Functional genomics of plant photosynthesis in the fast lane using Chlamydomonas reinhardtii

Oxygenic photosynthesis by algae and plants supports much of life on Earth. Several model organisms are used to study this vital process, but the unicellular green alga Chlamydomonas reinhardtii offers significant advantages for the genetic dissection of photosynthesis. Recent experiments with Chlamydomonas have substantially advanced our understanding of several aspects of photosynthesis, including chloroplast biogenesis, structure-function relationships in photosynthetic complexes, and environmental regulation. Chlamydomonas is therefore the organism of choice for elucidating detailed functions of the hundreds of genes involved in plant photosynthesis.     

Identification of mitochondrial proteins in membrane preparations from Chlamydomonas reinhardtii.

Preparations enriched in Chlamydomonas reinhardtii thylakoids have proven useful in the study of photosynthesis. Many of their polypeptides however remain unidentified. We report here on three of those, h1 (34 kDa), h2 (11 kDa), and P3 (63 kDa). h1, h2, and P3 are present in all tested mutants of C. reinhardtii lacking either one or several of the photosynthetic chain complexes or depleted in thylakoid membranes. h2 is an ascorbate-reducible, soluble c550-type cytochrome encoded in the nucleus. It cross-reacts immunologically with mitochondrial cytochromes c from various sources and contains a hexapeptide encoded in C. reinhardtii cytochrome c cDNA. P3, a nuclear-encoded peripheral protein, cross-reacts with various ATP synthase beta subunits. Its N-terminal sequence is encoded in C. reinhardtii mitochondrial beta subunit cDNA. h1 behaves as an integral hemoprotein; it is absent in a mitochondrial mutant that carries a deletion in apocytochrome b gene. We conclude that C. reinhardtii mitochondrial membranes copurify with thylakoid membranes. h1 is part of the cytochrome bc1 complex, h2 is cytochrome c, and P3 is the beta subunit of mitochondrial ATP synthase.     

Chlamydomonas reinhardtii: biological rationale for genomics

Chlamydomonas reinhardtii has been the subject of genetic, biochemical, cytological, and molecular analyses for over 50 years. It is an ideal model system for the study of flagella and basal bodies as well as the study of photosynthesis and chloroplast biogenesis, cell-cell recognition and fusion, phototaxis, and secretion. It is clear that many of the genes identified in Chlamydomonas have homologs in land plants as well as animals. Thus, a genomic approach in Chlamydomonas will provide another important avenue for the understanding of important biological processes.     

Effects of acetate on facultative autotrophy in Chlamydomonas reinhardtii assessed by photosynthetic measurements and stable isotope analyses

The green alga Chlamydomonas reinhardtii can grow photoautotrophically utilizing CO(2), heterotrophically utilizing acetate, and mixotrophically utilizing both carbon sources. Growth of cells in increasing concentrations of acetate plus 5% CO(2) in liquid culture progressively reduced photosynthetic CO(2) fixation and net O(2) evolution without effects on respiration, photosystem II efficiency (as measured by chlorophyll fluorescence), or growth. Using the technique of on-line oxygen isotope ratio mass spectrometry, we found that mixotrophic growth in acetate is not associated with activation of the cyanide-insensitive alternative oxidase pathway. The fraction of carbon biomass resulting from photosynthesis, determined by stable carbon isotope ratio mass spectrometry, declined dramatically (about 50%) in cells grown in acetate with saturating light and CO(2). Under these conditions, photosynthetic CO(2) fixation and O(2) evolution were also reduced by about 50%. Some growth conditions (e.g. limiting light, high acetate, solid medium in air) virtually abolished photosynthetic carbon gain. These effects of acetate were exacerbated in mutants with slowed electron transfer through the D1 reaction center protein of photosystem II or impaired chloroplast protein synthesis. Therefore, in mixotrophically grown cells of C. reinhardtii, interpretations of the effects of environmental or genetic manipulations of photosynthesis are likely to be confounded by acetate in the medium.     

