This document summarizes heterocyst differentiation in cyanobacteria. Heterocysts are specialized cells that allow cyanobacteria to fix nitrogen in aerobic conditions. In the absence of combined nitrogen, cyanobacteria like Anabaena produce heterocysts at regular intervals along filaments. Heterocysts stop producing oxygen and develop additional envelope layers to protect the nitrogenase enzyme from oxygen. They rely on nearby vegetative cells for energy and carbon in return for fixed nitrogen. 2-oxoglutarate signals nitrogen deprivation and the transcriptional regulator NtcA induces genes for heterocyst development and nitrogen assimilation.

1. HETEROCYST
DIFFERENTIATION
PRESENTED BY
SWEKSHA SINGH
M.SC BOTANY
1ST SEMESTER

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OVERVIEW
 Characteristic feature of cynobacteria.
 Heterocyst
 Types of Nitrogen fixing cyno
bacteria
 Akinete
 Heterocyst 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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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.

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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.

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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
.

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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

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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 size
2) Gradual diappearence of
Gas vacuole.

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Introduction - Heterocyst
Differentiation
 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

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 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

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 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

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 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 .
HETEROCYST IN
ANABAENA

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HETEROCYST IN
ANABAENA

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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-
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.
HETEROCYST IN
ANABAENA

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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