Dynamic Spatial Regulation in the Bacterial Cell
2000; Cell Press; Volume: 100; Issue: 1 Linguagem: Inglês
10.1016/s0092-8674(00)81686-4
ISSN1097-4172
Autores Tópico(s)Bacteriophages and microbial interactions
ResumoThe broad and rich history of prokaryotic biology in the 20th century has been driven by the power of genetics applied to vast populations of haploid cells. This history begins in 1943 with the statistical demonstration by Salvidor Luria and Max Delbrück that bacteria obey the Darwinian principles of random mutation and natural selection. This discovery set the stage over the decade that followed for using bacteria and their phage to elucidate the fundamental nature the genetic material. In 1944, Oswald Avery, Colin MacLeod, and Maclyn McCarty made the seminal discovery that the "transforming principle," which was known to alter the hereditary properties of streptococci, is DNA. This indicated that the genetic material is DNA, a concept that was reinforced a few years later by the work of Alfred Hershey and Martha Chase. Their experiments showed that phage T2 injects its DNA into host cells and hence that viral DNA is likely to suffice for viral propagation. During this extraordinary 10 year period, Joshua Lederberg and Edward Tatum discovered conjugation in bacteria, and Lederberg and Norton Zinder showed that certain phage can transduce DNA from one bacterium to another. Capping off the decade, James Watson and Francis Crick proposed in 1953 the double helical model for the structure for DNA, which immediately suggested a "possible copying mechanism for the genetic material." The pace of progress in prokaryotic biology quickened in the 1960s with the cracking of the genetic code, the discovery of messenger and transfer RNAs, the demonstration that the ribosome is the site of protein synthesis, the demonstration of the colinearity of the gene and the protein, and the development of the concept of the repressor. Studies on bacteria in the 1970s and 1980s led to profound insights into the machinery for DNA, RNA, and protein synthesis, complex metabolic pathways, regulatory dynamics, and the interactions between free-living cells and their environment. Yet a further leap in our understanding of the prokaryotic cell occurred in the final decade of the 20th century due in large measure to the availability of full bacterial genome sequences and the advent of prokaryotic cell biology. Within the limited venue of this review and given our mandate from the editors of Cell to look forward to the 21st century, we have chosen to concentrate on discoveries in the closing decade of the 20th century. Built on the foundation of the store of accumulated information about E. coli, the study of other bacteria that carry out a broader menu of behavior and cell differentiation patterns has led to new insights into the bacterial cell cycle and the regulatory networks that incorporate the spatial dimension into temporally controlled cellular events. Subcellular differentiation is not surprising in large and necessarily complex eukaryotic cells, where DNA, RNA, and proteins are actively moved around the cell in directed traffic patterns. What is surprising is that in the small (1–3 μm) bacterial cell, with no known directed transportation systems, dynamic subcellular localization of regulatory and structural proteins, in fact, accompanies, and in many cases directs, cell cycle progression and the generation of diverse cell types. The progression of the cell cycle reflects the basic genetic network that maintains life—it is the primitive brain of the cell. The business of a newly divided progeny cell is to grow, replicate its genome, ensure chromosome segregation, and carry out cytokinesis culminating in the production of two progeny with the same genetic makeup. The cellular machinery required to accomplish these tasks and the factors that regulate their ordered expression operate at different places in the cell and at different times in the cell cycle. For example, the replication complex has a specific cellular address, as do the chromosome segregation proteins, the origin and terminus of DNA replication, and the tubulin-like protein that is responsible for constriction at division. Similarly, the signal transduction proteins that regulate the initiation