Coupling of σG Activation to Completion of Engulfment during Sporulation of Bacillus subtilis Survives Large Perturbations to DNA Translocation and Replication

Spore formation in Bacillus subtilis is characterized by activation of RNA polymerase sigma factors, including the late-expressed σ^G. During spore formation an asymmetric division occurs, yielding the smaller prespore and the larger mother cell. At division, only 30% of the chromosome is in the prespore, and the rest is then translocated into the prespore. Following completion of engulfment of the prespore by the mother cell, σ^G is activated in the prespore. Here we tested the link between engulfment and σ^G activation by perturbing DNA translocation and replication, which are completed before engulfment. One approach was to have large DNA insertions in the chromosome; the second was to have an impaired DNA translocase; the third was to use a strain in which the site of termination of chromosome replication was relocated. Insertion of 2.3 Mb of Synechocystis DNA into the B. subtilis genome had the largest effect, delaying engulfment by at least 90 min. Chromosome translocation was also delayed and was completed shortly before the completion of engulfment. Despite the delay, σ^G became active only after the completion of engulfment. All results are consistent with a strong link between completion of engulfment and σ^G activation. They support a link between completion of chromosome translocation and completion of engulfment.     

Separation of chromosome termini during sporulation of Bacillus subtilis depends on SpoIIIE

Bacillus subtilis undergoes a highly distinctive division during spore formation. It yields two unequal cells, the mother cell and the prespore, and septum formation is completed before the origin-distal 70% of the chromosome has entered the smaller prespore. The mother cell subsequently engulfs the prespore. Two different probes were used to study the behavior of the terminus (ter) region of the chromosome during spore formation. Only one ter region was observed at the time of sporulation division. A second ter region, indicative of chromosome separation, was not distinguishable until engulfment was nearing completion, when one was in the mother cell and the other in the prespore. Separation of the two ter regions depended on the DNA translocase SpoIIIE. It is concluded that SpoIIIE is required during spore formation for chromosome separation as well as for translocation; SpoIIIE is not required for separation during vegetative growth.     

Use of asymmetric cell division and spoIIIE mutants to probe chromosome orientation and organization in Bacillus subtilis

Soon after the onset of sporulation in Bacillus subtilis , asymmetric cell division occurs to generate the differentiating prespore and mother cell types. Formation of the septum close to the cell pole initially bisects the nucleoid destined for the prespore, trapping only about one-third of the DNA in the small compartment. The remaining part of the chromosome is then transported through the septum. spoIIIE mutant cells fail to transfer the DNA and arrest with only partially segregated prespore chromosomes. Previous work has shown that the orientation of the chromosome at the time of septation is not random. Here, we use both physical and genetic methods to characterize the trapped DNA. The results show that the chromosome has a very specific orientation at the time of septation, consistent with the action of a centromere-like sequence near oriC . They also demonstrate that the chromosome is folded, or otherwise organized, in a highly ordered manner.     

Prespore cell inducing factor, ψ factor,controls both prestalk and prespore gene expression in Dictyostelium development

Prespore cell‐inducing (psi, ψ) factor (PsiA), encoded by the psiA gene of Dictyostelium, is a secreted signal glycoprotein that induces prespore cell differentiation when added to monolayer cultures. In situ hybridization during normal development showed that the psiA gene is highly expressed in scattered cells at the mound stage and in prespore cells at the onset of culmination. The conventional prespore‐cell marker genes, cotC and pspA, were expressed normally in psiA^? and psiA overexpressing strains. Expressions of rnrB and cudA are repressed in the prestalk cells of a wild type slug to render prespore specific pattern. However, a promoter‐reporter fusion gene, rnrB:lacZ, showed an ectopic expression in the prestalk cells of the psiA^? strain while cudA(psp):lacZ did so in those of the psiA overexpressing strain. Overexpression of psiA delayed expression of the prestalk specific gene, ecmB, during development, while knocking out psiA promoted its expression. In addition, overexpression inhibited DIF‐1‐induced stalk formation in monolayer cultures. Together with the known prespore inducing activity, the results indicate that PsiA regulates both prespore and prestalk/stalk cell differentiation. These results indicate that PsiA is also involved in prestalk cell differentiation.     

