Introduction to Bacterial Cell Division:
A typical cell division in bacteria consist of four common steps of signal that drives cell to divide, DNA replication that coincides with the M phase of division (S=M), DNA segregation and Cytokinesis.
Different modes of bacterial cell division:
Lateral cell elongation: MreB mediates this process, which arranges peptidoglycan production proteins in a helical manner along the cell wall. Prior to constriction, the newly produced peptidoglycan passes through zonal insertion in the mid-cell and is reliant on FtsZ. Two extensively studied model organisms, Bacillus subtilis and Escherichia coli, form symmetric rods with a single polar axis.
Peptidoglycan is absent from chlamydiae and planctomycetes. They don’t divide in a way that depends on FtsZ. A holdfast fixed at the head of a lengthy stalk allows many Planctomycetes species to grow adhering to surfaces.
Similar to other stalked Alphaproteobacteria such as Rhizobia and Caulobacter, daughter cells have a polar or subpolar flagellum at birth and subsequently grow a stalk at or close to the flagellar pole. Actinobacteria mostly follow fragmentation but in some Actinobacteria like Nocardia, Propionobacteria, polar elongation occurs.
Preseptal growth is observed in certain bacterial species where peptidoglycan is produced at the future division site before the complete formation of the Z-ring. A notable example of this growth pattern is found in Agrobacterium tumefaciens.
Among bacteria, binary fission stands out as the predominant strategy for cellular reproduction, representing the most frequently occurring mode of division.
Steps of binary fission in E coli:
In E. coli, binary fission is the process by which a cell divides into two about equal progeny cells. This process is used in the majority of bacteria. Steps in Binary fission are DNA replication, formation of divisome which is needed for cell elongation, cell septum formation, completion of septum with formation of distinct wall and cell separation.
Binary fission in E coli, like any other bacteria, is governed by the divisome, a ring-shaped structure that orchestrates synthesis of new cytoplasmic membrane and cell wall material until the cell doubles in length.
The bacterial chromosome is often attached to the plasma membrane around the center of the cell. Binary fission cannot occur until the cell separates its DNA copies to the opposite ends of the cell.
Functions of Key Proteins: FtsZ, ZipA, FtsA, and Others:
Following DNA replication and segregation of the nucleoids, a Z-ring is formed in the free space in the middle of the cell; It is made up of filaments and what appear to be filament bundles of FtsZ in addition to the anchor proteins ZipA and FtsA, which allow FtsZ filaments to adhere to the inside of the cytoplasmic membrane. The Z-ring then assembles a large number of additional proteins to create a mature divisome, which facilitates the septum’s development and two-way cell division. Numerous protein types make up the machinery of cell division, which assembles at the site of subsequent division. An integral component of this system is the protein FtsZ. The ring is formed by polymerization of about 10,000 FtsZ.
FtsZ
The FtsZ ring functions as microtubules, providing a track that is traversed by a motor protein that is responsible for the progressive shortening of the ring during cytokinesis.
Once the FtsZ ring forms, additional components of the cell division machinery assemble at that site. This complex is precisely positioned to ensure the cell divides without harming the DNA. During division, the cytoplasm is evenly separated, and a new cell wall forms between the two resulting cells.
The interaction of FtsZ with the cytoplasmic membrane at the future division site is an obligatory early step in the sequence of events that ends in septa formation and cell separation. Zip A anchors FtsZ to cytoplasmic membrane. in a relatively early point in the division cycle, the ZipA–FtsZ interaction takes place in the midcell site.
Apart from FtsZ and ZipA, cytokinesis requires at least seven other protein components in the Assembly of the Cytokinesis Machinery. These comprise six integral cytoplasmic membrane proteins (FtsI, FtsQ, FtsL, FtsW, FtsN, and FtsK) and one peripheral membrane protein (FtsA).
FtsA
FtsA is ATP hydrolyzing enzyme that provides energy for assembly of many proteins into division plane.
Specific functions of many Fts proteins are not clearly understood.
FtsI is an enzyme involved in building the murein (peptidoglycan) layer, specifically during the formation of the division septum. However, it is not necessary for murein synthesis that occurs as the cell grows in length.
FtsQ have the essential role of its large periplasmic domain but no quite of the membranespanning and cytoplasmic domains.
The cytoskeletal protein MreB is an actin analog that winds as a coil through the long axis of a rod-shaped cell, making contact with the cytoplasmic membrane in several locations. New cell wall synthesis takes place here. The synthesis of the lateral cell wall, which needs FtsL to act as PBP, is guided by MreB and FtsZ. FtsL plays role in division specific peptidoglycan synthesis.
Peptidoglycan Synthesis and Cell Wall Remodeling:
During peptidoglycan synthesis, old peptidoglycan is severed to allow formation of new peptidoglycan. Autolysins create openings at cell wall lysing b 1,4 glycosidic bond and new cell wall materials is added to repair holes. Bactoprenol crosses the membrane and enters the periplasm with peptidoglycan precursors. In this case, transglycosylase fixes the developing glycan backbone and adds a precursor. The peptide linkages between the glycan backbones are fixed by transpeptidase.
Thus, the formation of a septum that extends from the cell’s periphery toward its center causes the chromosomes to pull apart at opposite ends of the cell as the membrane enlarges and distributes cytoplasmic components to two developing cells. Once the new cell walls are in place, the daughter cells split apart and the division process is finished.
Fig: Cell division in E. coli
References:
- Haeusser, D. P., & Margolin, W. (2016). Splitsville: structural and functional insights into the dynamic bacterial Z ring. Nature reviews. Microbiology, 14(5), 305–319. https://doi.org/10.1038/nrmicro.2016.26