Comparative kinetics and inhibition of a carbonic anhydrase from Chlamydomonas reinhardii

A native carbonic anhydrase (CA) from Chlamydomonas reinhardii exhibited an apparent molecular weight of 165,000. Denaturation under reducing conditions yielded a single polypeptide, mol. wt 42,000, while the same treatment under nonreducing conditions yielded higher molecular weight aggregates at least two of which could be reactivated to exhibit enzyme activity. In CO2 hydration the enzyme has a Km value of 3.4 mM and a relatively high turnover number. It is very sensitive to inhibition by sulfonamide and moderately sensitive to anion inhibition. In general, the C. reinhardii enzyme resembles many higher plant CAs in its existence as an oligomer. However, it is more like the CAs from animal species in its catalytic versatility and sensitivity to sulfonamides.

Protein-protein interactions in the 18S ATPase of Chlamydomonas outer dynein arms

When outer-row dynein arms are extracted from Chlamydomonas flagellar axonemes, they dissociate into two ATPase complexes with sedimentation coefficients of 12S and 18S. We immunized mice with 18S dynein and generated a library of monoclonal antibodies against the polypeptides in this complex. Antibodies were selected which specifically recognize the 18S alpha- and beta-heavy chains and the 83,000-dalton and 70,000-dalton intermediate chains. These antibodies were isolated and characterized for their ability to recognize determinants on both denatured antigens and native 18S dynein; 18S dynein was dissociated in stepwise fashion into smaller aggregates with ionic and nonionic detergents and the resulting subcomplexes were isolated by precipitation with specific monoclonal antibodies. The smallest aggregates isolated were heterodimers between the alpha-chain and a 16,000-dalton light chain and between the two intermediate chains. Additional close associations of the beta-heavy chain with an 18,000-dalton light chain and 70,000-dalton intermediate chain, and a weaker interaction between the intermediate chain heterodimer and light chains of 21,000 daltons and 12,500 daltons, were also observed. We present a model of 18S dynein substructure based upon this information.

Complete nucleotide sequence and mRNA-mapping of the large subunit gene of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) from Chlamydomonas moewusii

Nucleotide (nt) sequence of the large subunit (LS) gene of ribulose-1,5-bisphosphate carboxylase/oxygenase from the green alga, Chlamydomonas moewusii, and mapping of transcription ends was achieved by two new strategies. The deduced LS sequence of 475 amino acid residues was compared with similar genes from six other species; cyanobacteria, land plants and a related alga (C. reinhardtii). The most conserved regions are the three ribulose bisphosphate binding sites and the CO2 activator site. The nt sequence conservation outside the coding region is limited to only three segments within the 5'-flanking region: a region of tandem repeats, TATAA box and ribosome-binding site. Termination point of transcription is an 'A' residue 3' to the first of two 18-nt inverted repeats, which has the potential to form a stem-loop hairpin structure. The possible role of these potential regulatory features for transcription and translation, and similar structures in other LS genes is presented.

The sequence of the chloroplast atpB gene and its flanking regions in Chlamydomonas reinhardtii

The chloroplast (cp)-encoded CF1 ATPase beta-subunit gene (atpB) of Chlamydomonas reinhardtii and its flanking regions have been sequenced. The derived amino acid (aa) sequence is highly homologous to that of the beta-subunit gene in Escherichia coli, bovine heart mitochondria, and higher plant cp. In contrast to all other cp genomes, the CF1 epsilon subunit gene (atpE) does not lie at the 3' end of the atpB gene but maps to a position 92 kb away in the other single-copy region. Northern blots confirm that the beta subunit is not encoded as part of a dicistronic message as it is in higher plants. The region just upstream from the atpB gene in C. reinhardtii contains two small open reading frames (ORFs) and not the gene for the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase as is found in cp genomes of higher plants. No transcripts for either ORF were detected, but the codon usage in these ORFs as well as in the atpB gene follows the unique pattern of codon usage previously seen in other cp genes in C. reinhardtii.