The plastid genome-encoded Ycf4 protein functions as a nonessential assembly factor for photosystem I in higher plants

Photosystem biogenesis in the thylakoid membrane is a highly complicated process that requires the coordinated assembly of nucleus-encoded and chloroplast-encoded protein subunits as well as the insertion of hundreds of cofactors, such as chromophores (chlorophylls, carotenoids) and iron-sulfur clusters. The molecular details of the assembly process and the identity and functions of the auxiliary factors involved in it are only poorly understood. In this work, we have characterized the chloroplast genome-encoded ycf4 (for hypothetical chloroplast reading frame no. 4) gene, previously shown to encode a protein involved in photosystem I (PSI) biogenesis in the unicellular green alga Chlamydomonas reinhardtii. Using stable transformation of the chloroplast genome, we have generated ycf4 knockout plants in the higher plant tobacco (Nicotiana tabacum). Although these mutants are severely affected in their photosynthetic performance, they are capable of photoautotrophic growth, demonstrating that, different from Chlamydomonas, the ycf4 gene product is not essential for photosynthesis. We further show that ycf4 knockout plants are specifically deficient in PSI accumulation. Unaltered expression of plastid-encoded PSI genes and biochemical analyses suggest a posttranslational action of the Ycf4 protein in the PSI assembly process. With increasing leaf age, the contents of Ycf4 and Y3IP1, another auxiliary factor involved in PSI assembly, decrease strongly, whereas PSI contents remain constant, suggesting that PSI is highly stable and that its biogenesis is restricted to young leaves.     

An exogenous chloroplast genome for complex sequence manipulation in algae

We demonstrate a system for cloning and modifying the chloroplast genome from the green alga, Chlamydomonas reinhardtii. Through extensive use of sequence stabilization strategies, the ex vivo genome is assembled in yeast from a collection of overlapping fragments. The assembled genome is then moved into bacteria for large-scale preparations and transformed into C. reinhardtii cells. This system also allows for the generation of simultaneous, systematic and complex genetic modifications at multiple loci in vivo. We use this system to substitute genes encoding core subunits of the photosynthetic apparatus with orthologs from a related alga, Scenedesmus obliquus. Once transformed into algae, the substituted genome recombines with the endogenous genome, resulting in a hybrid plastome comprising modifications in disparate loci. The in vivo function of the genomes described herein demonstrates that simultaneous engineering of multiple sites within the chloroplast genome is now possible. This work represents the first steps toward a novel approach for creating genetic diversity in any or all regions of a chloroplast genome.     

Chlamydomonas reinhardtii and photosynthesis: genetics to genomics

Genetic and physiological features of the green alga Chlamydomonas reinhardtii have provided a useful model for elucidating the function, biogenesis and regulation of the photosynthetic apparatus. Combining these characteristics with newly developed molecular technologies for engineering Chlamydomonas and the promise of global analyses of nuclear and chloroplast gene expression will add a new perspective to views on photosynthetic function and regulation.     

A novel pathway of cytochrome c biogenesis is involved in the assembly of the cytochrome b6f complex in arabidopsis chloroplasts

We recently characterized a novel heme biogenesis pathway required for heme c(i)' covalent binding to cytochrome b6 in Chlamydomonas named system IV or CCB (cofactor assembly, complex C (b6f), subunit B (PetB)). To find out whether this CCB pathway also operates in higher plants and extend the knowledge of the c-type cytochrome biogenesis, we studied Arabidopsis insertion mutants in the orthologs of the CCB genes. The ccb1, ccb2, and ccb4 mutants show a phenotype characterized by a deficiency in the accumulation of the subunits of the cytochrome b6f complex and lack covalent heme binding to cytochrome b6. These mutants were functionally complemented with the corresponding wild type cDNAs. Using fluorescent protein reporters, we demonstrated that the CCB1, CCB2, CCB3, and CCB4 proteins are targeted to the chloroplast compartment of Arabidopsis. We have extended our study to the YGGT family, to which CCB3 belongs, by studying insertion mutants of two additional members of this family for which no mutants were previously characterized, and we showed that they are not functionally involved in the CCB system. Thus, we demonstrate the ubiquity of the CCB proteins in chloroplast heme c(i)' binding.     