Noc protein binds to specific DNA sequences to coordinate cell division with chromosome segregation

Coordination of chromosome segregation and cytokinesis is crucial for efficient cell proliferation. In Bacillus subtilis, the nucleoid occlusion protein Noc protects the chromosomes by associating with the chromosome and preventing cell division in its vicinity. Using protein localization, ChAP‐on‐Chip and bioinformatics, we have identified a consensus Noc‐binding DNA sequence (NBS), and have shown that Noc is targeted to about 70 discrete regions scattered around the chromosome, though absent from a large region around the replication terminus. Purified Noc bound specifically to an NBS in vitro. NBSs inserted near the replication terminus bound Noc–YFP and caused a delay in cell division. An autonomous plasmid carrying an NBS array recruited Noc–YFP and conferred a severe Noc‐dependent inhibition of cell division. This shows that Noc is a potent inhibitor of division, but that its activity is strictly localized by the interaction with NBS sites in vivo. We propose that Noc serves not only as a spatial regulator of cell division to protect the nucleoid, but also as a timing device with an important role in the coordination of chromosome segregation and cell division.     

Differentiation inducing factors in Dictyostelium discoideum: A novel low molecular factor functions at an early stage(s) of differentiation

There are reports that secreted factor(s) are involved in prespore cell differentiation in Dictyostelium discoideum, but the structures and functions of the various factors have not been elucidated. Previously, we described two prespore cell‐inducing factors in conditioned medium; one was a glycoprotein named prespore cell‐inducing factor (ψ factor, or PSI‐1), and the other, a heat stable dialyzable factor(s). In the present paper, we purified and characterized the most potent prespore cell‐inducing activity in dialysates. The factor began to be secreted after the onset of starvation and stopped being secreted once the cells had aggregated, which was earlier than the onset of the ψ factor gene expression. In addition, unlike ψ factor, its secretion did not appear to depend on activation of protein kinase A. Interestingly, the purified factor not only induced prespore cell specific genes such as pspA and cotC but also a prestalk‐cell specific gene, ecmB in vitro. The purified factor is tentatively designated polyketide‐like factor (PLF), because it seems to be a novel polyketide with 208 Da. Half maximal induction of prespore cell was obtained with 26 nmol/L of PLF. We propose that PLF plays a key role in the acquisition of differentiation commitment, before the ψ factor induces specifically prespore cell differentiation.     

Specific amino acid substitutions in β strand S2 of FtsZ cause spiraling septation and impair assembly cooperativity in Streptomyces spp.

Bacterial cell division is orchestrated by the Z ring, which is formed by single‐stranded treadmilling protofilaments of FtsZ. In Streptomyces, during sporulation, multiple Z rings are assembled and lead to formation of septa that divide a filamentous hyphal cell into tens of prespore compartments. We describe here mutant alleles of ftsZ in Streptomyces coelicolor and Streptomyces venezuelae that perturb cell division in such a way that constriction is initiated along irregular spiral‐shaped paths rather than as regular septa perpendicular to the cell length axis. This conspicuous phenotype is caused by amino acid substitutions F37I and F37R in β strand S2 of FtsZ. The F37I mutation leads, instead of regular Z rings, to formation of relatively stable spiral‐shaped FtsZ structures that are capable of initiating cell constriction. Further, we show that the F37 mutations affect the polymerization properties and impair the cooperativity of FtsZ assembly in vitro. The results suggest that specific residues in β strand S2 of FtsZ affect the conformational switch in FtsZ that underlies assembly cooperativity and enable treadmilling of protofilaments, and that these features are required for formation of regular Z rings. However, the data also indicate FtsZ‐directed cell constriction is not dependent on assembly cooperativity.     

Cell cycle and sporulation in Bacillus subtilis

Recent work on cell division and chromosome orientation and partitioning in Bacillus subtilis has provided insights into cell cycle regulation during growth and development. The cell cycle is an integral part of development and entrance into sporulation is modulated by signals that transmit the status of DNA integrity, chromosome replication and segregation. In addition, B. subtilis modifies cell division and DNA segregation to establish cell-type-specific gene expression during sporulation.     

PomZ,a ParA‐like protein,regulates Z‐ring formation and cell division in Myxococcus xanthus

Accurate positioning of the division site is essential to generate appropriately sized daughter cells with the correct chromosome number. In bacteria, division generally depends on assembly of the tubulin homologue FtsZ into the Z‐ring at the division site. Here, we show that lack of the ParA‐like protein PomZ in Myxococcus xanthus resulted in division defects with the formation of chromosome‐free minicells and filamentous cells. Lack of PomZ also caused reduced formation of Z‐rings and incorrect positioning of the few Z‐rings formed. PomZ localization is cell cycle regulated, and PomZ accumulates at the division site at midcell after chromosome segregation but prior to FtsZ as well as in the absence of FtsZ. FtsZ displayed cooperative GTP hydrolysis in vitro but did not form detectable filaments in vitro. PomZ interacted with FtsZ in M. xanthus cell extracts. These data show that PomZ is important for Z‐ring formation and is a spatial regulator of Z‐ring formation and cell division. The cell cycle‐dependent localization of PomZ at midcell provides a mechanism for coupling cell cycle progression and Z‐ring formation. Moreover, the data suggest that PomZ is part of a system that recruits FtsZ to midcell, thereby, restricting Z‐ring formation to this position.     