Alpha-tubulin acetylase activity in isolated Chlamydomonas flagella

We have previously shown that the alpha-tubulin of Chlamydomonas flagella is synthesized as a precursor which is modified by acetylation in the flagellum during flagellar assembly. In this report, we show the presence of an alpha-tubulin acetylase activity in isolated Chlamydomonas flagella that is highly specific for alpha-tubulin of both mammalian brain and Chlamydomonas.

DNA-mediated transformation of Chlamydomonas reinhardi cells: use of aminoglycoside 3'-phosphotransferase as a selectable marker

Using a modified vector, we developed a method for DNA-mediated transformation of Chlamydomonas reinhardi with increased efficiency. The vector contained the yeast 2 microns origin of replication as a heterologous replicon. The aminoglycoside 3'-phosphotransferase (APH) gene linked to the simian virus 40 early promoter was used as an antibiotic selectable marker. The C. reinhardi transformants were resistant to 12 micrograms of G418 or 150 micrograms of kanamycin per ml. A quick-blot mRNA analysis demonstrated the presence of RNase-sensitive transcripts from the APH gene in the transformants, suggesting that the acquisition of antibiotic resistance was due to the expression of the APH gene. Southern blot analysis revealed the presence of free plasmid DNA in the transformant. The transforming vector was recovered by transforming recipient bacteria with the total DNA extracted from the C. reinhardi transformant.

Characterization of monoclonal antibodies against Chlamydomonas flagellar dyneins by high-resolution protein blotting

Monoclonal antibodies that recognize individual polypeptides of the outer arm dyneins of Chlamydomonas flagella were obtained and used to study the structural relationships between the various polypeptides. Immunoblot analysis showed that the gamma heavy chain of 12S dynein and the alpha and beta heavy chains and Mr 69,000 intermediate chain of 18S dynein each contain immunoreactive sites not found in the other dynein chains or in any other axonemal protein. We also used these antibodies to investigate possible structural similarities between dynein polypeptides from Chlamydomonas and phylogenetically distant species. No crossreactivity was observed with antibodies against either the alpha, beta, or gamma heavy chains, demonstrating that each Chlamydomonas heavy chain has structural features distinct from those present in dyneins from the other species tested. However, one antibody against the Mr 69,000 polypeptide recognized an intermediate chain (Mr 76,000) of latent-activity dynein-1 from the sea urchin Tripneustes gratilla. This result provides further evidence that 18S dynein and latent-activity dynein-1 are related. In the course of the above studies, we modified existing procedures to achieve efficient transfer of high molecular weight proteins from NaDodSO4/polyacrylamide gels to nitrocellulose sheets, and to detect small quantities of protein on nitrocellulose. Our modified procedure for staining total protein on nitrocellulose is rapid, inexpensive, and as sensitive as silver-staining of polyacrylamide gels. These methods should be useful to investigators working with small amounts of protein or requiring resolution of closely migrating polypeptides after transfer to nitrocellulose.

In vitro synthesis and assembly of the peripheral subunits of coupling factor CF1 (alpha and beta) by thylakoid-bound ribosomes

Bispecific antisera were prepared to a mixture of thylakoid membrane polypeptides 4.1 and 4.2. The identity of these polypeptides as the alpha and beta subunits of coupling factor (CF1) was established based on the cross-reactivity of the antisera toward CF1 from peas and by an analysis of the thm-24 mutant of Chlamydomonas which lacks the CF1 ATPase. Photochemical labeling of thylakoid membranes with hydrophobic and hydrophilic fluorescent probes indicated that these polypeptides did not significantly penetrate the membrane bilayer. Immunoprecipitation of the translation products of thylakoid-bound and soluble ribosomes showed the thylakoids to be the major site of synthesis of the polypeptides. Immunoprecipitation of the products of translation of total cellular RNA in a reticulocyte lysate showed no evidence for substantially higher molecular weight precursors. Further analysis of the thylakoid-bound synthesis of alpha and beta revealed that some of the in vitro synthesized polypeptides had been incorporated into the CF0-CF1 complex based on their release from membranes with trypsin and copurification with the CF0-CF1 ATPase.

