scholarly journals Ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) is essential for growth of the methanotroph Methylococcus capsulatus Bath.

2021 ◽  
Author(s):  
Calvin A. Henard ◽  
Chao Wu ◽  
Wei Xiong ◽  
Jessica M. Henard ◽  
Brett Davidheiser-Kroll ◽  
...  
Keyword(s):  
Gas Flow ◽  
Carbon Sources ◽  
Calvin Cycle ◽  
Pentose Phosphate ◽  
Methylococcus Capsulatus ◽  
Bisphosphate Carboxylase ◽  
Key Enzymes ◽  
C Isotopes ◽  
Genes Encoding ◽  
Assimilation Capacity

The ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) enzyme found in plants, algae, and an array of autotrophic bacteria is also encoded by a subset of methanotrophs, but its role in these microbes has largely remained elusive. In this study, we identified that CO 2 was requisite for RubisCO-encoding Methylococcus capsulatus Bath growth in a bioreactor with continuous influent and effluent gas flow. RNA sequencing identified active transcription of several carboxylating enzymes, including key enzymes of the Calvin and serine cycles, that could mediate CO 2 assimilation during cultivation with both CH 4 and CO 2 as carbon sources. Marker-exchange mutagenesis of M. capsulatus Bath genes encoding key enzymes of potential CO 2 -assimilating metabolic pathways indicated that a complete serine cycle is not required while RubisCO is essential for growth of this bacterium. 13 CO 2 tracer analysis showed that CH 4 and CO 2 enter overlapping anaplerotic pathways and implicated RubisCO as the primary enzyme mediating CO 2 assimilation in M. capsulatus Bath. Notably, we quantified the relative abundance of 3-phosphoglycerate and ribulose-1,5-bisphosphate 13 C isotopes, which supported that RubisCO-produced 3-phosphoglycerate is primarily converted to ribulose-1-5-bisphosphate via the oxidative pentose phosphate pathway in M. capsulatus Bath. Collectively, our data establish that RubisCO and CO 2 play essential roles in M. capsulatus Bath metabolism. This study expands the known capacity of methanotrophs to fix CO 2 via RubisCO, which may play a more pivotal role in the Earth’s biogeochemical carbon cycling and greenhouse gas regulation than previously recognized. Further, M. capsulatus Bath and other CO 2 -assimilating methanotrophs represent excellent candidates for use in the bioconversion of biogas waste streams that consist of both CH 4 and CO 2 . Importance The importance of RubisCO and CO 2 in M. capsulatus Bath metabolism is unclear. In this study, we demonstrated that both CO 2 and RubisCO are essential for M. capsulatus Bath growth. 13 CO 2 tracing experiments supported that RubisCO mediates CO 2 fixation and a non-canonical Calvin cycle is active in this organism. Our study provides insights into the expanding knowledge of methanotroph metabolism and implicates dual CH 4 /CO 2 -utilizing bacteria as more important players in the biogeochemical carbon cycle than previously appreciated. In addition, M. capsulatus and other methanotrophs with CO 2 assimilation capacity represent candidate organisms for the development of biotechnologies to mitigate the two most abundant greenhouse gases CH 4 and CO 2 .

scholarly journals CO2 Assimilation, Photosynthetic Enzymes, and Carbohydrates of `Concord' Grape Leaves in Response to Iron Supply

2004 ◽  
Vol 129 (5) ◽  
pp. 738-744 ◽  
Author(s):  
Li-Song Chen ◽  
Brandon R. Smith ◽  
Lailiang Cheng
Keyword(s):  
Sucrose Concentration ◽  
Calvin Cycle ◽  
Sucrose Phosphate Synthase ◽  
Nonstructural Carbohydrates ◽  
Bisphosphate Carboxylase ◽  
Chl A ◽  
Photosynthetic Enzymes ◽  
Fe Content ◽  
Significant Difference ◽  
Assimilation Capacity

