Wednesday, 20 June 2012

CO2 concentrating mechanisms in cyanobacteria: molecular components, their diversity and evolution by Badger

Cyanobacteria have existed as oxygenic photosynthetic bacteria on earth for at least 2.7 billion years (Buick, 1992). During that time they have endured a changing gaseous environment where CO2 has declined and O2 has risen. This has imposed evolutionary pressure on them to evolve strategies for efficiently acquiring inorganic carbon for photosynthesis. In response to this they have developed an effective photosynthetic CO2concentrating mechanism (CCM) for improving the carboxylation by their relatively inefficient Rubiscos (Badger and Price, 1992Price et al., 1998;Kaplan and Reinhold, 1999). This CCM is perhaps the most effective of any photosynthetic organism, concentrating CO2 up to 1000‐fold around the active site of Rubisco. In the past few years, there has been a rapid increase in the understanding of the mechanisms and genes involved in cyanobacterial CCMs. In addition, there has been a recent expansion in the availability of complete cyanobacterial genome sequences, thus increasing the potential to examine questions regarding both the evolution and diversity of components of the CCM across cyanobacterial species. This paper reviews current understanding of the mechanisms and genes underlying the operation of the cyanobacterial CCM, and takes the opportunity to employ comparative genomics to shed light on the evolution and diversity of the CCM among cyanobacterial species.

The cyanobacterial CCM model

  • cayboxysome is a protein micro-compartment in cell
  • contains Rubisco and carboxysomal carbonic anhydrase (CA)
  • CA converts an accumuilated cytosolic pool of HCO32- into CO2 in teh carboxysome
  • CO2 generation coupled with diffusive restriction to efflus from carboxysome leads to localised elevation of Co2 around active site of Rubisco in carboxysome
  • Substrate, HCO32- is accumulated in cytosol by active CO2 and HCO32- transporters
  • transporters are on plasma membrane and thlakoid membrane
  • exist in low and high affinity transporter forms
  • Cyanobacteria can improve affinity for inorganic carbon (Ci) at limiting Ci levels
  • due to changes in synthesis and properties of high and low affinity Ci transporters assoc with cells

Fig. 1. A generalized model for the cyanobacterial CCM. Shown on the figure are the Rubisco‐containing carboxysomes with the carboxysomal carbonic anhydrase (CA) and an associated diffusional resistance to CO2 efflux. The accumulation of HCO--3in the cytosol is achieved through the action of a number of CO2and HCO--3 uptake systems.

  • carbxosyomes are protein bodies
  • surrounded by a protein shell
  • contains Rubisco
  • for CO2 fixation to occur in beta-carboxysomes, it is proposed that HCO32- diffuses thru protein shell of carboxysome
  • low activity of CA inside str catalyses CO2 formation from HCO32- at high rates
  • saturate carboxylation reaction of Rubisco
  • models suggest HCO32- diffuses into carboxysome interior
  • CO2 generated by a specialised CA
  • CO2 can be elevated in carboxsysome with a diffusion barrier eg protein shell
  • restrict CO2 diffusion out of carboxysome

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