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ScienceWeek
MICROBIOLOGY: ON THE INTERNAL ORGANIZATION OF BACTERIA
The following points are made by Jakob Mueller-Jensen and Kenn Gerdes (Science 2004 306:987):
1) Bacteria are endowed with a considerable degree of internal organization. Thanks to fluorescence microscopy, it is now clear that many bacterial components -- DNA as well as proteins -- are found in specific subcellular locations. Indeed, the discovery of prokaryotic homologs of both tubulin and actin, which are key components of eukaryotic cellular organization, has overturned the textbook credo that cytoskeletons are exclusive to eukaryotes [1]. Garner et al [2] have provided the first quantitative biochemical study of a bacterial actin-like protein called ParM. They demonstrate that ParM is a dynamic polymerization engine that drives the segregation of DNA plasmids during bacterial cell division.
2) Accumulating evidence suggests that bacterial proteins related to tubulin and actin may be involved in bacterial cell division and DNA segregation. For example, FtsZ, a bacterial homolog of tubulin, is the principal component of the cytokinetic Z-ring that constricts the middle of the dividing bacterium [3]. MinCD is an inhibitor of Z-ring assembly that oscillates rapidly from pole to pole, ensuring that the Z-ring forms only at the bacterial midcell position [4]. The MinCD proteins are organized into extended membrane-associated coiled structures that wind throughout the bacterium between the two poles [5]. Rod-shaped bacteria contain actin homologs such as MreB, which forms helical filaments just beneath the bacterial cell surface that help to determine cell shape and may contribute to DNA segregation. Both the FtsZ ring and the MreB helices are highly dynamic structures that undergo continuous cycles of remodeling.
3) Bacterial plasmids are autonomous genetic elements composed of DNA that encode genes conferring resistance to, for example, antibiotics or heavy metals. Such plasmids must be segregated evenly between the daughter cells during bacterial cell division. The par (partitioning) locus of the R1 drug-resistance plasmid encodes three components: a centromere-like site in the DNA (parC), a DNA-binding protein (ParR), and a protein with adenosine triphosphatase (ATPase) activity (ParM). These components form a minimalist mitotic spindle, which positions pairs of plasmids at opposite ends of the rod-shaped bacterium, ensuring equal distribution of the plasmids between the daughter cells.
4) An important clue to how the R1 par operon works came from the discovery that ParM forms dynamic filamentous structures that extend along the longitudinal axis of the bacterium Escherichia coli. These ParM filaments are visible in only about 40% of a bacterial population at any given time, indicating that they are transient. R1 plasmids are located at the ParM filament tips. This implies that polymerization of ParM molecules into filaments could be the driving force that pushes the plasmids to opposite poles of dividing rod-shaped bacteria.
References (abridged):
1. J. Lowe, F. van den Ent, L. A. Amos, Annu. Rev. Biophys. Biomol. Struct. 33, 177 (2004)
2. E. C. Garner, C. S. Campbell, R. D. Mullins, Science 306, 1021 (2004)
3. E. F. Bi, J. Lutkenhaus, Nature 354, 161 (1991)
4. D. M. Raskin, P. A. de Boer, Proc. Natl. Acad. Sci. U.S.A. 96, 4971 (1999)
5. Y. L. Shih, T. Le, L. Rothfield, Proc. Natl. Acad. Sci. U.S.A. 100, 7865 (2003)
Science http://www.sciencemag.org
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MICROBIOLOGY: ON THE SHAPES OF BACTERIA
The following points are made by William Margolin (Current Biology 2004 14:R242):
1) Eukaryotic cells have three main types of cytoskeletal element: microtubules, composed of tubulin; microfilaments, composed of actin; and intermediate filaments, made up of proteins from a number of different families. A decade ago, the evolutionary origin of these cytoskeletal systems was unknown. But a great deal of structural and biochemical evidence now points to FtsZ and MreB as being the prokaryotic ancestors of tubulin and actin, respectively [1,2]. This suggests, not only that eukaryotic cytoskeletal proteins arose early in evolution, but also that bacteria have a cytoskeleton.
2) Whereas microtubules are involved in locomotion, mitosis and some aspects of cytokinesis, FtsZ seems to be limited to cytokinesis in prokaryotes and some eukaryotic organelles [3,4]. Actin is involved in many processes, including locomotion, cell growth, and cytokinesis, whereas MreB proteins function in cell growth and shape, and have an as yet unclear role in chromosome segregation [5]. The cell shape function of the MreB family of proteins is striking. For example, inactivating a MreB homolog in normally cylindrical bacteria, such as Escherichia coli or Bacillus subtilis, causes the cells to lose their cylindrical shape and become the "default" round shape.
