Analyses of CIA5, the master regulator of the carbon-concentrating mechanism in <i>Chlamydomonas reinhardtii</i>, and its control of gene expression

Abstract

In numerous studies, the CIA5 gene of Chlamydomonas reinhardtii Dangeard has been shown to control the expression of several "CO 2 -responsive genes" when cells are shifted to higher or lower levels of CO 2 . Using DNA microarray analyses with arrays containing 2764 unique cDNA sequences, we have demonstrated that several additional genes are controlled by the CIA5 gene, some increasing in expression when CO 2 levels are lowered and others decreasing. Not all genes that respond to changes in CO 2 concentrations are controlled by CIA5. For example, the RH1 gene, is markedly induced when both wild-type and cia5 mutant cells are shifted to high levels of CO 2 . We demonstrate that cycloheximide (an inhibitor of cytoplasmic protein synthesis) has no apparent effect on the initial induction of CO 2 -responsive genes, suggesting constitutive presence of all the molecular machinery needed by the cell to immediately respond to changes in CO 2 levels. This observation is consistent with our earlier suggestions that CIA5 or another key component(s) of the carbon-concentrating mechanism must be rapidly "activated" (or "inactivated") as part of the response of C. reinhardtii to changes in external CO 2 levels. We present new, direct evidence that CIA5 is localized to the nucleus in both low- and high-CO 2 conditions.Key words: CIA5, CCM, carbon-concentrating mechanism, photosynthesis, gene regulation, Chlamydomonas reinhardtii, CO 2 .