An ATP-dependent supercoiling topoisomerase of Chlamydomonas reinhardtii affects accumulation of specific chloroplast transcripts

We have found that Chlamydomonas reinhardtii cells contain an ATP-dependent topoisomerase activity that supercoils circular DNA in vitro. Subsequent addition of a type I topoisomerase eliminates the supercoils. Like bacterial gyrase, this activity is inhibited by low concentrations of novobiocin (less than 0.1 microM) and by nalidixic acid (less than 0.1 microM). We have examined the effects of these topoisomerase inhibitors on accumulation of various chloroplast transcripts in vivo. Novobiocin differentially affected such transcripts; some transcripts became more abundant while many others were reduced in the presence of this drug. Nalidixic acid on the other hand caused many transcripts to become more abundant albeit to varying degrees. Inhibitors of this algal topoisomerase specifically stimulate a family of related transcripts which we have previously shown to be under light-dark control. We discuss how the inhibitors of this topoisomerase might exert their in vivo effects.

Functional determinants in transit sequences: import and partial maturation by vascular plant chloroplasts of the ribulose-1,5-bisphosphate carboxylase small subunit of Chlamydomonas

The precursor of the ribulose-1,5-bisphosphate carboxylase small subunit and other proteins from Chlamydomonas reinhardtii are efficiently transported into chloroplasts isolated from spinach and pea. Thus, similar determinants specify precursor-chloroplast interactions in the alga and vascular plants. Removal of all or part of its transit sequence was found to block import of the algal small subunit into isolated chloroplasts. Comparison of available sequences revealed a nine amino acid segment conserved in the transit sequences of all small subunit precursors. A protease in the vascular plant chloroplasts recognized this region in the Chlamydomonas precursor and produced an intermediate form of the small subunit. We propose that processing of the small subunit precursor involves at least two proteolytic events; only one of these has been evolutionarily conserved.

Identification and partial DNA sequence of the gene for the alpha-subunit of the ATP synthase complex of Chlamydomonas reinhardii chloroplasts

The partial DNA sequences of two unidentified genes flanking the gene for the large subunit of ribulose bisphosphate carboxylase of Chlamydomonas reinhardii have been reported [(1982) J. Mol. Biol. 162, 775-793]. Based on a comparison of the derived amino acid sequence of one of these genes with the corresponding sequences from Nicotiana tabacum chloroplast DNA and the E. coli atp (unc) operon, one Chlamydomonas gene is identified as coding for the alpha-subunit of the ATP synthase complex.

Subfractionation of Chlamydomonas 18 S dynein into two unique subunits containing ATPase activity

The 18 S dynein of the outer arm of Chlamydomonas flagella contains two different heavy polypeptide chains (Mr approximately equal to 340,000), two intermediate chains (Mr = 69,000 and 78,000), and eight light chains (Mr = 8,000-20,000). We report here that when purified 18 S dynein is dialyzed against a low ionic strength solution, it is dissociated into two smaller subunits which can then be separated and purified by sucrose density gradient centrifugation. One subunit contains one of the heavy chains and a Mr = 16,000 light chain; the other contains the other heavy chain and the remaining intermediate and light chains. Both subunits have ATPase activity. When recombined in the presence of 5-25 mM KCl, the subunits reassemble to form a particle similar to native 18 S dynein; neither subunit by itself can reform such a particle. 18 S dynein is therefore a heteropolymer containing two compositionally distinct subunits. Because the complete outer arm contains both 12 S and 18 S dyneins, the arm must have a total of three sites of ATP binding and hydrolysis: one associated with 12 S dynein and two with 18 S dynein.

delta-Aminolevulinic acid-synthesizing enzymes need an RNA moiety for activity

When Chlamydomonas enzymes that convert glutamate to delta-aminolevulinic acid are separated into three fractions, one of the fractions contains RNA, and the RNA moiety is needed for the enzyme activity.

