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 photosynthetic organisms

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.

Thank You