Own-rooted 1-year-old `Concord' grapevines (Vitis labruscana Bailey) were fertigated twice weekly for 11 weeks with 1, 10, 20, 50, or 100 μm iron (Fe) from ferric ethylenediamine di (o-hydroxyphenylacetic) acid (Fe-EDDHA) in a complete nutrient solution. As Fe supply increased, leaf total Fe content did not show a significant change, whereas active Fe (extracted by 2,2′-dipyridyl) content increased curvilinearly. Chlorophyll (Chl) content increased as Fe supply increased, with a greater response at the lower Fe rates. Chl a: b ratio remained relatively constant over the range of Fe supply, except for a slight increase at the lowest Fe treatment. Both CO2 assimilation and stomatal conductance increased curvilinearly with increasing leaf active Fe, whereas intercellular CO2 concentrations decreased linearly. Activities of key enzymes in the Calvin cycle, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), NADP-glyceraldehyde-3-phosphate dehydrogenase (GAPDH), phosphoribulokinase (PRK), stromal fructose-1,6-bisphosphatase (FBPase), and a key enzyme in sucrose synthesis, cytosolic FBPase, all increased linearly with increasing leaf active Fe. No significant difference was found in the activities of ADP-glucose pyrophosphorylase (AGPase) and sucrose phosphate synthase (SPS) of leaves between the lowest and the highest Fe treatments, whereas slightly lower activities of AGPase and SPS were observed in the other three Fe treatments. Content of 3-phosphoglycerate (PGA) increased curvilinearly with increasing leaf active Fe, whereas glucose-6-phosphate (G6P), fructose-6-phosphate (F6P), and the ratio of G6P: F6P remained unchanged over the range of Fe supply. Concentrations of glucose, fructose, sucrose, starch, and total nonstructural carbohydrates (TNC) at both dusk and predawn increased with increasing leaf active Fe. Concentrations of starch and TNC at any given leaf active Fe content were higher at dusk than at predawn, but both glucose and fructose showed the opposite trend. No difference in sucrose concentration was found at dusk or predawn. The export of carbon from starch breakdown during the night, calculated as the difference between dusk and predawn measurements, increased as leaf active Fe content increased. The ratio of starch to sucrose at both dusk and predawn also increased with increasing leaf active Fe. In conclusion, Fe limitation reduces the activities of Rubisco and other photosynthetic enzymes, and hence CO2 assimilation capacity. Fe-deficient grapevines have lower concentrations of nonstructural carbohydrates in source leaves and, therefore, are source limited.

scholarly journals Carbon Assimilation and Carbohydrate Metabolism of `Concord' Grape (Vitis labrusca L.) Leaves in Response to Nitrogen Supply

2003 ◽  
Vol 128 (5) ◽  
pp. 754-760 ◽  
Author(s):  
Li-Song Chen ◽  
Lailiang Cheng
Keyword(s):  
Calvin Cycle ◽  
Starch Degradation ◽  
Similar Response ◽  
Vitis Labrusca ◽  
Carbon Export ◽  
Bisphosphate Carboxylase ◽  
Key Enzymes ◽  
N Limitation ◽  
Wide Range ◽  
Leaf N

One-year-old grapevines (Vitis labrusca L. `Concord') were supplied twice weekly for 5 weeks with 0, 5, 10, 15, or 20 mm nitrogen (N) in a modified Hoagland's solution to generate a wide range of leaf N status. Both light-saturated CO2 assimilation at ambient CO2 and at saturating CO2 increased curvilinearly as leaf N increased. Although stomatal conductance showed a similar response to leaf N as CO2 assimilation, calculated intercellular CO2 concentrations decreased. On a leaf area basis, activities of key enzymes in the Calvin cycle, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), NADP-glyceraldehyde-3-phosphate dehydrogenase (GAPDH), phosphoribulokinase (PRK), and key enzymes in sucrose and starch synthesis, fructose-1,6-bisphosphatase (FBPase), sucrose phosphate synthase (SPS), and ADP-glucose pyrophosphorylase (AGPase), increased linearly with increasing leaf N content. When expressed on a leaf N basis, activities of the Calvin cycle enzymes increased with increasing leaf N, whereas activities of FBPase, SPS, and AGPase did not show significant change. As leaf N increased, concentrations of glucose-6-phosphate (G6P), fructose-6-phosphate (F6P), and 3-phosphoglycerate (PGA) increased curvilinearly. The ratio of G6P/F6P remained unchanged over the leaf N range except for a significant drop at the lowest leaf N. Concentrations of glucose, fructose, and sucrose at dusk increased linearly with increasing leaf N, and there was no difference between predawn and dusk measurements. As leaf N increased, starch concentration increased linearly at dusk, but decreased linearly at predawn. The calculated carbon export from starch degradation during the night increased with increasing leaf N. These results showed that 1) grapes leaves accumulated less soluble carbohydrates under N-limitation; 2) the elevated starch level in low N leaves at predawn was the result of the reduced carbon export from starch degradation during the night; and 3) the reduced capacity of CO2 assimilation in low N leaves was caused by the coordinated decreases in the activities of key enzymes involved in CO2 assimilation as a result of direct N limitation, not by the indirect feedback repression of CO2 assimilation via sugar accumulation.