Bacterial cell division: regulating Z-ring formation

The earliest stage of cell division in bacteria is the formation of a Z ring, composed of a polymer of the FtsZ protein, at the division site. Z rings appear to be synthesized in a bi‐directional manner from a nucleation site (NS) located on the inside of the cytoplasmic membrane. It is the utilization of a NS specifically at the site of septum formation that determines where and when division will occur. However, a Z ring can be made to form at positions other than at the division site. How does a cell regulate utilization of a NS at the correct location and at the right time? In rod‐shaped bacteria such as Escherichia coli and Bacillus subtilis, two factors involved in this regulation are the Min system and nucleoid occlusion. It is suggested that in B. subtilis, the main role of the Min proteins is to inhibit division at the nucleoid‐free cell poles. In E. coli it is currently not clear whether the Min system can direct a Z ring to the division site at mid‐cell or whether its main role is to ensure that division inhibition occurs away from mid‐cell, a role analogous to that in B. subtilis. While the nucleoid negatively influences Z‐ring formation in its vicinity in these rod‐shaped organisms, the exact relationship between nucleoid occlusion and the ability to form a mid‐cell Z ring is unresolved. Recent evidence suggests that in B. subtilis and Caulobacter crescentus, utilization of the NS at the division site is intimately linked to the progress of a round of chromosome replication and this may form the basis of achieving co‐ordination between chromosome replication and cell division.     

Septation and chromosome segregation during sporulation in Bacillus subtilis

The early stages of sporulation in Bacillus subtilis incorporate a modified, highly asymmetric cell division. It is now clear that most, if not all, of the components of the vegetative division machinery are used also for asymmetric division. However, the machinery for chromosome segregation may differ significantly between vegetative growth and sporulation. Several interesting checkpoint mechanisms couple cell cycle events to gene expression early in sporulation. This review summarises important advances in the understanding of chromosome segregation and cell division at the onset of sporulation in B.subtilis in the past three years.     

Separation and properties of prestalk and prespore cells of Dictyostelium discoideum

We describe a method of separating prestalk and prespore cells of Dictyostelium discoideum slugs using a self-generating Percoll gradient. This method gives quantitative recovery of cells and good purity. Separated prestalk and prespore cells possess different levels of the enzymes UDP galactose :polysaccharide transferase, cAMP phosphodiesterase and glycogen phosphorylase. We have used this method, as well as mechanical dissection of slugs, to examine the fate of separated prestalk and prespore cells in Dictyostelium strains that are able to give rise to mature stalk and spore cells in cell monolayers. The results from such experiments provide direct evidence that prestalk and prespore cells from the migrating slug stage are programmed to differentiate into stalk and spore cells respectively.     

Staphylococcus aureus requires at least one FtsK/SpoIIIE protein for correct chromosome segregation

Faithful coordination between bacterial cell division and chromosome segregation in rod‐shaped bacteria, such as Escherichia coli and Bacillus subtilis, is dependent on the DNA translocase activity of FtsK/SpoIIIE proteins, which move DNA away from the division site before cytokinesis is completed. However, the role of these proteins in chromosome partitioning has not been well studied in spherical bacteria. Here, it was shown that the two Staphylococcus aureus FtsK/SpoIIIE homologues, SpoIIIE and FtsK, operate in independent pathways to ensure correct chromosome management during cell division. SpoIIIE forms foci at the centre of the closing septum in at least 50% of the cells that are close to complete septum synthesis. FtsK is a multifunctional septal protein with a C‐terminal DNA translocase domain that is not required for correct chromosome management in the presence of SpoIIIE. However, lack of both SpoIIIE and FtsK causes severe nucleoid segregation and morphological defects, showing that the two proteins have partially redundant roles in S. aureus.     

