Periplasmic carbonic anhydrase structural gene (Cah1) mutant in chlamydomonas reinhardtii

Historical perspective on microalgal and cyanobacterial acclimation to low- and extremely high-CO(2) conditions

Reports in the 1970s from several laboratories revealed that the affinity of photosynthetic machinery for dissolved inorganic carbon (DIC) was greatly increased when unicellular green microalgae were transferred from high to low-CO(2) conditions. This increase was due to the induction of carbonic anhydrase (CA) and the active transport of CO(2) and/or HCO(3) (-) which increased the internal DIC concentration. The feature is referred to as the 'CO(2)-concentrating mechanism (CCM)'. It was revealed that CA facilitates the supply of DIC from outside to inside the algal cells. It was also found that the active species of DIC absorbed by the algal cells and chloroplasts were CO(2) and/or HCO(3) (-), depending on the species. In the 1990s, gene technology started to throw light on the molecular aspects of CCM and identified the genes involved. The identification of the active HCO(3) (-) transporter, of the molecules functioning for the energization of cyanobacteria and of CAs with different cellular localizations in eukaryotes are examples of such successes. The first X-ray structural analysis of CA in a photosynthetic organism was carried out with a red alga. The results showed that the red alga possessed a homodimeric beta-type of CA composed of two internally repeating structures. An increase in the CO(2) concentration to several percent results in the loss of CCM and any further increase is often disadvantageous to cellular growth. It has recently been found that some microalgae and cyanobacteria can grow rapidly even under CO(2) concentrations higher than 40%. Studies on the mechanism underlying the resistance to extremely high CO(2) concentrations have indicated that only algae that can adopt the state transition in favor of PS I could adapt to and survive under such conditions. It was concluded that extra ATP produced by enhanced PS I cyclic electron flow is used as an energy source of H(+)-transport in extremely high-CO(2) conditions. This same state transition has also been observed when high-CO(2) cells were transferred to low CO(2) conditions, indicating that ATP produced by cyclic electron transfer was necessary to accumulate DIC in low-CO(2) conditions.

Identification and regulation of high light-induced genes in Chlamydomonas reinhardtii

We have used restriction fragment differential display for isolating genes of the unicellular green alga Chlamydomonas reinhardtii that exhibit elevated expression on exposure of cells to high light. Some of the high light-activated genes were also controlled by CO2 concentration. Genes requiring both elevated light and low CO2 levels for activation encoded both novel polypeptides and those that function in concentrating inorganic carbon (extracellular carbonic anhydrase, low CO2-induced protein, ABC transporter of the MRP subfamily). All the genes in this category were shown to be under the control of Cia5, a protein that regulates the responses of C. reinhardtii to low-CO2 conditions. Genes specifically activated by high light, even under high-CO2 conditions, encoded a 30 kDa chloroplast membrane protein, a serine hydroxymethyltransferase, a nuclease, and two proteins of unknown function. Experiments using DCMU, an inhibitor of photosynthetic electron transport, and mutants devoid of either photosystem I or photosystem II activity, showed aberrant expression of all the genes regulated by both CO2 and high light, suggesting that redox plays a role in controlling their expression. In contrast, there was little effect of DCMU or lesions that block photosynthetic electron transport on the activity of genes that were specifically controlled by high light.

Insertional mutants of Chlamydomonas reinhardtii that require elevated CO(2) for survival

Aquatic photosynthetic organisms live in quite variable conditions of CO(2) availability. To survive in limiting CO(2) conditions, Chlamydomonas reinhardtii and other microalgae show adaptive changes, such as induction of a CO(2)-concentrating mechanism, changes in cell organization, increased photorespiratory enzyme activity, induction of periplasmic carbonic anhydrase and specific polypeptides (mitochondrial carbonic anhydrases and putative chloroplast carrier proteins), and transient down-regulation in the synthesis of Rubisco. The signal for acclimation to limiting CO(2) in C. reinhardtii is unidentified, and it is not known how they sense a change of CO(2) level. The limiting CO(2) signals must be transduced into the changes in gene expression observed during acclimation, so mutational analyses should be helpful for investigating the signal transduction pathway for low CO(2) acclimation. Eight independently isolated mutants of C. reinhardtii that require high CO(2) for photoautotrophic growth were tested by complementation group analysis. These mutants are likely to be defective in some aspects of the acclimation to low CO(2) because they differ from wild type in their growth and in the expression patterns of five low CO(2)-inducible genes (Cah1, Mca1, Mca2, Ccp1, and Ccp2). Two of the new mutants formed a single complementation group along with the previously described mutant cia-5, which appears to be defective in the signal transduction pathway for low CO(2) acclimation. The other mutations represent six additional, independent complementation groups.

Insertional mutants of Chlamydomonas reinhardtii that require elevated CO(2) for survival

Aquatic photosynthetic organisms live in quite variable conditions of CO(2) availability. To survive in limiting CO(2) conditions, Chlamydomonas reinhardtii and other microalgae show adaptive changes, such as induction of a CO(2)-concentrating mechanism, changes in cell organization, increased photorespiratory enzyme activity, induction of periplasmic carbonic anhydrase and specific polypeptides (mitochondrial carbonic anhydrases and putative chloroplast carrier proteins), and transient down-regulation in the synthesis of Rubisco. The signal for acclimation to limiting CO(2) in C. reinhardtii is unidentified, and it is not known how they sense a change of CO(2) level. The limiting CO(2) signals must be transduced into the changes in gene expression observed during acclimation, so mutational analyses should be helpful for investigating the signal transduction pathway for low CO(2) acclimation. Eight independently isolated mutants of C. reinhardtii that require high CO(2) for photoautotrophic growth were tested by complementation group analysis. These mutants are likely to be defective in some aspects of the acclimation to low CO(2) because they differ from wild type in their growth and in the expression patterns of five low CO(2)-inducible genes (Cah1, Mca1, Mca2, Ccp1, and Ccp2). Two of the new mutants formed a single complementation group along with the previously described mutant cia-5, which appears to be defective in the signal transduction pathway for low CO(2) acclimation. The other mutations represent six additional, independent complementation groups.