of DNA replication, and the proteins that discriminate midcell and quarter cell sites for the position of the division plane, are consigned to, and dynamically relocate from, specific cellular locations. Superimposed on this cell cycle framework are functions unique to specific cells, such as those that are responsible for the inherent generation of cellular diversity. The polar placement of regulatory factors in predivisional cells can dictate the asymmetry that, upon cell division, leads to diverse cell types (26Horvitz R.H Herskowitz I Mechanisms of asymmetric cell division two Bs or not two Bs, that is the question.Cell. 1992; 68: 237-258Abstract Full Text PDF PubMed Scopus (426) Google Scholar, 31Jacobs C Shapiro L Microbial asymmetric cell division localization of cell fate determinants.Curr. Opin. Genet. Dev. 1998; 8: 386-391Crossref PubMed Scopus (23) Google Scholar). Essentially, localized factors allow different parts of the cell to express unique functions. The integration of the cell cycle program with cellular responses to outside events is also a critical factor in prokaryotic cell fate determination. Thus, in some bacteria, starvation signals entry into a developmental pathway, but this is an adaptation of the cell cycle genetic network. Added complexity during development is provided by the recent discoveries that dynamic changes in protein location influence gene expression and thus the induction and spatial orientation of morphogenetic events. The message is now clear—the regulation of the bacterial cell, in all of its functions, must be understood within the context of a three-dimensional grid. Studies on cytokinesis and chromosome segregation reinforce the view that the bacterial cell must be considered in three dimensions (Figure 1). The first and most crucial step in cell division is the assembly of the tubulin-like protein FtsZ into a cytokinetic ring (Z ring) at the center of the cell (Figure 1A; 55Lutkenhaus J Addinall S.G Bacterial cell division and the Z ring.Annu. Rev. Biochem. 1997; 66: 93-116Crossref PubMed Scopus (396) Google Scholar). The Z ring, in turn, recruits in an ordered sequence a series of additional proteins that mediate division. In addition, as in the formation of microtubules in eukaryotic cells, two proteins, ZipA and EzrA, which influence its polymerization and its depolymerization, aid proper assembly of the Z ring (21Hale C.A de Boer P.A.J Direct binding of FtsZ to ZipA, an essential component of the septal ring structure that mediates cell division in E. coli.Cell. 1997; 88: 175-185Abstract Full Text Full Text PDF PubMed Scopus (381) Google Scholar, 46Levin P.A Kurtser I.G Grossman A.D Identification and characterization of a negative regulator of FtsZ ring formation in Bacillus subtilis.Proc. Natl. Acad. Sci. USA. 1999; 96: 9642-9647Crossref PubMed Scopus (160) Google Scholar, 69RayChaudhuri D ZipA is a MAP-Tau homolog and is essential for structural integrity of the cytokinetic FtsZ ring during bacterial cell division.EMBO J. 1999; 18: 2372-2383Crossref PubMed Scopus (143) Google Scholar). Importantly, however, neither ZipA nor EzrA inform FtsZ where the midcell is; medially sited Z rings assemble in their absence. Hence, we are left with two topological mysteries: how does a bacterial cell know where its middle is, and what is the nature of the medial mark that FtsZ recognizes and that triggers its polymerization? This mystery is deepened in some bacteria, such as Caulobacter, where cytokinesis occurs asymmetrically at every division (Figure 1B). Even bacteria, such as E. coli and B. subtilis, that divide in the middle, have the potential to undergo cytokinesis near the cell poles. The use of these polar division sites is normally suppressed by a cell division inhibitor (MinCD), which is composed of the proteins MinC and MinD (11de Boer P.A Crossley R.E Rothfield L.I A division inhibitor and a topological specificity factor coded for by the minicell locus determine proper placement of the division septum in E. coli.Cell. 1989; 56: 641-649Abstract Full Text PDF PubMed Scopus (572) Google Scholar). How does MinCD discriminate between polar sites and the cell middle? In E. coli, MinCD localizes near the poles and away from the cell center (Figure 2). This polar localization occurs in a highly dynamic manner. MinCD molecules first cluster near one pole, then at the other, and then back again with a periodicity of only tens of seconds (27Hu Z Lutkenhaus J Topological regulation of cell division in Escherichia coli involves rapid pole to pole oscillation of the division inhibitor MinC under the control of MinD and minE.Mol. Microbiol. 