scholarly journals Physiological Control and Regulation of the Rhodobacter capsulatus cbb Operons

1998 ◽  
Vol 180 (16) ◽  
pp. 4258-4269 ◽  
Author(s):  
George C. Paoli ◽  
Padungsri Vichivanives ◽  
F. Robert Tabita
Keyword(s):  
Rhodobacter Capsulatus ◽  
Pentose Phosphate ◽  
Open Reading Frames ◽  
Physiological Control ◽  
Gene Encoding ◽  
Bisphosphate Carboxylase ◽  
Promoter Fusion ◽  
Fructose 1,6 Bisphosphatase ◽  
Mutant Strains ◽  
Genes Encoding

ABSTRACT The genes encoding enzymes of the Calvin-Benson-Bassham (CBB) reductive pentose phosphate pathway in Rhodobacter capsulatus are organized in at least two operons, each preceded by a separate cbbR gene, encoding potential LysR-type transcriptional activators. As a prelude to studies ofcbb gene regulation in R. capsulatus, the nucleotide sequence of a 4,537-bp region, which includedcbbR II, was determined. This region contained the following open reading frames: a partial pgmgene (encoding phosphoglucomutase) and a complete qorgene (encoding NADPH:quinone oxidoreductase), followed by cbbR II, cbbF (encoding fructose 1,6-bisphosphatase), cbbP (encoding phosphoribulokinase), and part of cbbT (encoding transketolase). Physiological control of the CBB pathway and regulation of the R. capsulatus cbb genes were studied by using a combination of mutant strains and promoter fusion constructs. Characterization of mutant strains revealed that either form I or form II ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO), encoded by the cbbLS andcbbM genes, respectively, could support photoheterotrophic and autotrophic growth. A strain with disruptions in both cbbL and cbbM could not grow autotrophically and grew photoheterotrophically only when dimethyl sulfoxide was added to the culture medium. Disruption ofcbbP resulted in a strain that did not synthesize form II RubisCO and had a phenotype similar to that observed in the RubisCO-minus strain, suggesting that there is only onecbbP gene in R. capsulatus and that this gene is cotranscribed with cbbM. Analysis of RubisCO activity and synthesis in strains with disruptions in eithercbbR I orcbbR II, and β-galactosidase determinations from wild-type and mutant strains containing cbb Ip- andcbb IIp-lacZ fusion constructs, indicated that the cbb I andcbb II operons of R. capsulatus are within separate CbbR regulons.

scholarly journals Metabolic Signals That Lead to Control of CBB Gene Expression in Rhodobacter capsulatus

2002 ◽  
Vol 184 (7) ◽  
pp. 1905-1915 ◽  
Author(s):  
Mary A. Tichi ◽  
F. Robert Tabita
Keyword(s):  
Gene Expression ◽  
Rhodobacter Capsulatus ◽  
Pentose Phosphate ◽  
Gene Encoding ◽  
Bisphosphate Carboxylase ◽  
Starting Point ◽  
Mutant Strains ◽  
Genes Encoding ◽  
Metabolic Signal ◽  
Made In

ABSTRACT Various mutant strains were used to examine the regulation and metabolic control of the Calvin-Benson-Bassham (CBB) reductive pentose phosphate pathway in Rhodobacter capsulatus. Previously, a ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO)-deficient strain (strain SBI/II) was found to show enhanced levels of cbb I and cbb II promoter activities during photoheterotrophic growth in the presence of dimethyl sulfoxide. With this strain as the starting point, additional mutations were made in genes encoding phosphoribulokinase and transketolase and in the gene encoding the LysR-type transcriptional activator, CbbRII. These strains revealed that a product generated by phosphoribulokinase was involved in control of CbbR-mediated cbb gene expression in SBI/II. Additionally, heterologous expression experiments indicated that Rhodobacter sphaeroides CbbR responded to the same metabolic signal in R. capsulatus SBI/II and mutant strain backgrounds.

scholarly journals Genetic and Physiological Characterization of Fructose-1,6-Bisphosphate Aldolase and Glyceraldehyde-3-Phosphate Dehydrogenase in the Crabtree-Negative Yeast Kluyveromyces lactis

2022 ◽  
Vol 23 (2) ◽  
pp. 772
Author(s):  
Rosaura Rodicio ◽  
Hans-Peter Schmitz ◽  
Jürgen J. Heinisch
Keyword(s):  
Kluyveromyces Lactis ◽  
Regulation Of Gene Expression ◽  
Carbon Sources ◽  
Pentose Phosphate ◽  
Gene Encoding ◽  
Physiological Characterization ◽  
Genes Encoding ◽  
Fructose 1,6 Bisphosphate