Photosynthetic water oxidation in cytochrome b(559) mutants containing a disrupted heme-binding pocket

The role of cytochrome b(559) in photosynthetic oxygen evolution has been investigated in three chloroplast mutants of Chlamydomonas reinhardtii, in which one of the two histidine axial ligands to the heme, provided by the alpha subunit, has been replaced by the residues methionine, tyrosine, and glutamine. Photosystem two complexes functional for oxygen evolution could be assembled in the methionine and tyrosine mutants up to approximately 15% of wild type levels, whereas no complexes with oxygen evolution activity could be detected in the glutamine mutant. PSII supercomplexes isolated from the tyrosine and methionine mutants were as active as wild type in terms of light-saturated rates of oxygen evolution but in contrast to wild type contained no bound heme despite the presence of the alpha subunit. Oxygen evolution in the tyrosine and methionine mutants was, however, more sensitive to photoinactivation than the WT. Overall, these data establish unambiguously that a redox role for the heme of cytochrome b(559) is not required for photosynthetic oxygen evolution. Instead, our data provide new evidence of a role for cytochrome b(559) in the protection of the photosystem two complex in vivo.     

cis- and trans-Acting determinants for translation of psbD mRNA in Chlamydomonas reinhardtii

Chloroplast translation is mediated by nucleus-encoded factors that interact with distinct cis-acting RNA elements. A U-rich sequence within the 5' untranslated region of the psbD mRNA has previously been shown to be required for its translation in Chlamydomonas reinhardtii. By using UV cross-linking assays, we have identified a 40-kDa RNA binding protein, which binds to the wild-type psbD leader, but is unable to recognize a nonfunctional leader mutant lacking the U-rich motif. RNA binding is restored in a chloroplast cis-acting suppressor. The functions of several site-directed psbD leader mutants were analyzed with transgenic C. reinhardtii chloroplasts and the in vitro RNA binding assay. A clear correlation between photosynthetic activity and the capability to bind RNA by the 40-kDa protein was observed. Furthermore, the data obtained suggest that the poly(U) region serves as a molecular spacer between two previously characterized cis-acting elements, which are involved in RNA stabilization and translation. RNA-protein complex formation depends on the nuclear Nac2 gene product that is part of a protein complex required for the stabilization of the psbD mRNA. The sedimentation properties of the 40-kDa RNA binding protein suggest that it interacts directly with this Nac2 complex and, as a result, links processes of chloroplast RNA metabolism and translation.     

Identification of novel mitochondrial protein components of Chlamydomonas reinhardtii. A proteomic approach

Pure mitochondria of the photosynthetic alga Chlamydomonas reinhardtii were analyzed using blue native-polyacrylamide gel electrophoresis (BN-PAGE). The major oxidative phosphorylation complexes were resolved: F(1)F(0)-ATP synthase, NADH-ubiquinone oxidoreductase, ubiquinol-cytochrome c reductase, and cytochrome c oxidase. The oligomeric states of these complexes were determined. The F(1)F(0)-ATP synthase runs exclusively as a dimer, in contrast to the C. reinhardtii chloroplast enzyme, which is present as a monomer and subcomplexes. The sequence of a 60-kD protein, associated with the mitochondrial ATP synthase and with no known counterpart in any other organism, is reported. This protein may be related to the strong dimeric character of the algal F(1)F(0)-ATP synthase. The oxidative phosphorylation complexes resolved by BN-PAGE were separated into their subunits by second dimension sodium dodecyl sulfate-PAGE. A number of polypeptides were identified mainly on the basis of their N-terminal sequence. Core I and II subunits of complex III were characterized, and their proteolytic activities were predicted. Also, the heterodimeric nature of COXIIA and COXIIB subunits in cytochrome c oxidase was demonstrated. Other mitochondrial proteins like the chaperone HSP60, the alternative oxidase, the aconitase, and the ADP/ATP carrier were identified. BN-PAGE was also used to approach the analysis of the major chloroplast protein complexes of C. reinhardtii.     