1999; 34: 82-90Crossref PubMed Scopus (354) Google Scholar, 68Raskin D.M de Boer P.A.J Rapid pole-to-pole oscillation of a protein required for directing division to the middle of E. coli.Proc. Natl. Acad. Sci. USA. 1999; 96: 4971-4976Crossref PubMed Scopus (570) Google Scholar)! This oscillation depends on a third protein MinE, which is present as a medially positioned ring adjacent to the Z ring (67Raskin D.M de Boer P.A.J The MinE ring an FtsZ-independent cell structure required for selection of the correct division site in E. coli.Cell. 1997; 91: 685-694Abstract Full Text Full Text PDF PubMed Scopus (199) Google Scholar, 68Raskin D.M de Boer P.A.J Rapid pole-to-pole oscillation of a protein required for directing division to the middle of E. coli.Proc. Natl. Acad. Sci. USA. 1999; 96: 4971-4976Crossref PubMed Scopus (570) Google Scholar). How and why MinCD makes these extraordinarily rapid, oscillatory movements is not clear, but the end result is that over time the division inhibitor is at its lowest concentration at midcell and at its highest concentration at the cell caps. Thus, minicell formation, but not binary fission, is prevented. Once again, we see that the bacterial cell is a grid in which proteins that govern the position of the division plane are deployed at particular sites within the cell in a dynamic manner that changes (sometimes with great rapidity) over time. Cells of all kinds must coordinate cell division with the faithful segregation of the newly duplicated genetic material to each daughter cell. Eukaryotic cells have an elaborate spindle apparatus that drives sister chromosomes apart and that ensures their capture by each daughter cell. Bacteria, in contrast, have no conspicuous mitotic machine. The challenge of elucidating the mechanism by which chromosomes are segregated with high fidelity in prokaryotes has been relatively intractable due to the small size of the nucleoid and the absence of methods to study its organization. Newly developed cytological methods, however, have made it possible to visualize specific sites on the chromosome and their movement over the course of the cell cycle. For example, tandem copies of the lactose operon operator lacO can be introduced into the chromosome at specific sites and then decorated with lactose repressor that has been fused to the green fluorescent protein (GFP) (20Gordon G.S Sitnikov D Webb C.D Teleman A Straight A Losick R Murray A.W Wright A Chromosome and low copy plasmid segregation in E. coli visual evidence for distinct mechanisms.Cell. 1997; 90: 1113-1121Abstract Full Text Full Text PDF PubMed Scopus (324) Google Scholar, 80Webb C.D Teleman A Gordon S Straight A Belmont A Lin D.C.-H Grossman A.D Wright A Losick R Bipolar localization of the replication origin region of chromosomes in vegetative and sporulating cells of B. subtilis.Cell. 1997; 88: 667-674Abstract Full Text Full Text PDF PubMed Scopus (305) Google Scholar). This makes it possible to visualize in living cells any site on the chromosome at which the operator cassette has been inserted. Similarly, GFP can be fused to a protein that naturally binds near the origin of replication (18Glaser P Sharpe M.E Raether B Perego M Ohlsen K Errington J Dynamic, mitotic-like behavior of a bacterial protein required for accurate chromosome partitioning.Genes Dev. 1997; 11: 1160-1168Crossref PubMed Scopus (270) Google Scholar, 50Lin D.C.H Levin P.A Grossman A.D Bipolar localization of a chromosome partition protein in Bacillus subtilis.Proc. Natl. Acad. Sci. USA. 1997; 94: 4721-4726Crossref PubMed Scopus (210) Google Scholar, 58Mohl D.A Gober J.W Cell cycle–dependent polar localization of chromosome partitioning proteins in Caulobacter crescentus.Cell. 1997; 88: 675-684Abstract Full Text Full Text PDF PubMed Google Scholar). Finally, specific sites on the chromosome can be visualized with high resolution in fixed cells by use of fluorescence in situ hybridization (FISH) (Niki and