Developmental potential of isolated Dictyostelium myxamoebae

Myxamoebae of the cellular slime mold Dictyostelium discoideum were isolated as single cells from several developmental stages. The course of differentiation of the isolated cells was assayed by the synthesis or loss of prespore organelles readily detectable by electron microscopy. It was determined by these marker organelles that single-cell isolates from all stages tested are incapable of differentiating or redifferentiating. However, prespore cells isolated from a migrating slug dedifferentiated into vegetative cells if cell division occurred.     

Glycoantigen expression is regulated both temporally and spatially during development in the cellular slime molds Dictyostelium discoideum and D. mucoroides

Six monoclonal antibodies were isolated which react with common antigens shared by multiple glycoconjugate species in the cellular slime mold Dictyostelium discoideum. Based on competition of antibody binding by glycopeptides and simple sugars, and inhibition of antibody binding by antigen pretreatment with Na periodate, it is argued that at least five of the six antibodies recognize epitopes which contain carbohydrate. These epitopes are consequently referred to as glycoantigens (GAs).Three of the GAs are expressed during growth and throughout the developmental cycle, but are eventually enriched in prestalk and stalk cells. The remaining three are expressed only during and/or after aggregation and are exclusively expressed or highly enriched in prespore cells and spores. These conclusions are derived from Western blot immunoanalysis of purified cell types, immunofluorescence, and EM immunocytochemistry.The two GAs found only in prespore cells appear to be exclusively enclosed within prespore vesicles. The third GA of this type, which is only enriched in prespore cells compared to prestalk cells, is also found in other vesicle types as well as on the cell surface.Two of the GAs enriched in prestalk cells are initially found in all cells of the slug. They are undetectable in spores and prominent in stalk cells. The third GA, though found in the interiors of both prestalk and prespore cells, is enriched on the cell surface of prestalk cells.The chief characteristics of expression of four of these GAs are conserved in the related species D. mucoroides. This species is characterized by continuous trans differentiation of prespore cells into prestalk cells. This shows that the prespore cells maintain specific mechanisms for turning over their cell type specific GAs and that prestalk cells express a specific mechanism for inducing at least one of their cell-type specific GAs.These observations identify specific carbohydrate structures (as GAs) whose synthesis, subsequent localization and turnover are developmentally regulated. The exclusive association of two GAs with prespore vesicles identifies these GAs as markers for this organelle and raises questions regarding the functional significance of this association. The restricted cell surface localization of the other four GAs, together with data from cell adhesion studies, suggest the possibility of a potential role for these GAs in intercellular recognition leading to cell sorting.This paper is dedicated to the memory of the late Daniel McMahon.     

A DNA‐binding protein defines the precise region of chromosome capture during Bacillus sporulation

During sporulation, Bacillus subtilis divides around the nucleoid near one cell pole, initially capturing approximately one quarter of one chromosome in the newly formed forespore compartment. While it is known that a specific region of the nucleoid is reproducibly captured in the forespore, the mechanism underlying the precision of capture is unknown. Here we describe a role for RefZ, a DNA‐binding protein that regulates FtsZ, and its cognate binding motifs (RBMs) in defining the specific region of chromosome initially captured in the forespore. RefZ is conserved across the Bacillus genus and remains functional as an inhibitor of cell division in a species‐swapping experiment. The RBMs are also conserved in their positioning relative to oriC across Bacillus, suggesting that the function of the RBMs is both important and position‐dependent in the genus. In B. subtilis, the RBMs flank the region of the chromosome captured at the time of cell division, and we find that RefZ binds the five oriC‐proximal RBMs with similar apparent affinity in units of two and four. refZ and RBM mutants capture chromosomal regions normally excluded from the forespore, suggesting that RefZ‐RBM complexes play a role in regulating the position of cell division relative to the chromosome during sporulation.     

Cell cycle coordination and regulation of bacterial chromosome segregation dynamics by polarly localized proteins

What regulates chromosome segregation dynamics in bacteria is largely unknown. Here, we show in Caulobacter crescentus that the polarity factor TipN regulates the directional motion and overall translocation speed of the parS/ParB partition complex by interacting with ParA at the new pole. In the absence of TipN, ParA structures can regenerate behind the partition complex, leading to stalls and back‐and‐forth motions of parS/ParB, reminiscent of plasmid behaviour. This extrinsic regulation of the parS/ParB/ParA system directly affects not only division site selection, but also cell growth. Other mechanisms, including the pole‐organizing protein PopZ, compensate for the defect in segregation regulation in ΔtipN cells. Accordingly, synthetic lethality of PopZ and TipN is caused by severe chromosome segregation and cell division defects. Our data suggest a mechanistic framework for adapting a self‐organizing oscillator to create motion suitable for chromosome segregation.