Regulation of cyanobacterial CO₂-concentrating mechanisms through transcriptional induction of high-affinity C_i-transport systems

Approximately 50% of global CO₂-based productivity is now attributed to the activity of phytoplankton, including ocean-dwelling cyanobacteria. In response to inherent restrictions on the rate of CO ₂ supply in the aquatic environment, cyanobacteria have evolved a very efficient means of capturing inorganic carbon (C_i), as either CO₂ or HCO₃^- for photosynthetic carbon fixation. This capturing mechanism, known as a CO₂-concentrating mechanism (CCM), involves the operation of active CO₂ and HCO ₃^- transporters and results in the concentration of CO₂ around RuBisCO, in a unique microcompartment called the carboxysome. The CCM exhibits two basic physiological states: a constitutive, low-affinity state; and a high-affinity state, which is induced in response to C_i limitation. Many of the genetic components of the CCM, including genes encoding C_i transporters, have been identified. It is apparent that the expression of genes encoding the inducible, high-affinity C_i transporters is particularly sensitive to C_i availability, and we are now interested in defining how cyanobacterial cells sense and respond to C_i limitation at the transcriptional level. Current theories include direct sensing of external C_i; sensing of internal C_i-pool fluctuations; and detection of changes in photorespiratory intermediates, carbon metabolites, or redox potential. At present, there is no consensual view. We have investigated the physiological and transcriptional responses of CCM mutants and wildtype strains to pharmacological treatments and various light, O₂, and C_i regimes. Our data suggest that perception of C_i limitation by a cyanobacterial cell is either directly or indirectly related to the size of the internal C_i pool within the cell, in an oxygen-dependent manner.