Regulation of Carbonic Anhydrase Expression in Chlamydomonas

Abstract

The photosynthetic characteristics of the green alga Chlamydomonas reinhardtii as well as other algae and cyanobacteria are dependent upon the inorganic carbon concentration experienced by the alga during growth (1). Cells grown at air levels of CO2 (0,03%) have a much higher affinity for inorganic carbon (DIC) than cells grown at high levels of CO2 (>1%). Since the CO2 concentration during growth of the alga has no effect on either the Km(CO2) of the principal CO2 fixing enzyme, RuBP carboxylase/ oxygenase (Rubisco) or the mechanism of photosynthetic CO2 fixation, low DIC adapted algae must employ another method for increasing the efficiency of CO2 utilization (2). It is now apparent that the high affinity for DIC exhibited by these algae can be traced to a system that effectively saturates Rubisco with CO2 and eliminates O2 inhibition of photosynthesis. This system is dependent upon two separate components: carbonic anhydrase (CA), an enzyme primarily localized in the periplasmic space and responsible for maintaining HCO3/CO2 equilibrium; and an inorganic carbon concentrating mechanism that actively transports inorganic carbon into the cell (2), Mutants of Chlamydomonas that are deficient in CA activity or DIC transport exhibit low affinity for inorganic carbon (3,4). Growth at high levels of DIC inhibits the expression of CA activity and DIC transport (5).