Hirago, 1998; 35Jensen R.B Shapiro L The Caulobacter crescentus smc gene is required for cell cycle progression and chromosome segregation.Proc. Natl. Acad. Sci. USA. 1999; 96: 10661-10666Crossref PubMed Scopus (178) Google Scholar). In their classic paper on the regulation of DNA replication, Jacob, Brenner, and Cuzin (30Jacob F Brenner S Cuzin F On the regulation of DNA replication in bacteria.Cold Spring Harb. Symp. Quant. Biol. 1963; 28: 329-348Crossref Google Scholar) imagined that replication origins would attach at the midcell and be pushed apart by growth of the cell wall in between. But now we know by direct inspection that just the opposite is the case: during the cell cycle, newly duplicated origins of replication abruptly separate from each other, moving toward opposite ends of the cell (Figure 1; 18Glaser P Sharpe M.E Raether B Perego M Ohlsen K Errington J Dynamic, mitotic-like behavior of a bacterial protein required for accurate chromosome partitioning.Genes Dev. 1997; 11: 1160-1168Crossref PubMed Scopus (270) Google Scholar, 20Gordon G.S Sitnikov D Webb C.D Teleman A Straight A Losick R Murray A.W Wright A Chromosome and low copy plasmid segregation in E. coli visual evidence for distinct mechanisms.Cell. 1997; 90: 1113-1121Abstract Full Text Full Text PDF PubMed Scopus (324) Google Scholar, 47Lewis P.J Errington J Direct evidence for active segregation of oriC regions of the Bacillus subtilis chromosome and co-localization with the SpoOJ partitioning protein.Mol. Microbiol. 1997; 25: 945-954Crossref PubMed Scopus (158) Google Scholar, 50Lin D.C.H Levin P.A Grossman A.D Bipolar localization of a chromosome partition protein in Bacillus subtilis.Proc. Natl. Acad. Sci. USA. 1997; 94: 4721-4726Crossref PubMed Scopus (210) Google Scholar, 80Webb C.D Teleman A Gordon S Straight A Belmont A Lin D.C.-H Grossman A.D Wright A Losick R Bipolar localization of the replication origin region of chromosomes in vegetative and sporulating cells of B. subtilis.Cell. 1997; 88: 667-674Abstract Full Text Full Text PDF PubMed Scopus (305) Google Scholar, 81Webb C.D Graumann P.L Kahana J.A Teleman A.A Silver P.A Losick R Use of time-lapse microscopy to visualize rapid movement of the replication origin region of the chromosome during the cell cycle in Bacillus subtilis.Mol. Microbiol. 1998; 28: 883-892Crossref PubMed Scopus (170) Google Scholar, 60Niki H Hiraga S Polar localization of the replication origin and terminus in E. coli nucleoids during chromosome partitioning.Genes Dev. 1998; 12: 1036-1045Crossref PubMed Scopus (173) Google Scholar, 35Jensen R.B Shapiro L The Caulobacter crescentus smc gene is required for cell cycle progression and chromosome segregation.Proc. Natl. Acad. Sci. USA. 1999; 96: 10661-10666Crossref PubMed Scopus (178) Google Scholar). Meanwhile, the terminus is preferentially located near the cell middle. What is the nature of the motor that drives segregation? Recent evidence suggests that multiple force-generating processes may contribute to chromosome movement. The remarkable discovery that DNA polymerase remains relatively stationary over the course of the cell cycle has prompted the idea that the replication machinery is a factory through which the DNA is threaded and duplicated (as opposed to a locomotive that chugs down the DNA) (42Lemon K.P Grossman A.D Localization of bacterial DNA polymerase evidence for a factory model of replication.Science. 1998; 282: 1516-1519Crossref PubMed Scopus (405) Google Scholar). If so, then perhaps the cell harnesses the force of replication itself in driving replication origins apart. This leads to two questions: how are newly duplicated origins directed away from each other, and is there a tether that anchors them at or near the cell poles? Once origins are separated, the remainder of the chromosomes must be pushed or pulled apart to achieve complete segregation. An attractive possibility is that this is accomplished by compaction of the chromosomes. Bacterial chromosomes must indeed be highly compacted because their contour length is 1000 times longer than the nucleoid into which they are folded. At the same time, the order of genes across the nucleoid seems, at first approximation, to preserve the order of genes in the chromosome: that is, the replication origin and the terminus