3) Consistent with the view that MreB plays a role in imposing a more complex cylindrical cell architecture, naturally round bacteria such as streptococci do not have mreB homologs in their genomes. However, rhizobia and corynebacteria do not have MreB, but are still cylindrically shaped, possibly because they grow at their tips like filamentous fungi [5]. Their rod shape may depend on other proteins that are yet to be discovered.
4) If MreB proteins are required for maintaining a cylindrical shape in most bacteria, then what might confer what appear to be more complex shapes, such as helices? For example, vibrioid ("comma") or helical shapes are characteristic of a relatively small number of bacterial species. Is there a gene to keep a helical shape from reverting to a straight cylinder, analogous to the need for mreB in cylindrical shape? Or is helical shape the default, with specific genes being required for straight cylinders?
References (abridged):
1. van den Ent, F., Amos, L.A., and Lowe, J. (2001). Prokaryotic origin of the actin cytoskeleton. Nature 413, 39-44
2. L÷we, J. and Amos, L.A. (1998). Crystal structure of the bacterial cell-division protein FtsZ. Nature 391, 203-206
3. Margolin, W. (2000). Themes and variations in prokaryotic cell division. FEMS Microbiol. Rev. 24, 531-548
4. Osteryoung, K.W. and Nunnari, J. (2004). The division of endosymbiotic organelles. Science 302, 1698-1704
5. Daniel, R.A. and Errington, J. (2003). Control of cell morphogenesis in bacteria: two distinct ways to make a rod-shaped cell. Cell 113, 767-776
Current Biology http://www.current-biology.com
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MICROBIOLOGY: BACTERIAL CHROMOSOME DYNAMICS
The following points are made by David J. Sherratt (Science 2003 301:780):
1) Central to the life process is the organization of DNA into chromosomes that allow the DNA to be managed in concert with other cellular activities. Negative supercoiling of DNA and SMC (structural maintenance of chromosomes) proteins play ubiquitous roles in chromosome organization, whereas the use of histones in chromosome compaction and organization is restricted to eukaryotes and archaea. The integrity of DNA is maintained using conserved repair and recombinational mechanisms, and highly accurate DNA replication is a prelude to the faithful transmission of newly replicated chromosomes to daughter cells before cell division.
2) The mechanism of DNA replication and its control through initiation is conserved through all branches of life, although bacterial chromosomes control replication from a single origin per chromosome, whereas eukaryote chromosomes contain multiple origins. Ubiquitous AAA proteins [adenosine triphosphatases (ATPases) associated with various cellular activities] play key roles in eukaryote and prokaryote DNA replication initiation, replication fork progression, recombination and chromosome segregation, by using the energy of ATP hydrolysis to translocate DNA, separate DNA strands, and remodel nucleoprotein complexes.
3) The segregation of bacterial chromosomes, commonly containing 1000 to 5000 genes within a single DNA molecule, may seem to be less of a challenge than the segregation of the multiple chromosomes of eukaryote cells. However, bacteria have no mitotic apparatus and segregate chromosomes to daughter cells as they replicate. Furthermore, some bacteria have multiple chromosomes, and many contain plasmids that encode proteins crucial to that organism's life-style. The replication and segregation to daughter bacterial cells of multiple replicating units (replicons) must be precisely controlled and coordinated with the cell cycle if the descendants of a cell are to maintain the genetic identity of their parent. Remarkably, some plasmids have evolved to be transferred to and maintained within diverse bacterial species, indicating autonomy of replication control, though they retain an ability to interact with diverse host environments.
4) In summary: Bacterial chromosomes are highly compacted structures and share many properties with their eukaryote counterparts, despite not being organized into chromatin or being contained within a cell nucleus. Proteins conserved across all branches of life act in chromosome organization, and common mechanisms maintain genome integrity and ensure faithful replication. The principles that underlie chromosome segregation in bacteria and eukaryotes share similarities, although bacteria segregate DNA as it replicates and lack a eukaryote-like mitotic apparatus for segregating chromosomes. This may be because the distances that newly replicated bacterial chromosomes move apart before cell division are small as compared to those in eukaryotes. Bacteria specify positional information, which determines where cell division will occur and which places the replication machinery and chromosomal loci at defined locations that change during cell cycle progression.
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