Heterocyst differentiation

HETEROCYST DIFFERENTIATION

PRESENTED BY

SWEKSHA SINGHM.SC BOTANY 1ST SEMESTER

© 2009 Killpack Group All Rights Reserved

OVERVIEW

Characteristic feature of cynobacteria.

Heterocyst Types of Nitrogen fixing cyno bacteria Akinete Heterocyst differentiation in cynobacteria (Blue green algae)


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.


HETEROCYST

Heterocyst 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.


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.


NITROGEN 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

.


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


AKINETE

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-1) Increase in size2) Gradual diappearence ofGas vacuole.


Introduction - HeterocystDifferentiation

Important reasons for differentiation: Adaption to environmental conditions Expressing different functions at different

times in the life cycle Thus differentiation can be triggered by: Environmental signal The specialized cell types is the outcome of

complex regulatory pathways: Altered gene expression differential protein stability differential protein localization


In 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.

HETEROCYST IN CYANOBACTERIA


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.

HETEROCYST IN ANABAENA


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 .





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-oxoglutarate’s 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.



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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Heterocyst differentiation