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Original- "Heterocyst"
Single heterocysts develop about every 9-15 cells, producing a one-dimensional pattern along the filament. The interval between heterocysts remains approximately constant even though the cells in the filament are dividing. The bacterial filament can be seen as a multicellular organism with two distinct yet interdependent cell types. Such behavior is highly unusual in prokaryotes and may have been the first example of multicellular patterning in evolution. Once a heterocyst has formed it cannot revert to a vegetative cell. Certain heterocyst-forming bacteria can differentiate into spore-like cells called akinetes or motile cells called hormogonia, making them the most phenotyptically versatile of all prokaryotes.

The mechanism of controlling heterocysts is thought to involve the diffusion of an inhibitor of differentiation called patS. Heterocyst formation is inhibited in the presence of a fixed nitrogen source, such as ammonium or nitrate. Heterocyst maintenance is dependent on an enzyme called hetN. The bacteria may also enter a symbiotic relationship with certain plants. In such a relationship, the bacteria do not respond to the availability of nitrogen, but to signals produced by the plant. Up to 60% of the cells can become heterocysts, providing fixed nitrogen to the plant in return for fixed carbon.

The following sequences take place in formation of heterocysts from a vegetative cell:
 * The cell enlarges.
 * Granular inclusions decrease.
 * Photosynthetic lammele reorients.
 * The wall finally becomes triple-layered. These three layers develop outside the cell's outer layer.
 * The middle layer is homogeneous.
 * The inner layer is laminated.
 * The senescent heterocyst undergoes vacuolation and finally breaks off from the filament causing fragmentation. These fragments are called hormogonia and undergo asexual reproduction.

Edit-"heterocyst"
Single heterocysts develop about every 9-15 cells, producing a one-dimensional pattern along the filament. The interval between heterocysts remains approximately constant even though the cells in the filament are dividing. The bacterial filament can be seen as a multi-cellular organism with two distinct yet interdependent cell types. Such behavior is highly unusual in prokaryotes and may have been the first example of multi-cellular patterning in evolution. Once a heterocyst has formed it cannot revert to a vegetative cell. Certain heterocyst-forming bacteria can differentiate into spore-like cells called akinetes or motile cells called hormogonia, making them the most phenotyptically versatile of all prokaryotes.

Gene Expression
In low nitrogen environments, heterocyst differentiation is triggered by the transcriptional regulator NctA. NctA influences heterocyst differentiation by signaling proteins involved in the process of heterocyst differentiation. For instance, NctA controls the expression of several genes including HetR which is crucial for heterocyst differentiation. It is crucial as it up-regulates other genes such as hetR, patS, hepA by binding to their promoter and thus acting as a transcription factor.It is also worthy to note that the expression of nctA, and HetR are dependent on each other and their presence promotes heterocyst differentiation even in the presence of nitrogen.It has also been recently found that other genes such as PatA, hetP regulate heterocyst differentiation. PatA patterns the heterocysts along the filaments, and it is also important for cell division.PatS influences the heterocyst patterning by inhibiting heterocyst differentiation when a group of differentiating cells come together to form a pro- heterocyst (immature heterocyst). Heterocyst maintenance is dependent on an enzyme called hetN. Heterocyst formation is inhibited by the presence of a fixed nitrogen source, such as ammonium or nitrate.

Symbiotic relationships

The bacteria may also enter a symbiotic relationship with certain plants. In such a relationship, the bacteria do not respond to the availability of nitrogen, but to signals produced by the plant for heterocyst differentiation. Up to 60% of the cells can become heterocyst, providing fixed nitrogen to the plant in return for fixed carbon. The signal produced by the plant, and the stage of heterocyst differentiation it affects is unknown. Presumably, the symbiotic signal generated by the plant acts before NctA activation as hetR is required for symbiotic heterocyst differentiation. For the symbiotic association with the plant, nctA is needed as the bactria with mutated nctA can’t infect plants.

Negarrez (talk) 06:26, 20 November 2017 (UTC)

References