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Query: HUMANGGP:003739 (CO2)
 48,959 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

A previously described Mendelian mutant of Chlamydomonas reinhardi, ac i72, exhibiting altered ribulosebisphosphate carboxylase activity and unable to grow on minimal medium is examined for changes in ribulosebisphosphate oxygenase activity. The ribulosebisphosphate oxygenase activity of the enzyme purified from both wild type and ac i72 is compared over a pH range from 7.0 to 9.5. Both enzymes exhibit maximum activity at pH 9.0. However, the ac i72 enzyme is twice as active as the wild type enzyme at a physiological pH of 7.0. The studies in vivo of the products of CO2 fixation of ac i72 and wild type cells in the presence of high and low O2 concentration shows that due to a lower level of carboxylation, the ac i72 cells fix CO2 at half the rate of wild type cells. In ac i72, 24% of the photosynthetically fixed 14C is channelled into the water-soluble fraction as opposed to 6% in wild type. Thin-layer chromatography of the water-soluble fraction showed extensive accumulation of components of the glycolate pathway in ac i72 as compared to wild type. This indicates that the oxygenase activity of the enzyme prevails in ac i72 in vivo. Since a high concentration of glycolate is toxic to cells of C. reinhardi, the high oxygenase activity of ac i72 provides an explanation for the inability of ac i72 to grow phototrophically even though its rate of CO2 fixation is half that of wild type. This toxicity to glycolate is overcome by growth under amber illumination or low O2 concentration.
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PMID:Characterization of the oxygenase activity in a mutant of Chlamydomonas reinhardi exhibiting altered ribulosebisphosphate carboxylase. 0 67

1. The effects of carbon dioxide, oxygen and pH on the inversion intensity of phototaxis of Chlamydomonas reinhardtii have been investigated. 2. With decreasing with CO2 tension the inversion intensity is decreased. 3. The gassing with CO2 can be substituted by hydrogencarbonate only to a small extent (20%). On the other hand, the effect of decreasing CO2 tension can be prevented also only in part by adjusting the pH to about 6.5-7.0. Thus the effect of CO2 on the inversion intensity of phototaxis is obviously a composite of a true CO2 effect and an effect of the concomitant pH change. 4. Oxygen has only a slight effect. In presence of oxygen (air) the phototactic reaction values are somewhat lower than in its absence. 5. Under certain conditions circadian rhythms seem to be initiated by changing oxygen as well as CO2 tensions. 6. Based on these results some contradictory results of older investigations are discussed.
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PMID:Effect of external factors on phototaxis of Chlamydomonas reinhardtii. II. Charbon dioxide, oxygen and pH. 1 29

The carbon isotope discrimination properties of a representative of each of the three types of photosynthetic bacteria Chlorobium thiosulfatophilum, Rhodospirillum rubrum and Chromatium and of the C3-alga Chlamydomonas reinhardii were determined by measuring the ratio of 13CO2 to 12CO2 incorporated during photoautotrophic growth. 2. Chromatium and R. rubrum had isotope selection properties similar to those of C3-plants, whereas Chlorobium was significantly different. 3. The results suggest that Chromatium and R. rubrum assimilate CO2 mainly via ribulose 1,5-diphosphate carboxylase and the associated reactions of the reductive pentose phosphate cycle, whereas Chlorobium utilizes other mechanisms. Such mechanisms would include the ferredoxin-linked carboxylation enzymes and associated reactions of the reductive carboxylic acid cycle.
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PMID:Mechanisms of CO2 fixation in bacterial photosynthesis studied by the carbon isotope fractionation technique. 40 96

Urea transport into the unicellular green alga Chlamydomonas reinhardi was investigated to further our understanding of controls operating on urea catabolism in this organism. Transport into cells grown with acetate and deprived of ammonia is a saturable process, mediated by at least two systems operating maximally at different external urea concentrations. The lower concentration system, with an apparent Km for urea of 5.1 micron, was the object of detailed study. Transport of urea from a saturating concentration (57 micron) into ammonia- and acetate-grown cells freshly suspended in ammonia-limited medium was not detected. Upon further culturing in the absence of ammonia, derepression occurred with transport ability, first appearing at about 1 h , reaching a maximum at about 2 h, and maintaining this maximum at least 5 h. In contrast to this, CO2-grown cells became derepressed more slowly, and maximum transport ability was not maintained. Addition of ammonia or methylamine (5 mM) during nitrogen deprivation prevented further increases in transport ability and caused loss of previously acquired transport ability. Cycloheximide (10 microng/ml) had a similar effect. Energy uncouplers or dark, anaerobic conditions depressed transport. By these criteria, transport from low urea concentrations is mediated by a process that requires protein synthesis and activation by cellular energy, and the process has a rapid rate of turnover and of deactivation by ammonia.
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PMID:Transport of urea at low concentrations in Chlamydomonas reinhardi. 85 84