are located at opposite ends of the nucleoid with other sites on the chromosome located in between (60Niki H Hiraga S Polar localization of the replication origin and terminus in E. coli nucleoids during chromosome partitioning.Genes Dev. 1998; 12: 1036-1045Crossref PubMed Scopus (173) Google Scholar, 78Teleman A.A Graumann P.L Lin D.C.H Grossman A.D Losick R Chromosome arrangement within a bacterium.Curr. Biol. 1998; 8: 1102-1109Abstract Full Text Full Text PDF PubMed Scopus (164) Google Scholar). Thus, the chromosome is folded in a highly ordered manner. A clue to understanding how this compaction occurs is the discovery that many bacteria have a member of the eukaryotic family of chromosome-condensing proteins known as SMC proteins. B. subtilis and Caulobacter cells mutant for SMC, and E. coli cells mutant for a similar but nonhomologous protein MukB, have diffuse chromosomes (61Niki H Jaffé A Imamura R Ogura T Hiraga S The new gene mukB codes for a 177 kd protein with the coiled-coil domains involved in chromosome partitioning in E. coli.EMBO J. 1991; 10: 183-193Crossref PubMed Scopus (306) Google Scholar, 5Britton R.A Lin D.C Grossman A.D Characterization of a prokaryotic SMC protein involved in chromosome partitioning.Genes Dev. 1998; 12: 1254-1259Crossref PubMed Scopus (238) Google Scholar, 35Jensen R.B Shapiro L The Caulobacter crescentus smc gene is required for cell cycle progression and chromosome segregation.Proc. Natl. Acad. Sci. USA. 1999; 96: 10661-10666Crossref PubMed Scopus (178) Google Scholar). B. subtilis SMC and E. coli MukB mutants generate a high proportion of anucleate cells, whereas a Caulobacter SMC mutant exhibits a cell cycle block just before division. This may reflect a cell cycle checkpoint that precludes anucleate cell formation. Thus, compaction proteins evidently play a key role in chromosome segregation. Further facilitating chromosome segregation in B. subtilis and Caulobacter (but not E. coli) is another class of compaction proteins, members of the ParB family of partition proteins (29Ireton K Gunther IV, N.W Grossman A.D spo0J is required for normal chromosome segregation as well as the initiation of sporulation in Bacillus subtilis.J. Bacteriol. 1994; 176: 5320-5329Crossref PubMed Scopus (287) Google Scholar, 18Glaser P Sharpe M.E Raether B Perego M Ohlsen K Errington J Dynamic, mitotic-like behavior of a bacterial protein required for accurate chromosome partitioning.Genes Dev. 1997; 11: 1160-1168Crossref PubMed Scopus (270) Google Scholar, 50Lin D.C.H Levin P.A Grossman A.D Bipolar localization of a chromosome partition protein in Bacillus subtilis.Proc. Natl. Acad. Sci. USA. 1997; 94: 4721-4726Crossref PubMed Scopus (210) Google Scholar, 58Mohl D.A Gober J.W Cell cycle–dependent polar localization of chromosome partitioning proteins in Caulobacter crescentus.Cell. 1997; 88: 675-684Abstract Full Text Full Text PDF PubMed Google Scholar). In Caulobacter ParB binds to a region of the chromosome near the origin (58Mohl D.A Gober J.W Cell cycle–dependent polar localization of chromosome partitioning proteins in Caulobacter crescentus.Cell. 1997; 88: 675-684Abstract Full Text Full Text PDF PubMed Google Scholar), and in B. subtilis the replication origin region is decorated with multiple copies of a ParB homolog called Spo0J (49Lin D.C.-H Grossman A.D Identification and characterization of a bacterial chromosome partitioning site.Cell. 1998; 92: 675-685Abstract Full Text Full Text PDF PubMed Scopus (327) Google Scholar). Spo0J binds to scattered sites in the vicinity of the origin and may cause this region to fold up into a higher ordered structure. ParB proteins also play a role in the inheritance of certain minichromosomes, such as the fertility plasmid F. These minichromosomes localize to the midcell prior to replication and to the cell quarter points following duplication (20Gordon G.S Sitnikov D Webb C.D Teleman A Straight A Losick R Murray A.W Wright A Chromosome and low copy plasmid segregation in E. coli visual evidence for distinct mechanisms.Cell. 1997; 90: 1113-1121Abstract Full Text Full Text PDF PubMed Scopus (324) Google Scholar, 59Niki H Hiraga S Subcellular distribution of actively partitioning F plasmid during the cell division cycle in E. coli.Cell. 1997; 90: 951-957Abstract Full Text Full