The photoaffinity probe 8-azidoadenosine 5'-triphosphate selectively labels the heavy chain of Chlamydomonas 12 S dynein

Chlamydomonas 12 S dynein, which makes up part of the outer arm of the flagellar axoneme, consists of three polypeptides of 330,000, 22,000, and 18,000 daltons. We have used 8-azidoadenosine 5'-triphosphate (8-N3ATP), a photoaffinity analog of ATP, to investigate which of the dynein polypeptides contains the site of ATP hydrolysis. 8-N3ATP is a competitive inhibitor of the hydrolysis of ATP by 12 S dynein and is hydrolyzed by 12 S dynein in an ATP- and vanadate-sensitive fashion, indicating that it binds to the 12 S dynein hydrolytic site in the same way as ATP. When dynein was incubated with [gamma-32P]- or [alpha-32P]8-N3ATP in the presence of UV light to activate the azido moiety, the analog was incorporated into 12 S dynein's heavy polypeptide chain, but not its light chains. The incorporation was UV-dependent, was blocked by addition of ATP or vanadate plus ADP to the reaction mixture, and did not occur in heat-denatured dynein. These results strongly suggest that the hydrolytic site of 12 S dynein is contained in its heavy chain.

Partial purification and characterization of the major AP endonuclease from Chlamydomonas reinhardi

The major AP endonuclease from Chlamydomonas reinhardi has been partially purified and characterized. The enzyme has a molecular weight of about 38 000 as measured by molecular sieving. There is an absolute requirement for a divalent cation, with magnesium being better than manganese. The activity is stimulated by dithiothreitol and Triton X-100. The activity is sensitive to ionic strength, as 50 mM NaCl or KCl results in 70% inhibition. The enzyme is specific for apurinic and apyrimidinic (AP) sites and does not cleave DNA that has been damaged by ultraviolet light, methyl methanesulfonate, osmium tetroxide or sodium bisulfite. There is no deficiency in the AP endonuclease activity in extracts prepared from two mutants of Chlamydomonas that are sensitive to both ultraviolet light and methyl methanesulfonate. There was no evidence for induction of AP endonuclease after exposure of the cells to methyl methanesulfonate.

Evidence for solvent-induced conformational changes of the soluble Dunaliella chloroplast coupling factor 1 (CF1)

The ATPase activity of the chloroplast coupling factor 1 (CF1) isolated from the green alga Dunaliella is completely latent. A brief heat treatment irreversibly induces a Ca2+-dependent activity. The Ca2+-dependent ATPase activity can be reversibly inhibited by ethanol, which changes the divalent cation dependency from Ca2+ to Mg2+. Both the Ca2+-dependent and Mg2+-dependent ATPase activities of heat-treated Dunaliella CF1 are inhibited by monospecific antisera directed against Chlamydomonas reinhardi CF1. However, when assayed under identical conditions, the Ca2+-dependent ATPase activity is significantly more sensitive to inhibition by the antisera than is the Mg2+-dependent activity. These data are interpreted as indicating that soluble Dunaliella CF1 can exist in a variety of conformations, at least one of which catalyzes a Ca2+-dependent ATPase and two or more of which catalyze an Mg2+-dependent ATPase.