TCA1, a single nuclear-encoded translational activator specific for petA mRNA in Chlamydomonas reinhardtii chloroplast

We isolated seven allelic nuclear mutants of Chlamydomonas reinhardtii specifically blocked in the translation of cytochrome f, a major chloroplast-encoded subunit of the photosynthetic electron transport chain encoded by the petA gene. We recovered one chloroplast suppressor in which the coding region of petA was now expressed under the control of a duplicated 5' untranslated region from another open reading frame of presently unknown function. Since we also recovered 14 nuclear intragenic suppressors, we ended up with 21 alleles of a single nuclear gene we called TCA1 for translation of cytochrome b(6)f complex petA mRNA. The high number of TCA1 alleles, together with the absence of genetic evidence for other nuclear loci controlling translation of the chloroplast petA gene, strongly suggests that TCA1 is the only trans-acting factor. We studied the assembly-dependent regulation of cytochrome f translation--known as the CES process--in TCA1-mutated contexts. In the presence of a leaky tca1 allele, we observed that the regulation of cytochrome f translation was now exerted within the limits of the restricted translational activation conferred by the altered version of TCA1 as predicted if TCA1 was the ternary effector involved in the CES process.     

Chloroplast genes in Chlamydomonas affecting organelle ribosomes. Genetic and biochemical analysis of analysis of antibiotic-resistant mutants at several gene loci.

Six chloroplast gene mutants of Chlamydomonas reinhardtii resistant to spectinomycin, erythromycin, or streptomycin have been assessed for antibiotic resistance of their chloroplast ribosomes. Four of these mutations clearly confer high levels of antibiotic resistance on the chloroplast ribosomes both in vivo. Although one mutant resistant to streptomycin and one resistant to spectinomycin have chloroplast ribosomes as sensitive to antibiotics as those of wild type in vivo, these mutations can be shown to alter the wildtype sensitivity of chloroplast ribosomes in polynucleotide-directed amino acid incorporation in vitro. Genetic analysis of these six chloroplast mutants and three similar mutants (Sager, 1972), two of which have been shown to affect chloroplast ribosomes (Mets and Bogorad, 1972; Schlanger and Sager, 1974), indicates that in Chlamydomonas at least three chloroplast gene loci can affect streptomycin resistance of chloroplast ribosomes and that two can affect erythromycin resistance. The three spectinomycin-resistant mutants examined appear to be alleles at a single chloroplast gene locus, but may represent mutations at two different sites within the same gene. Unlike wild type, the streptomycin and spectinomycin resistant mutants which have chloroplast ribosomes sensitive to antibiotics in vivo, grow well in the presence of antibiotic by respiring exogenously supplied acetate as a carbon source, and have normal levels of cytochrome oxidase activity and cyanide-sensitive respiration. We conclude that mitochondrial protein synthesis in these mutants is resistant to these antibiotics, whereas in wild type it is sensitive. To explain the behavior of these two chloroplast gene mutants as well as other one-step mutants which are resistant both photosynthetically and when respiring acetate in the dark, we have postulated that a mutation in a single chloroplast gene may result in alteration of both chloroplast and mitochondrial ribosomes. Mitochondrial resistance would appear to be the minimal necessary condition for survival of all such mutants, and antibiotic-resistant chloroplast ribosomes would be necessary for survival only under photosynthetic conditions.     