A mutant, ac i72, of Chlamydomonas reinhardi possessing an altered ribulosebisphosphate carboxylase and unable to grow on minimal medium has been isolated and characterized. Comparison of ribulosebisphosphate carboxylase purified from both wild type and ac i72 strains is given. The enzyme from ac i72 shows alterations in several characteristics: (a) the specific activity is reduced to 35% that of wild type, (b) the V for both substrates is reduced 3-6 fold, (c) the Mg2+ requirement for maximal activity is 3 times greater, (d) the inhibitory effect of Cl- is greater, and (e) the isoelectric point is changed (6.0 for wild type and 5.8 for ac i72). However, the ribulosebisphosphate carboxylase from ac i72 is identical to that from wild type with respect to pH requirement, temperature sensitivity, subunit structure, and sedimentation characteristic. Other photosynthetic properties of wild type and ac i72 cells were also compared. CO2 fixation in ac i72 in vivo is reduced proportionally to the reduction in activity of the enzyme, but the level of O2 evolution is the same as in wild-type cells. Photosynthetic electron transport, 70-S ribosome content, and chlorophyll content are unaltered in ac i72. The chloroplast ultrastructure of ac i72 cells is distinctly different from that of wild-type cells. The inheritance of the mutation is Mendelian.
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PMID:A mutant strain of Chlamydomonas reinhardi exhibiting altered ribulosebisphosphate carboxylase. 124 67

From high-CO2 (5% CO2) grown unicellular green alga, Chlamydomonas reinhardtii, carbonic anhydrase (CA) was isolated by affinity chromatography and characterized. Isolated CA was identified as an isozyme (CA2) which is the product from the second gene CAH2 by peptide sequencing. The CA2 was inactivated by dithiothreitol. This treatment caused dissociation of CA2 into the large (38 kDa) and small subunits (4243 Da). The molecular mass of the CA2 holoenzyme measured by low-angle laser light-scattering photometry and precision differential refractometry combined with gel-filtration HPLC was 87.9 kDa. These results and gene structure indicate that CA2 is a heterotetramer consisting of two large and two small subunits linked by disulfide bonds like CA1, which is the CAH1 gene product. The specific activity of CA2 purified by anion-exchange HPLC was 3300 units per mg protein, which was approximately 1.6 times higher than that of CA1. Therefore, it was concluded that two structurally related isozymes, CA1 and CA2, are present in the wild type cells of C. reinhardtii and differentially regulated by the atmospheric CO2 concentration.
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PMID:Characterization of carbonic anhydrase isozyme CA2, which is the CAH2 gene product, in Chlamydomonas reinhardtii. 136 43

Synthesis of the photosystem II D1 protein as a precursor with a carboxyl-terminal extension occurs in almost all eukaryotic photosynthetic organisms examined so far, as well as in cyanobacteria. Processing of the D1 precursor has been recently postulated to play a regulatory role in the light-dependent migration of photosystem II units from the unstacked to the stacked thylakoids (Bowyer, J. M., Packer, J. C. L., McCormack, B. A., Whitelegge, J. P., Robinson, C., and Taylor, M. A. (1992) J. Biol. Chem. 267, 5424-5433). To test this hypothesis, site-directed mutagenesis and chloroplast transformation have been used to create a "preprocessed" mutant Chlamydomonas strain which synthesizes mature D1 protein directly. We have found that this strain is indistinguishable from wild type in terms of photosynthetic performance and cell doubling time under CO2- and light-saturated photoautotrophic growth conditions.
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PMID:The carboxyl-terminal extension of the D1 protein of photosystem II is not required for optimal photosynthetic performance under CO2- and light-saturated growth conditions. 151 1