Text PDF PubMed Scopus (163) Google Scholar). The N-terminal region of the ParB family member SopB contains topographic information that is responsible for localization to the cell quarter points (22Hanai R Liu R Benedetti P Caron P.R Lynch A.S Wang J.C Molecular dissection of a protein SopB essential for Escherichia coli F plasmid partition.J. Biol. Chem. 1996; 271: 17469-17475Crossref PubMed Scopus (54) Google Scholar). Meanwhile, the C-terminal region binds to a centromere-like sequence called sopC in the F plasmid, thereby tethering F to the quarter points of the cell. SopB has an additional property that illustrates the conceptual power of viewing the bacterial cell in three dimensions. When bound to sopC, SopB blocks the transcription of DNA that abuts the centromere (56Lynch A.S Wang J.C SopB protein-mediated silencing of genes linked to the sopC locus of Escherichia coli F plasmid.Proc. Natl. Acad. Sci. USA. 1995; 92: 1896-1900Crossref PubMed Scopus (105) Google Scholar). One model for this silencing phenomenon holds that the centromere is a nucleation site for the polymerization of the partition protein, which extends into flanking DNA (71Rodionov O Lobocka M Yarmolinsky M Silencing of genes flanking the P1 plasmid centromere.Science. 1999; 283: 546-549Crossref PubMed Scopus (181) Google Scholar). However, the discovery that SopB localizes to a particular place in the cell has given rise to an alternative model (40Kim S.K Wang J.C Gene silencing via protein-mediated subcellular localization of DNA.Proc. Natl. Acad. Sci. USA. 1999; 96: 8557-8561Crossref PubMed Scopus (34) Google Scholar). This alternative model posits that the plasmid is tethered via its centromere to a velcro-like patch of SopB molecules on the cell membrane. DNA flanking the centromere would then bind to the velcro-like patch in a non-sequence-specific manner. Rather than polymerization from a nucleation site, a high localized concentration of SopB causes interaction with DNA flanking the centromere. Thus, the concept of the bacterial cell as a spatial grid offers a new way to think about an old problem. The generation of diverse cell types is a fundamental task for all organisms that carry out developmental programs or produce cells with unique functions. In both prokaryotes and eukaryotes, a cell division that yields progeny cells with distinct cell fates can occur by the asymmetric distribution of regulatory factors and structural proteins in the predivisional cell. This intrinsic asymmetry is an inseparable function of the cell cycle regulatory network. The asymmetric partition of cell fate determinants depends on their directed localization to discrete regions of the cell prior to division. Thus, a spatial vector becomes an integral part of cell cycle mechanics. For example, during Drosophila neurogenesis, the directional alignment of the neuroblast mitotic spindle is a critical component in the asymmetric localization of factors that result, upon division, in the production of progeny with different cell fates positioned with the correct spatial vectors in the embryo (54Lu B Jan L.Y Jan Y.-N Asymmetric cell division lessons from flies and worms.Curr. Opin. Genet. Dev. 1998; 8: 392-399Crossref PubMed Scopus (46) Google Scholar). In budding yeast, actin cables form a railway system at a specific time in the cell cycle that is used to deliver cell fate determinants to the site of a new bud (52Long R.M Singer R.H Meng X Gonzales I Nasmyth K Jansen R.P Mating type switching in yeast controlled by asymmetric localization of ASH1 in RNA.Science. 1997; 277: 383-387Crossref PubMed Scopus (398) Google Scholar, 77Takizawa P.A Sil A Swedlow J.R Herskowitz I Vale R.D Actin dependent localization of an RNA encoding a cell-fate determinant in yeast.Nature. 1997; 389: 90-93Crossref PubMed Scopus (303) Google Scholar). How then in a prokaryotic cell, with no obvious directional tracks, is intrinsic asymmetry accomplished? In Caulobacter, the cell cycle is inherently asymmetric and the challenge is to understand how this asymmetry is maintained generation after generation. An asymmetric predivisional cell undergoes cytokinesis to give rise to dissimilar progeny cells, the swarmer and the stalked cell (for review, see 83Wheeler R.T Gober J.W Shapiro L Protein localization during the Caulobacter crescentus cell cycle.Curr. Opin. Microbiol. 