Effects of electron mediator charge properties on the reaction kinetics of hydrogenase from Chlamydomonas

Anions modulate hydrogenase activity in cell-free preparations of Chlamydomonas reinhardtii, and this modulation is greatly influenced by the charge properties of the redox agent included to mediate electron transfer to hydrogenase. With cationic methyl viologen as the electron mediator, anions stimulate the maximum velocity of H2 production (e.g., a 320% increase in the presence of 1 M NaCl) but have little effect on the Km for methyl viologen. Conversely, when hydrogenase activity is mediated by polyanionic metatungstate or ferredoxin, H2 production is strongly inhibited by anions (e.g., 70-77% inhibition by 0.2 M NaCl). This inhibition is primarily due to a reduced affinity of hydrogenase for these mediators (as evidenced by a large increase in Km values), rather than a change in the maximum velocity of the reaction. Anions have little effect on the kinetics of hydrogenase activity mediated by zwitterionic sulfonatopropyl viologen, a redox agent with a nearly neutral net charge. These results suggest the presence of a cationic region near the active site of hydrogenase. This cationic region, probably due to lysine and/or arginine residues, may serve in vivo to facilitate the interaction between hydrogenase and ferredoxin, the polyanionic, physiological electron mediator.

Identification of the alpha and beta subunits of the chloroplast coupling factor one in Chlamydomonas reinhardi

The alpha and beta subunits of the Chlamydomonas reinhardi coupling factor one have been identified by an immunochemical method and by the reaction of dicyclohexylcarbodiimide with the beta subunit. Antibodies raised against the C. reinhardi subunit with the highest apparent molecular weight react with the alpha subunit of spinach coupling factor one and antibodies raised against the beta subunit of the spinach chloroplast enzyme cross-react with the C. reinhardi subunit of lower apparent molecular weight. Dicyclohexylcarbodiimide also reacts with this subunit. We conclude therefore that the two subunits of highest apparent molecular weight can be named alpha and beta in order of decreasing apparent molecular weight, in contrast to the nomenclature suggested by Piccioni, R. G., Bennoun, P. and Chua, N.-H. [(1981) Eur. J. Biochem. 117, 93-102].

Nucleotide sequences of two pea cDNA clones encoding the small subunit of ribulose 1,5-bisphosphate carboxylase and the major chlorophyll a/b-binding thylakoid polypeptide

Two major chloroplast proteins are encoded by nuclear genes and synthesized on free cytoplasmic ribosomes: the small subunit of ribulose 1,5-bisphosphate carboxylase and the apoprotein components of the chlorophyll a/b light harvesting complex. We have recently reported the isolation of two cDNA clones from pea which encode both the small subunit of ribulose 1,5-bisphosphate carboxylase (pSS15) and the polypeptide 15 (pAB96), the major chlorophyll a/b binding protein (Broglie, R., Bellemare, G., Bartlett, S., Chua, N.-H., and Cashmore, A. R. (1981) Proc. Natl. Acad. Sci. U.S.A. 78, 7304-7308). To further characterize these clones, we determined their nucleotide sequence. Clone pSS15 contains a 691-base pair cDNA insert which encodes the entire 123 amino acids of the mature small subunit protein. In addition, this clone also encodes 33 amino acids of the NH2-terminal transit peptide extension and 148 nucleotides of the 3' noncoding region preceding the poly(A)tail. A second cDNA clone (pAB96) contains an 833-nucleotide insert which encodes most of polypeptide 15. The DNA sequence of this cloned cDNA was used to deduce the previously undetermined amino acid sequence of this integral thylakoid membrane protein. The nucleotide sequence of the cDNA clone, pSS15, should provide information concerning the role of the transit sequence in the transport of cytoplasmically synthesized chloroplast proteins. Similarly, the deduced amino acid sequence of polypeptide 15 will provide information for predicting its orientation in thylakoid membranes as well as its role in binding chlorophyll.