The Arabidopsis immutans mutation affects plastid differentiation and the morphogenesis of white and green sectors in variegated plants

The immutans (im) variegation mutant of Arabidopsis has green and white leaf sectors due to the action of a nuclear recessive gene, IMMUTANS (IM). This gene encodes the IM protein, which is a chloroplast homolog of the mitochondrial alternative oxidase. Because the white sectors of im accumulate the noncolored carotenoid, phytoene, IM likely serves as a redox component in phytoene desaturation. In this paper, we show that IM has a global impact on plant growth and development and is required for the differentiation of multiple plastid types, including chloroplasts, amyloplasts, and etioplasts. IM promoter activity and IM mRNAs are also expressed ubiquitously in Arabidopsis. IM transcript levels correlate with carotenoid accumulation in some, but not all, tissues. This suggests that IM function is not limited to carotenogenesis. Leaf anatomy is radically altered in the green and white sectors of im: Mesophyll cell sizes are dramatically enlarged in the green sectors and palisade cells fail to expand in the white sectors. The green im sectors also have significantly higher than normal rates of O(2) evolution and elevated chlorophyll a/b ratios, typical of those found in "sun" leaves. We conclude that the changes in structure and photosynthetic function of the green leaf sectors are part of an adaptive mechanism that attempts to compensate for a lack of photosynthesis in the white leaf sectors, while maximizing the ability of the plant to avoid photodamage.

Physiological and molecular biological characterization of intracellular carbonic anhydrase from the marine diatom Phaeodactylum tricornutum

A single intracellular carbonic anhydrase (CA) was detected in air-grown and, at reduced levels, in high CO(2)-grown cells of the marine diatom Phaeodactylum tricornutum (UTEX 642). No external CA activity was detected irrespective of growth CO(2) conditions. Ethoxyzolamide (0.4 mM), a CA-specific inhibitor, severely inhibited high-affinity photosynthesis at low concentrations of dissolved inorganic carbon, whereas 2 mM acetazolamide had little effect on the affinity for dissolved inorganic carbon, suggesting that internal CA is crucial for the operation of a carbon concentrating mechanism in P. tricornutum. Internal CA was purified 36.7-fold of that of cell homogenates by ammonium sulfate precipitation, and two-step column chromatography on diethylaminoethyl-sephacel and p-aminomethylbenzene sulfone amide agarose. The purified CA was shown, by SDS-PAGE, to comprise an electrophoretically single polypeptide of 28 kD under both reduced and nonreduced conditions. The entire sequence of the cDNA of this CA was obtained by the rapid amplification of cDNA ends method and indicated that the cDNA encodes 282 amino acids. Comparison of this putative precursor sequence with the N-terminal amino acid sequence of the purified CA indicated that it included a possible signal sequence of up to 46 amino acids at the N terminus. The mature CA was found to consist of 236 amino acids and the sequence was homologous to beta-type CAs. Even though the zinc-ligand amino acid residues were shown to be completely conserved, the amino acid residues that may constitute a CO(2)-binding site appeared to be unique among the beta-CAs so far reported.

CHLAMYDOMONAS AS A MODEL ORGANISM

The unicellular green alga Chlamydomonas offers a simple life cycle, easy isolation of mutants, and a growing array of tools and techniques for molecular genetic studies. Among the principal areas of current investigation using this model system are flagellar structure and function, genetics of basal bodies (centrioles), chloroplast biogenesis, photosynthesis, light perception, cell-cell recognition, and cell cycle control. A genome project has begun with compilation of expressed sequence tag data and gene expression studies and will lead to a complete genome sequence. Resources available to the research community include wild-type and mutant strains, plasmid constructs for transformation studies, and a comprehensive on-line database.

The Cia5 gene controls formation of the carbon concentrating mechanism in Chlamydomonas reinhardtii

Wild-type Chlamydomonas reinhardtii cells shifted from high concentrations (5%) of CO2 to low, ambient levels (0.03%) rapidly increase transcription of mRNAs from several CO2-responsive genes. Simultaneously, they develop a functional carbon concentrating mechanism that allows the cells to greatly increase internal levels of CO2 and HCO3-. The cia5 mutant is defective in all of these phenotypes. A newly isolated gene, designated Cia5, restores transformed cia5 cells to the phenotype of wild-type cells. The 6,481-bp gene produces a 5.1-kb mRNA that is present constitutively in light in high and low CO2 both in wild-type cells and the cia5 mutant. It encodes a protein that has features of a putative transcription factor and that, likewise, is present constitutively in low and high CO2 conditions. Complementation of cia5 can be achieved with a truncated Cia5 gene that is missing the coding information for 54 C-terminal amino acids. Unlike wild-type cells or cia5 mutants transformed with an intact Cia5 gene, cia5 mutants complemented with the truncated gene exhibit constitutive synthesis of mRNAs from CO2-responsive genes in light under both high and low CO2 conditions. These discoveries suggest that posttranslational changes to the C-terminal domain control the ability of CIA5 to act as an inducer and directly or indirectly control transcription of CO2-responsive genes. Thus, CIA5 appears to be a master regulator of the carbon concentrating mechanism and is intimately involved in the signal transduction mechanism that senses and allows immediate responses to fluctuations in environmental CO2 and HCO3- concentrations.