To understand the CO2-concentrating mechanism in cyanobacteria, a genomic DNA fragment that complements a temperature-sensitive high-CO2 (5%)-requiring mutant of Synechococcus PCC7942 has been isolated. An open reading frame (ORF272) encoding a polypeptide of 272 amino acids (Mr, 30,184) was found within the genomic region located 20 kilobases downstream from the genes for ribulose-1,5-bisphosphate carboxylase/oxygenase (rbcLS). Insertion of a kanamycin-resistance gene cartridge within the ORF272 in wild-type cells led to a high-CO2-requiring phenotype. Strains carrying a gene disabled by insertional mutagenesis accumulated inorganic carbon in the cells, but they could not fix it efficiently, even though ribulose-1,5-bisphosphate carboxylase activity was comparable to that of the wild-type strain. Therefore, the ORF272 was designated as a gene icfA, which is essential to inorganic carbon fixation. Furthermore, the predicted icfA gene product shared significant sequence similarities with plant chloroplast carbonic anhydrases (CAs) from pea (22%) and spinach (22%) and also with the Escherichia coli cynT gene product (31%), which was recently identified to be E. coli CA. These results indicate that the putative CA encoded by icfA is essential to photosynthetic carbon dioxide fixation in cyanobacteria and that plant chloroplast CAs may have evolved from a common ancestor of the prokaryotic CAs, which are distinct from mammalian CAs and Chlamydomonas periplasmic CAs.
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PMID:A gene homologous to chloroplast carbonic anhydrase (icfA) is essential to photosynthetic carbon dioxide fixation by Synechococcus PCC7942. 158 76

A new isoenzyme of carbonic anhydrase has been isolated and purified from Chlamydomonas reinhardtii. This carbonic anhydrase is composed of two nonidentical subunits with apparent molecular masses of 39 and 4.5 kDa and is located in the periplasmic space. This is the second periplasmic carbonic anhydrase found in C. reinhardtii. Two genes, CAH1 and CAH2, which code for carbonic anhydrase, have been recently described by Fujiwara et al. (Fujiwara, S., Fukuzawa, H., Tachiki, A., and Miyachi, S. (1990) Proc. Natl. Acad, Sci. U.S.A. 87, 9779-9783). The CAH1 gene codes for a periplasmic carbonic anhydrase which is induced under low CO2 conditions and is well characterized. The carbonic anhydrase characterized in this report was isolated from a mutant that is unable to synthesize the CAH1 gene product. Amino acid sequencing demonstrates that this newly isolated carbonic anhydrase is the CAH2 gene product. This is the first report of another functional carbonic anhydrase in C. reinhardtii.
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PMID:Partial characterization of a new isoenzyme of carbonic anhydrase isolated from Chlamydomonas reinhardtii. 190 96

Photosynthesis-deficient mutant 45-3B of the green alga Chlamydomonas reinhardtii contains a chloroplast mutation that causes valine-331 to be replaced by alanine within the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase. This amino acid substitution occurs in loop 6 of the alpha/beta-barrel active site, three residues distant from catalytic lysine-334. The mutation reduces the specific activity of the enzyme and also reduces its CO2/O2 specificity factor by 42%, but the amount of holoenzyme is unaffected. In a previous study, an intragenic-suppressor mutation, named S40-9D, was selected that causes threonine-342 to be replaced by isoleucine, thereby increasing the CO2/O2 specificity of the mutant enzyme by 36%. To determine which other residues might be able to complement the original mutation, nine additional genetically independent revertants have now been analyzed. Another intragenic suppressor, represented by mutation S61-2J, causes glycine-344 to be replaced by serine. This change increases the CO2/O2 specificity of the mutant enzyme by 25%. Of the revertants recovered and analyzed, the mutant enzyme was improved only due to true reversion or by intragenic suppression mediated by substitutions at residues 342 or 344. Changes in the physical properties of the two pairs of complementing substitutions indicate that steric effects within loop 6 are responsible for the observed changes in the CO2/O2 specificity of the enzyme.
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PMID:Complementing amino acid substitutions within loop 6 of the alpha/beta-barrel active site influence the CO2/O2 specificity of chloroplast ribulose-1,5-bisphosphate carboxylase/oxygenase. 190 74


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