Introduction to Bacterial Binary Fission:
The genetic material of bacteria is often divided equally into two daughter cells during a vegetative cell division process called binary fission.
Throughout the cell division cycle, the bacterium must first pinpoint the central region where division will take place. This midcell location is then marked and primed for the upcoming process of cytokinesis. Subsequently, the division septum is constructed through the synchronized inward growth of several structural layers: the cytoplasmic membrane, the tough peptidoglycan (murein) wall, and—specifically in Gram-negative organisms like Escherichia coli—the outer membrane of the cell envelope.
Binary fission involves the following steps: DNA replication, divisome creation (which is necessary for cell elongation), cell septum formation, septum completion with the formation of a separate wall, and cell separation.
Divisome- Structure and Significance:
The divisome is a sophisticated multicomponent molecular mechanism that controls cell division in bacteria. It typically has a ring-like shape and allows a septum to form between the daughter cells. Until the cell doubles in length, divosome creation coordinates the synthesis of new cell wall and cytoplasmic membrane material.
Z-ring Formation and FtsZ Polymerization:
- A Z-ring is created in the free space in the center of the cell after DNA replication and nucleoid segregation. In addition to the anchor proteins FtsA and ZipA, which enable FtsZ filaments to adhere to the interior of the cytoplasmic membrane, it is composed of filaments and what seem to be filament bundles of FtsZ. In order to establish the maturation of the divisome and aid the creation of the septum and cell division in two, Z-ring then enlists a sizable number of additional proteins.
It is thought that the essential protein for bacterial cell division is FtsZ. It is the first to be drawn to the site of division and, in conjunction with anchor proteins, serves as a framework for other divisome proteins. - Firstly, the FtsZ is distributed in the entire cytoplasm. As replication begins, chromosome begins to separate. In this stage, FtsZ polymerization occurs.
- At the site of future cell division, FtsZ polymerizes into filamentous structures that come together to form a distinctive ring called the Z ring. Membrane-bound proteins that interact with FtsZ, such ZipA and FtsA, affix this ring to the inner surface of the cell envelope.
- These elements come together to form the proto-ring, an early assembly.
- Approximately 10,000 FtsZ molecules contribute to constructing this ring. The proto-ring then serves as a scaffold, attracting and organizing additional proteins needed for divisome formation.
Role of Anchor and divisome Proteins:
- Under typical conditions, FtsA and ZipA are essential for the sequential recruitment of these downstream division proteins.
- FtsA forms complex in membrane that hydrolyses ATP to provide energy for divisome assembly. Zip A is connected to Cytoplasmic membrane and anchors FtsZ ring to membrane.
- The mature divisome is formed in the second stage when the proto-ring attracts proteins like FtsN and FtsBLQ as well as enzymes like FtsI that are involved in cell wall (septum) formation.
- Divisome generates area where cell is going to divide. When the chromosome is completely divided, FtsZ ring is formed in middle where cell will divide. Divisome assembly triggers initiation of inner membrane constriction by GTP hydrolysis.
- The position of the Z ring is directed by MinCDE system. MinD is bound to the membrane and it recruits MinC. Membrane localization of MinC is necessary for FtsZ inhibition.
- MinE oscillates from pole to pole and it polymerises MinCD dimers in pole which allows FtsZ ring to form in middle of cell.
- FtsI is transpeptidase that mediates transpeptidation during peptidoglycan formation.
- FtsK separates chromosome during cell division.
- Z ring constriction and coordinated peptidoglycan production work together to achieve septal invagination.
- FtsZ completes constriction. Membrane gets pinched off and peptidoglycan and cell wall synthesis continues until scission between two cells happens that gets separated as peptidoglycan layer and cell wall synthesis is completely made.
- FtsA recruits FtsN. The periplasmic region of FtsN is directed toward the peptidoglycan layer, which helps stabilize its positioning and may also activate FtsI, thereby promoting the production of septal peptidoglycan.
- An actin homolog, the cytoskeletal protein MreB coils around the long axis of a rod-shaped cell and contacts the cytoplasmic membrane multiple times. These are locations where new cell walls are made.
Peptidoglycan Synthesis and Remodeling:
Old peptidoglycan is broken off to make room for new peptidoglycan to form during peptidoglycan production. Autolysins create openings at cell wall lysing b 1,4 glycosidic bond and new cell wall materials is added to repair holes. Bactoprenol transfer peptidoglycan precursors across the membrane into periplasm. Here, transglycosylase fix and add precursor to the nascent glycan backbone. Tanspeptidase fix the peptide bonds between the glycan backbones
So, FtsI for septal growth must couple their enzymatic activity to membrane constriction and localized cell wall hydrolysis. In non-chaining species, hydrolases work to separate bacterial daughter cells after membrane constriction and septation.
 Fig: FtsZ and the division of prokaryotic cells and organelles
Fig: FtsZ and its related subassemblies involved in divisome formation
References:
- Du, S., & Lutkenhaus, J. (2017). Assembly and activation of the Escherichia coli divisome. Molecular microbiology, 105(2), 177–187. https://doi.org/10.1111/mmi.13696
- Margolin W. (2005). FtsZ and the division of prokaryotic cells and organelles. Nature reviews. Molecular cell biology, 6(11), 862–871. https://doi.org/10.1038/nrm1745
- Rico, A. I., Krupka, M., & Vicente, M. (2013). In the beginning, Escherichia coli assembled the proto-ring: an initial phase of division. The Journal of biological chemistry, 288(29), 20830–20836. https://doi.org/10.1074/jbc.R113.479519