Circularization of linear viroid RNA via 2'-phosphomonoester, 3', 5'-phosphodiester bonds by a novel type of RNA ligase from wheat germ and Chlamydomonas

A novel type of RNA ligase activity in extracts of wheat germ or Chlamydomonas requires 2', 3'-cyclic phosphate and 5'-phosphate ends for ligation to form a 2'-phosphomonoester, 3',5'-phosphodiester bond. Using 5'-3 2P-labeled linear PSTV, we demonstrate that RNase T1-nicked viroid predominantly forms (formula; see text) U-bonds. Natural linear PSTV, however, forms mainly (formula; see text) A-bonds upon enzymatic circularization. We show that natural linear PSTV RNA has nicks between C181 and A182, or between C348 and A349, and that consequently C181 and C348 carry 2',3'-cyclophosphate termini.

Substructure of the outer dynein arm

The substructure of the outer dynein arm has been analyzed in quick-frozen deep-etch replicas of Tetrahymena and Chlamydomonas axonemes. Each arm is found to be composed of five morphologically discrete components: an elliptical head; two spherical feet; a slender stalk; and an interdynein linker. The feet make contact with the A microtubule of each doublet; the stalk contacts the B microtubule; the head lies between the feet and stalk; and the linker associates each arm with its neighbor. The spatial relationships between these five components are found to be distinctly different in rigor (ATP-depleted) versus relaxed (ATP- or vanadate plus ATP-treated) axonemes, and the stalk appears to alter its affinity for the B microtubule in the relaxed state. Images of living cilia attached to Tetrahymena cells show that the relaxed configuration is adopted in vivo. We relate our observations to morphological and experimental studies reported by others and propose several models that suggest how this newly described dynein morphology may relate to dynein function.

Membrane lipid metabolism in Chlamydomonas reinhardtii 137+ and Y-1: I. Biochemical localization and characterization of acyltransferase activities

The acyltransferases involved in the synthesis of the chloroplast membrane glycerolipids were analysed biochemically in dark-grown and greening Chlamydomonas reinhardtii y-1 as well as in the synchronous wild-type algae (strain 137+) and wild-type membranes. Using oleoyl-CoA as a substrate, three acyltransferase enzyme activities were detected. Glycerol-3-phosphate (glycerol-3-P) acyltransferase exhibited a pH optimum of 8.0 and was inhibited by addition of N-ethylmaleimide (MalNEt). Lysophosphatidate (PtdLys) acyltransferase exhibited a pH optimum of 7.0 and was not affected by the addition of MalNEt. From preliminary analyses, the activity at pH 5.5 appeared to be associated with dihydroxyacetone phosphate acyltransferase activity. Both glycerol-3-P and PtdLys acyltransferases were analysed further and found to be present in dark-grown and light-induced y-1 cells as well as in synchronous 137+ cells and their photosynthetic membranes. Both enzyme activities were enriched at least 10-fold in the photosynthetic membranes of 137+ chloroplasts relative to the activities present in the whole cells. This enrichment is indicative of their intrinsic localization in the thylakoids, suggesting that the photosynthetic membranes exhibit a greater degree of autonomy with respect to the synthesis of their membrane lipids than previously reported. A role for glycerol-3-P and PtdLys acyltransferases in the synthesis of the chloroplast membrane lipids is suggested further by the increases in both enzyme activities coincident with and preceding thylakoid biogenesis following light induction of dark-grown y-1 cells. Increased acyltransferase activity preceded the increase in the chlorophyll content of greening y-1 cells, which is a generally accepted marker for thylakoid synthesis. The increase in the PtdLys acyltransferase activity upon light-induction of the y-1 cells was both more immediate and more dramatic than the increase in glycerol-3-P acyltransferase activity. PtdLys acyltransferase activity was negligible in dark-grown cells and the dramatic increase upon light induction may be important in the subsequent initiation of chloroplast membrane lipid synthesis. On the basis of the localization of acyltransferase enzyme activities to the photosynthetic membranes of 137+ cells and the increase in acyltransferase activity both preceding and occurring in concert with thylakoid synthesis, we propose a direct role for the photosynthetic membranes in the synthesis of their membrane lipid components.