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CO2 Concentrating mechanisms in cyanobacteria: molecular
components, their diversity and evolution
Murray R.Badger & G. Dean Price
Presented by Shilpi Misra
Outline Cyanobacterial photosynthesis
CO2 concentrating Mechanism (CCM) model
Phylogeny of cyanobacteria
Molecular Components and their functions
Diversity in CCM components
Evolution of Cyanobacterial CCM
Polyphylectic Origin of CCMs
Cyanobacteria and photosynthesis
Cyanobacteria have existed as oxygenic photosynthetic bacteria on Earth for 2.7 billion years.
Changing environment from CO2 to O2 increased evolutionary pressure
They developed effective photosynthetic CO2 concentrating mechanism (CCM)
Most effective photosynthetic mechanism concentrating upto 1000 fold of CO2 around the active site of Rubisco.
Cyanobacterial CCM Model
HCO-3 is accumulated by
active transporters located in plasma membrane and thylakoid membrane.
Carboxysome are protein bodies surrounded by protein shell.
It contains Rubisco and carboxysomal carbonic anhydrase (CA).
CA converts HCO-3 into CO2.
Diffusive restriction to the efflux of CO2
Phylogeny of cyanobacteria Two ways of division – Ribosomal RNA subunit sequences &
type of Rubisco
Based on 16s ribosomal RNA genes cyanobacteria found to be different from non-green algae, green algae and higher plants.
Division based on type of Rubisco more appropriate with
respect to evolution of their CCMs.
Various photosynthetic bacteria have been grouped according to Form 1 Rubisco large subunit types into Form 1A,B, C and D.
Cyanobacteria are within Form 1A and 1B domains.
Phylogeny of cyanobacteriaRubisco Phylogeny
Form 1A
α- cyanobacteria
α-carboxysomes
Form 1B
β-cyanobacteria
β-carboxysomes
Carboxysome structure and phylogeny
Protein shell contains different types of polypeptides.
There is a difference in the polypeptides of α- cyanobacteria & β-cyanobacteria.
CcmK, L and O genes are contained in β-cyanobacterial genomes, CsoS1 and peptide A and B are found in α- cyanobacteria.
CO2 fixation in carboxysomes occurs with the help of Carbonic Anhydrase.
Carbonic Anhydrases Analysis of α, β and γ carbonic anhydrases showed a wide
diversity.
There is absence any clearly identifiable CA genes in either α or β cyanobacteria.
β carboxysomal CA gene was present in many but not in all β cyanobacteria.
β cyanobacteria may also possess α-CA.
This area still needs some explanation and research.
Ci (inorganic C) transporters
Regardless of the form Ci (HCO-3 or CO2), the species
accumulated is HCO-3
Four modes of Ci uptake have been identified:-
BCT1 HCO-3 transporter
Sodium-dependent HCO-3 uptake
NDH- 14
NDH-1 genes and CO2 uptake
NDH- 13
BCT1 HCO-3 transporter
First cyanobacterial Ci transporter
The BCT1 transporter is encoded by the cmpABCD operon & expressed under severe Ci limitation.
Cmp ABCD genes codes for four protein: Cmp A, B, C and D
Cmp A- HCO-3 binding protein
Cmp B- intrinsic membrane proteinCmp C- large extrinsic membrane protein with ATP binding
siteCmp D- smaller related protein with an ATP site
Sodium-dependent HCO-3 uptake
A gene isolated from cyanobacteria (sbtA) code for Na+ dependent transport activity induced under Ci-limitation.
Cyanobacteria possess a Na+/ HCO-3 symporter
This is energized by the inwardly directed Na+ gradient which is energized by Na+/H+ antiporter.
NDH-1 genes and CO2 uptake
NDH-1 dehydrogenase complex is involved in enabling CO2 uptake by cyanobacteria.
There are many numbers of NDH -1 genes in cyanobacteria which are present in single copies.
Out of these Ndh D3/D4, NdhF3/F4 components are involved in catalysing active CO2 uptake by converting CO2
to HCO-3
NDH-1 genes and CO2 uptake
chpX and chp Y are two other genes which are involved in enabling CO2 uptake
ndhF3/ndhD3/chpY genes code for NDH-13 complex that have high affinity for CO2 uptake
ndhF4/ndhD4/chpX genes code for NDH-14 complex involved in low affinity CO2 uptake.
Diversity in CCM components
Four types of cyanobacterial CCM strategies can be classified on the presence of carboxysomes, components of Ci transporters.
The presence of α and β carboxysomes are the most striking variation.
The high and low affinity for CO2 and HCO-3 transport
systems present in the cells.
Evolution of cyanobacterial CCM
Atmospheric CO2 levels when cyanobacteria first arose were higher than today.
Decline in CO2 levels doubling of O2 during Phanerozoic era.
Changes in CO2 and O2 caused evolution of CCM in cyanobacteria.
This era was the first time significant pressures were applied on photosynthetic organisms to develop CCMs.
Evolution of cyanobacterial CCM
First step to CCM was evolution of carboxysome structure for Rubisco.
Both cyanobacterial groups developed CCM independently rather than from a common ancestor.
Requirement of CA due to slow rate of conversion of HCO-3 to
CO2
Evolution of low and high affinity HCO-3 and CO2 systems.
Carboxysomes developed first and differentiated into α and β carboxysomes.
Polyphylectic origin of CCMs
If CO2 limitation was not imposed their won’t be any CCMs.
Then the cholorophyte & rhodophyte algae would also lack any CCM.
There are no homologues of cyanobacterial CCM genes in record.
Thus carboxysomes genes belong to only cyanobacteria and
proteobacteria.