1998; 1: 636-642Crossref PubMed Scopus (13) Google Scholar). As the name implies, the swarmer cell swims off to find nutrients propelled by a single polar flagellum. The initiation of chromosome replication is repressed in this cell for a constant fraction of the cell cycle. The progeny stalked cell functions as a nonmotile stem cell, immediately initiating DNA replication. The progeny swarmer cell initiates DNA replication only once it ejects its flagellum and grows a new stalk in its place. Thus, the swarmer-to-stalked cell transition is comparable to a G1-to-S transition. During S phase, the chemotaxis genes are activated and the resulting chemotaxis protein complex is localized to the new swarmer pole of the predivisional cell (1Alley M.R Maddock J.R Shapiro L Polar localization of a bacterial chemoreceptor.Genes Dev. 1992; 6: 825-836Crossref PubMed Scopus (168) Google Scholar), as are the proteins involved in flagellar biogenesis. At or near the completion of DNA replication, the ccrM gene, which encodes a DNA methyltransferase that is essential for viability, is transcribed, and its product rapidly brings the newly replicated chromosomes from hemimethylation to full methylation (76Stephens C Reisenauer A Wright R Shapiro L A cell cycle-regulated bacterial DNA methyltransferase is essential for viability.Proc. Natl. Acad. Sci. USA. 1996; 93: 1210-1214Crossref PubMed Scopus (163) Google Scholar). These cell cycle events and their time and place of execution are controlled by members of the two-component signal transduction protein family. The response regulator CtrA is the direct instrument of DNA replication control, the control of initiation of transcription of the cascade of flagellar genes, and the transcription of the CcrM DNA methyltransferase (65Quon K.C Marczynski G.T Shapiro L Cell cycle control by an essential bacterial two-component signal transduction protein.Cell. 1996; 84: 83-93Abstract Full Text Full Text PDF PubMed Scopus (366) Google Scholar). CtrA also contributes to the cell cycle–controlled transcription of the ftsZ gene encoding the tubulin-like cell division protein (39Kelly A.J Sackett M.J Din N Quardokus E Brun Y.V Cell cycle-dependent transcriptional and proteolytic regulation of FtsZ in Caulobacter.Genes Dev. 1998; 12: 880-893Crossref PubMed Scopus (170) Google Scholar). The CckA histidine kinase is responsible, directly or indirectly, for the phosphorylation of CtrA, to create the active CtrA∼P regulatory protein (32Jacobs C Domian I.J Maddock J.R Shapiro L Cell cycle-dependent polar localization of an essential bacterial histidine kinase that controls DNA replication and cell division.Cell. 1999; 97: 111-120Abstract Full Text Full Text PDF PubMed Scopus (198) Google Scholar). Not surprisingly, both of these proteins are essential for cell viability. What is surprising is that both of these proteins, in quite different ways, are spatially restricted at different stages of the cell cycle (Figure 3). In the predivisional cell, CtrA∼P is confined to the swarmer portion of the predivisional cell where it binds to the origin of replication and represses the initiation of DNA replication (66Quon K.C Yang B Domian I.J Shapiro L Marczynski G.T Negative control of bacterial DNA replication by a cell cycle regulatory protein that bends at the chromosomal origin.Proc. Natl. Acad. Sci. USA. 1998; 95: 120-125Crossref PubMed Scopus (254) Google Scholar), while CtrA∼P is cleared from the stalked portion of the predivisional cell by proteolysis (12Domian I.J Quon K.C Shapiro L Cell type-specific phosphorylation and proteolysis of a transcriptional regulator controls the G1-to-S transition in a bacterial cell cycle.Cell. 1997; 90: 415-424Abstract Full Text Full Text PDF PubMed Scopus (321) Google Scholar), allowing the initiation of DNA replication as soon as division has occurred. The eventual transition of the progeny swarmer cell into a new stalked cell is accompanied by the clearance of CtrA∼P by ClpP/ClpX-dependent proteolysis (33Jenal U Fuchs T An essential protease involved in bacterial cell cycle control.EMBO J. 1998; 17: 5658-5669Crossref PubMed Scopus (236) Google Scholar), freeing the replication origin for re
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