We therefore propose that these mutations be redesignated ftsZ(Rsa) for resistance to SulA. A procedure involving mutagenesis of ftsZ cloned on low-copy-number vectors was used to isolate three additional ftsZ(Rsa) mutations. DNA sequence analysis of these mutations revealed that they were distinct from the previously isolated mutations. One of these mutations, ftsZ3(Rsa), led to an altered FtsZ protein that could no longer support cell growth but still conferred the Rsa phenotype in the presence of ftsZ+. In addition to being resistant to SulA, all ftsZ(Rsa) mutations also conferred resistance to a LacZ-FtsZ hybrid protein (ZZ). One possibility is that FtsZ functions as a multimer and that FtsZ(Rsa) mutant proteins have an increased ability for multimerization, making them resistant to SulA and ZZ.

FtsZ regulates frequency of cell division in Escherichia coli

Cell division is regulated so that it occurs only once per cell cycle. In Escherichia coli, a rod-shaped bacterium, division normally takes place at the center of the long axis of the cell; however, in the minicell mutant, division can also take place at the cell pole. Such divisions take place at the expense of normal divisions, resulting in an overall increase in nucleated cell length. We report here that increasing the level of FtsZ can completely suppress the cell length of the minicell mutant by increasing the frequency at which cell division events take place. This result suggests that the level of FtsZ controls the frequency of cell division in E. coli.

Central role for the Escherichia coli minC gene product in two different cell division-inhibition systems

In Escherichia coli, selection of the proper division site at midcell requires the specific inhibition of septation at two other potential division sites, located at each of the cell poles. This site-specific inhibition of septation is mediated by the gene products of the minicell locus (the minB operon) that includes three genes, minC, minD, and minE. In this paper we show that one of the components of this division-inhibition system, the minC gene product, is also an essential component of another division-inhibition system, which is induced by derepression of the dicB gene and leads to inhibition of septation at all potential division sites. The two minC-dependent division-inhibition systems could be functionally distinguished by their different responses to the minE gene product. The results suggest a model in which a common mechanism, mediated by MinC, is responsible for the division block in a class of division-inhibition systems that can be independently activated by different proteins that determine the specific properties of these systems.

Regulation of cell division in E. coli

Recent investigation of some old cell division mutants of E. coli suggests that genes playing central roles in the regulation of division have been identified. The results suggest that cell division is triggered when a critical level of a single protein, FtsZ, is attained. The activity of this protein is channelled to the new division site by the activity of the min locus, which blocks access to old sites. Continued study of these genes should yield further insights into the cell division process.

Cellular defects caused by deletion of the Escherichia coli dnaK gene indicate roles for heat shock protein in normal metabolism

DnaK is a major heat shock protein of Escherichia coli and has been previously reported to be essential for growth at high temperatures. We systematically investigated the role of DnaK in cellular metabolism at a wide range of growth temperatures by analyzing cellular defects caused by deletion of the dnaK gene (delta dnaK52). At intermediate temperatures (30 degrees C), introduction of the delta dnaK52 allele into wild-type cells caused severe defects in cell division, slow growth, and poor viability of the cells. delta dnaK52 mutants were genetically unstable at 30 degrees C and frequently acquired secondary mutations. At high (42 degrees C) and low (11 and 16 degrees C) temperatures the delta dnaK52 allele could only be introduced into the subpopulation of wild-type cells that had duplicated the dnaK region of their chromosome. delta dnaK52 mutants isolated at 30 degrees C were cold sensitive as well as temperature sensitive for growth. Cell division defects of delta dnaK52 mutants at 30 degrees C were largely suppressed by overproduction of the FtsZ protein, which is normally required for septation during cell division; however, slow growth and poor viability at 30 degrees C and cold sensitivity and temperature sensitivity of growth were not suppressed, indicating that delta dnaK52 mutants had additional defective cellular functions besides cell division.

A lacZ-pbpB gene fusion coding for an inducible hybrid protein that recognizes localized sites in the inner membrane of Escherichia coli

An in-phase gene fusion consisting of the 5'-terminal 1,314 base pairs (bp) of the structural gene for beta-galactosidase (lacZ) and the 3'-terminal 1,644 bp of the structural gene coding for penicillin-binding protein 3 (pbpB) of Escherichia coli was constructed and cloned in the plasmid pDIAM64. The product of the fusion gene was a remarkably stable protein with an apparent molecular weight of 110,000 (p110) that retained the ability to covalently interact with beta-lactam antibiotics. The fusion protein was found associated with the membrane at low levels of induction, but it accumulated in the cytoplasm of cells induced for a long time as inclusion bodies of high density. Inclusion bodies were localized at defined positions corresponding to septal sites in all of the pDIAM64-containing strains tested except PAT84 and GD113 (which carry the ftsZ84 mutant allele). These findings indicate a possible role of the FtsZ protein in the integration of Pbp3 into the membrane and in septum localization during the cell division cycle.

Regulation of cell division in Escherichia coli K-12: probable interactions among proteins FtsQ, FtsA, and FtsZ

In Escherichia coli, the FtsQ, FtsA, and FtsZ proteins are believed to play essential roles in the regulation of cell division. Of the three proteins, FtsZ has received the most attention, particularly because of its interactions with SfiA. Double mutants which carry mutations located in the ftsQ, ftsA, or ftsZ gene in combination with the lon-1 mutation were constructed. In the presence of the lon-1 mutation, which is known to stabilize SfiA, the ftsQ1 mutant cells were not capable of forming colonies on a rich agar medium, whereas mutant cells harboring either one of the mutations grew well on this medium. Examination of lon-1 fts double-mutant cells for sensitivity to UV light revealed that those carrying the ftsA10 allele were resistant. It was also observed that in the presence of a multicopy plasmid containing a wild-type ftsZ gene, the ftsQ1 mutant filamented markedly following a nutritional shift-up and that the division rate of ftsZ84 mutant cells was slightly reduced when they harbored a wild-type ftsQ-containing plasmid. The possibility that the Fts proteins are interacting with one another and forming a molecular complex is discussed.

Analysis of cell division gene ftsZ (sulB) from gram-negative and gram-positive bacteria

The ftsZ (sulB) gene of Escherichia coli codes for a 40,000-dalton protein that carries out a key step in the cell division pathway. The presence of an ftsZ gene protein in other bacterial species was examined by a combination of Southern blot and Western blot analyses. Southern blot analysis of genomic restriction digests revealed that many bacteria, including species from six members of the family Enterobacteriaceae and from Pseudomonas aeruginosa and Agrobacterium tumefaciens, contained sequences which hybridized with an E. coli ftsZ probe. Genomic DNA from more distantly related bacteria, including Bacillus subtilis, Branhamella catarrhalis, Micrococcus luteus, and Staphylococcus aureus, did not hybridize under minimally stringent conditions. Western blot analysis, with anti-E. coli FtsZ antiserum, revealed that all bacterial species examined contained a major immunoreactive band. Several of the Enterobacteriaceae were transformed with a multicopy plasmid encoding the E. coli ftsZ gene. These transformed strains, Shigella sonnei, Salmonella typhimurium, Klebsiella pneumoniae, and Enterobacter aerogenes, were shown to overproduce the FtsZ protein and to produce minicells. Analysis of [35S]methionine-labeled minicells revealed that the plasmid-encoded gene products were the major labeled species. This demonstrated that the E. coli ftsZ gene could function in other bacterial species to induce minicells and that these minicells could be used to analyze plasmid-endoced gene products.