Heterocyst differentiation

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Heterocyst differentiation



2009 Killpack Group All Rights ReservedOVERVIEW

Characteristic feature of cynobacteria.HeterocystTypes of Nitrogen fixing cyno bacteriaAkineteHeterocyst differentiation in cynobacteria (Blue green algae)

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CHARACTERISTICS FEATURE OF CYNOBACTERIA.Cynobacteria are photosynthetic ,gram negative prokaryotes. Cynobacteria differ from photosynthetic bacteria in that the photosynthesis of cynobacteria result in release of oxygen derived from the splitting of water molecule in manner very similar to eukaryotic algae and higher plant. Cynobacteria produce unique structure, called heterocyst which function as the site for nitrogen fixation.

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HETEROCYSTHeterocyst are differentiated cell that are specialized for fixation of nitrogen in an aerobic environment.In heterocyst in the light photosystem 1generated ATP, but no production of O2 take place .The reductant move into heterocyst from vegetative cell, in return fixed nitrogen move from heterocyst to vegetative cell.

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Nitrogenase in the heterocyst is protected from inactivation by O2 by a variety of means, by enhancing respiration and by barring, the heterocyst envelope.In order to avoid inactivation of nitrogenase by oxygen ,heterocyst stops synthesizing oxygen and limit the entry of that gas, presumably because the vander waal radii of nitrogen and oxygen are similar 1.5A and 1.4A.

2009 Killpack Group All Rights ReservedNITROGEN FIXATION

Cynobacteria are truly autotropic in nature that they can fix not only CO2 but also gaseous nitrogen.Cynobacteria which can fix nitrogen can be broadly .

2009 Killpack Group All Rights Reserved CLASSIFICATION

Type 1 , include unicellular form such as Aphanothece and Gloeocapsa which can fix nitrgen both in aerobic and anaerobic (microaerophilic) condition.Type 2,include nonheterocystous filamentous form , such as Oscillatoria , which can fix it only under anaerobic condition .Type 3, the most important group , contain many heterocystous fillamentous form which can fix nitrogen both aerobically and anaerobically

2009 Killpack Group All Rights ReservedAKINETE Akinete is thick walled dormant cell derived from the enlargement of a vegetative cell . It is the resting cell of cynobacteria and unicellular fillamentous green algae.The akinete are filled with food reserve, and have a normal Cell wall surrounded with three layer of coat.Devlopement of akinete From vegetative cell involve-Increase in sizeGradual diappearence ofGas vacuole.

2009 Killpack Group All Rights ReservedCyanobacteria use two mechanisms to separate these activities: a biological circadian clock to separate them temporally, and multicellularity and cellular differentiation to separate them spatiallyaz cold spring harb

For example, the unicellularCyanothece sp. strain ATCC 51142 storesglycogen during the day and fixes nitrogen atnight (Toepel et al. 2008), whereas the filamentousTrichodesmium erythraeum IMS101 fixesnitrogen during the day in groups of specializedcells (Sandh et al. 2009).


Introduction - HeterocystDifferentiationImportant reasons for differentiation: Adaption to environmental conditions Expressing different functions at different times in the life cycleThus differentiation can be triggered by:Environmental signalThe specialized cell types is the outcome of complex regulatory pathways:Altered gene expressiondifferential protein stabilitydifferential protein localization

2009 Killpack Group All Rights ReservedIn the presence of a source of combined nitrogen such as nitrate or ammonium, Anabaena grows as long filaments containing hundreds of photosynthetic vegetative cells.In the absence of combined nitrogen, it produces heterocysts every ten to twenty vegetative cells along filaments.


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Heterocysts are typically distinguishable from vegetative cells:larger rounder shape diminished pigmentationthicker cell envelopesprominent cyanophycin granules at poles adjacent to vegetative cells


Oxygen-producing photosystem PSII is dismantled during differentiation and heterocysts devlopement.

Morphological changes include the deposition of two additional envelope layers around the heterocyst: an inner laminated layer composed of two heterocyst specific glycolipids (HGL)and an outer polysaccharide layer (HEP).

Heterocysts and vegetative cells are mutually interdependent. Because they lack photosystem II and carbon fixation, heterocysts are dependent on vegetative cells for a source of reductant and carbon, which is probably partially supplied as sucrose.


2009 Killpack Group All Rights ReservedThe additional envelope layers surroundingheterocysts help to protect the enzyme nitrogenasefrom oxygen

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Nitrogenase, sequestered within these cells, transformsdinitrogenintoammoniumat the expense of ATP and reductantboth generated by carbohydrate metabolism, a process that is supplemented, in the light, by the activity of PS I. Carbohydrate, probably in the form of sucrose, is synthesized in vegetative cells and moves into heterocysts. In return, nitrogen fixed in heterocysts moves into the vegetative cells, at least in part in the form ofamino acids. az wiki11

In Anabaena PCC7 120 vegetative cell must also supply glutamate to heterocyst which convert it to glutamine and other amino acid.

Heterocyst development is complete in about 20 hours at 30 C .


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Photosynthetic C fixation through the Calvin cycle (CC) occurs in the vegetative cells and could lead to Suc and glycogen biosynthesis. Heterocysts act as an important sink for carbohydrates from vegetative cells and as a source of fixed N (Wolk et al., 1994). In heterocysts, which could also synthesize glycogen and Suc, the reductants for N2and O2reduction are generated by the activity of the oxidative pentose-P cycle (OPPC), the NADPH heterocyst-specific ferrodoxin, and respiratory electron transport (RET), as well as the ATP synthesis by cyclic phosphorylation (PSI). Suc enzymes are indicated as (1) SuS; (2) A/N-Inv; (3A) SPS-A; (3B) SPS-B; and (4) SPP.KG,-Ketoglutarate. az http://www.plantphysiol.org/content/143/3/1385/F8.expansion12


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In cyanobacteria, 2-oxoglutarate, an intermediate in the Krebs cycle, constitutes the signal for nitrogen deprivation.The Krebs cycle in cyanobacteria is incomplete because of the lack of 2-oxoglutarate dehydrogenase 2-oxoglutarates main function is to serve as a precursor in a variety of biosynthetic reactions. It is the primary carbon skeleton for incorporation of ammonium and is considered the metabolic junction between carbon and nitrogen balance in cyanobacteria.


2009 Killpack Group All Rights Reservedcombined nitrogensuch as ammonium or nitrate inhibits thedifferentiation of heterocysts

An artificial analog of2-oxoglutarate, 2,2-difluoropentanoic acid, DFPA,added to medium resulted in heterocyst developmenteven in the presence of ammonium

Nitrogenlimitingconditions result in an increase in thelevels of 2-oxoglutarate showing that 2-oxoglutarate plays a key role incontrolling heterocyst development (Laurentet al. 2005)..

NtcA is conserved in all cyanobacteria and regulates a number of genes involved in carbon and nitrogen metabolism


Nitrogen limiting conditions result in an increase in the levels of 2-oxoglutarate.

NtcA, a transcriptional regulator belonging to the CRP (cyclic AMP receptor protein) family of proteins, senses 2-oxoglutarate levels.

In Anabaena PCC 7120, NtcA is required for the expression of the genes in pathways for ammonium and nitrate assimilation, as well as heterocyst development.Het R is the master regulator of heterocyst devlopement and play a key role in differentiation

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