PARENT SESSION

Symposium S4C Controlling CO₂: Stomates and carbon concentrating mechanisms
Tuesday August 31st, 2004 2:40 PM-4:40 PM Room 510A
Chair: George Espie
Co-Chair: Murray Badger

Identification of a new class of bicarbonate transporter from the marine cyanobacterium, Synechococcus PCC7002. G. Dean Price^*,1, Fiona Woodger¹, Tucker Loraine¹, Murray Badger¹, Susan Howitt⁵, ¹ Molecular Plant Physiology Group, Canberra, ACT, Australia⁵ Biochem & Molecular Biology, Canberra, ACT, Australia

ABSTRACT- Up to 50% of global CO₂-based productivity is associated with marine phytoplankton, with oceanic cyanobacteria contributing 40-80% of this marine productivity. In aquatic environments the CO₂ supply rate for photosynthesis can be severely restricted, compared to terrestrial environments, and in order to maintain the efficiency of photosynthetic carbon fixation cyanobacteria have evolved an efficient capturing mechanism for inorganic carbon (Ci; CO₂ & HCO₃-,) known as a CO₂ concentrating mechanism (CCM). This CCM involves the operation of active CO₂ and HCO₃^- transporters and results in the concentration of CO2 around Rubisco in a unique microcompartment called the carboxysome. In freshwater strains of cyanobacteria there are two CO₂ uptake systems and at least two HCO₃^- transporters employed to provide effective accumulation of Ci within the cell; the genes involved have been identified. However, in marine cyanobacteria the physiological characteristics and genetic identification of Ci transporters are less well understood. We have used the marine cyanobacterium, Synechococcus PCC7002 as a model marine cyanobacterium to characterize the properties of HCO₃^- transporters. We have confirmed thorough gene disruptions, and gain-of-function analyses, that the sbtA homolog codes for a HCO₃^- transporter, and significantly, we have identified a new HCO₃^- transporter, named bicA, that codes for a low affinity HCO₃^- transporter with a high flux property. Close homologs of bicA are present in the genome databases of all sequenced marine cyanobacteria. The potential role of the BicA transporter in marine systems will be discussed.

KEY WORDS: CO2 concentrating mechanism, Photosynthetic CO2 fixation, Bicarbonate transport, cyanobacteria