Abstract

Aquatic organisms, including Chlamydomonas reinhardtii, are faced with a variable supply of dissolved inorganic carbon (Ci). Accordingly, C. reinhardtii has the ability to acclimate to the changing Ci supply through a variety of responses, including induction of a CO₂ concentrating mechanism (CCM) when Ci is limiting. The CCM uses active Ci uptake to accumulate a high internal concentration of bicarbonate, which is dehydrated by a specific thylakoid carbonic anhydrase to supply CO₂, the substrate used in photosynthesis. In addition to the changes demonstrably related to the function of the CCM, C. reinhardtii exhibits several other acclimation responses to limiting Ci, such as changes in cellular organization and induction or upregulation of several genes. A key area currently under investigation is how C. reinhardtii cells recognize the change in Ci or CO₂ concentration, and transduce that signal into needed gene expression changes. Mutational analyses are proving very useful for learning more about the CCM and about the acclimation response to changes in Ci availability. Cloning of the gene disrupted in cia5, a mutant apparently unable to acclimate to limiting Ci, has opened opportunities for more rapid progress in understanding the signal transduction pathway. The Cia5 gene appears to encode a transcription factor that may control, either directly or indirectly, much of the gene expression responses to limiting Ci in C. reinhardtii. Several additional new mutants with potential defects in the signal transduction pathway have been isolated, including three new alleles of cia5.