The microalga Chlamydomonas reinhardtii as a platform for the production of human protein therapeutics

Microalgae are a diverse group of eukaryotic photosynthetic microorganisms. While microalgae play a crucial role in global carbon fixation and oxygen evolution, these organisms have recently gained much attention for their potential role in biotechnological and industrial applications, such as the production of biofuels. We investigated the potential of the microalga Chlamydomonas reinhardtii to be a platform for the production of human therapeutic proteins. C. reinhardtii is a unicellular freshwater green alga that has served as a popular model alga for physiological, molecular, biochemical and genetic studies. As such, the molecular toolkit for this microorganism is highly developed, including well-established methods for genetic transformation and recombinant gene expression. We transformed the chloroplast genome of C. reinhardtii with seven unrelated genes encoding for current or potential human therapeutic proteins and found that four of these genes supported protein accumulation to levels that are sufficient for commercial production. Furthermore, the algal-produced proteins were bioactive. Thus, the microalga C. reinhardtii has the potential to be a robust platform for human therapeutic protein production.     

Analysis of the nucleus-encoded and chloroplast-targeted rieske protein by classic and site-directed mutagenesis of Chlamydomonas.

Three mutants of the alga Chlamydomonas reinhardtii affected in the nuclear PETC gene encoding the Rieske iron-sulfur protein 2Fe-2S subunit of the chloroplast cytochrome b(6)f complex have been characterized. One has a stable deletion that eliminates the protein; two others carry substitutions Y87D and W163R that result in low accumulation of the protein. Attenuated expression of the stromal protease ClpP increases accumulation and assembly into b(6)f complexes of the Y87D and W163R mutant Rieske proteins in quantities sufficient for analysis. Electron-transfer kinetics of these complexes were 10- to 20-fold slower than those for the wild type. The deletion mutant was used as a recipient for site-directed mutant petC alleles. Six glycine residues were replaced by alanine residues (6G6A) in the flexible hinge that is critical for domain movement; substitutions were created near the 2Fe-2S cluster (S128 and W163); and seven C-terminal residues were deleted (G171och). Although the 6G6A and G171och mutations affect highly conserved segments in the chloroplast Rieske protein, photosynthesis in the mutants was similar to that of the wild type. These results establish the basis for mutational analysis of the nuclear-encoded and chloroplast-targeted Rieske protein of photosynthesis.     

The chloroplast ycf7 (petL) open reading frame of Chlamydomonas reinhardtii encodes a small functionally important subunit of the cytochrome b6f complex.

The small chloroplast open reading frame ORF43 (ycf7) of the green unicellular alga Chlamydomonas reinhardtii is cotranscribed with the psaC gene and ORF58. While ORF58 has been found only in the chloroplast genome of C.reinhardtii, ycf7 has been conserved in land plants and its sequence suggests that its product is a hydrophobic protein with a single transmembrane alpha helix. We have disrupted ORF58 and ycf7 with the aadA expression cassette by particle-gun mediated chloroplast transformation. While the ORF58::aadA transformants are indistinguishable from wild type, photoautotrophic growth of the ycf7::aadA transformants is considerably impaired. In these mutant cells, the amount of cytochrome b6f complex is reduced to 25-50% of wild-type level in mid-exponential phase, and the rate of transmembrane electron transfer per b6f complex measured in vivo under saturating light is three to four times slower than in wild type. Under subsaturating light conditions, the rate of the electron transfer reactions within the b6f complex is reduced more strongly in the mutant than in the wild type by the proton electrochemical gradient. The ycf7 product (Ycf7) is absent in mutants deficient in cytochrome b6f complex and present in highly purified b6f complex from the wild-type strain. Ycf7-less complexes appear more fragile than wild-type complexes and selectively lose the Rieske iron-sulfur protein during purification. These observations indicate that Ycf7 is an authentic subunit of the cytochrome b6f complex, which is required for its stability, accumulation and optimal efficiency. We therefore propose to rename the ycf7 gene petL.