The milk yeast Kluyveromyces lactis degrades glucose through glycolysis and the pentose phosphate pathway and follows a mainly respiratory metabolism. Here, we investigated the role of two reactions which are required for the final steps of glucose degradation from both pathways, as well as for gluconeogenesis, namely fructose-1,6-bisphosphate aldolase (FBA) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH). In silico analyses identified one gene encoding the former (KlFBA1), and three genes encoding isoforms of the latter (KlTDH1, KlTDH2, KlGDP1). Phenotypic analyses were performed by deleting the genes from the haploid K. lactis genome. While Klfba1 deletions lacked detectable FBA activity, they still grew poorly on glucose. To investigate the in vivo importance of the GAPDH isoforms, different mutant combinations were analyzed for their growth behavior and enzymatic activity. KlTdh2 represented the major glycolytic GAPDH isoform, as its lack caused a slower growth on glucose. Cells lacking both KlTdh1 and KlTdh2 failed to grow on glucose but were still able to use ethanol as sole carbon sources, indicating that KlGdp1 is sufficient to promote gluconeogenesis. Life-cell fluorescence microscopy revealed that KlTdh2 accumulated in the nucleus upon exposure to oxidative stress, suggesting a moonlighting function of this isoform in the regulation of gene expression. Heterologous complementation of the Klfba1 deletion by the human ALDOA gene renders K. lactis a promising host for heterologous expression of human disease alleles and/or a screening system for specific drugs.

scholarly journals CO2 Assimilation, Carbohydrate Metabolism, Xanthophyll Cycle, and the Antioxidant System of `Honeycrisp' Apple Leaves with Zonal Chlorosis

2004 ◽  
Vol 129 (5) ◽  
pp. 729-737 ◽  
Author(s):  
Li-Song Chen ◽  
Lailiang Cheng
Keyword(s):  
Excitation Energy ◽  
Carbohydrate Metabolism ◽  
Antioxidant System ◽  
Xanthophyll Cycle ◽  
Calvin Cycle ◽  
Thermal Dissipation ◽  
Nonstructural Carbohydrates ◽  
Bisphosphate Carboxylase ◽  
Key Enzymes ◽  
Fructose 1,6 Bisphosphatase

To determine the cause of a characteristic zonal chlorosis of `Honeycrisp' apple (Malus ×domestica Borkh.) leaves, we compared CO2 assimilation, carbohydrate metabolism, the xanthophyll cycle and the antioxidant system between chlorotic leaves and normal leaves. Chlorotic leaves accumulated higher levels of nonstructural carbohydrates, particularly starch, sorbitol, sucrose, and fructose at both dusk and predawn, and no difference was found in total nonstructural carbohydrates between predawn and dusk. This indicates that carbon export was inhibited in chlorotic leaves. CO2 assimilation and the key enzymes in the Calvin cycle, ribulose 1,5-bisphosphate carboxylase/oxygenase, NADP-glyceraldehyde-3-phosphate dehydrogenase, phosphoribulokinase, stromal fructose-1,6-bisphosphatase, and the key enzymes in starch and sorbitol synthesis, ADP-glucose pyrophosphorylase, cytosolic fructose-1,6-bisphosphatase, and aldose 6-phosphate reductase were significantly lower in chlorotic leaves than in normal leaves. However, sucrose phosphate synthase activity was higher in chlorotic leaves. In response to a reduced demand for photosynthetic electron transport, thermal dissipation of excitation energy (measured as nonphotochemical quenching of chlorophyll fluorescence) was enhanced in chlorotic leaves under full sun, lowering the efficiency of excitation energy transfer to PSII reaction centers. This was accompanied by a corresponding increase in both xanthophyll cycle pool size (on a chlorophyll basis) and conversion of violaxanthin to antheraxanthin and zeaxanthin. The antioxidant system, including superoxide dismutase and ascorbate peroxidase and the ascorbate pool and glutathione pool, was up-regulated in chlorotic leaves in response to the increased generation of reactive oxygen species via photoreduction of oxygen. These findings support the hypothesis that phloem loading and/or transport is partially or completely blocked in chlorotic leaves, and that excessive accumulation of nonstructural carbohydrates may cause feedback suppression of CO2 assimilation via direct interference with chloroplast function and/or indirect repression of photosynthetic enzymes.

scholarly journals Metabolic and Molecular Events Occurring during Chromoplast Biogenesis

2011 ◽  
Vol 2011 ◽  
pp. 1-13 ◽  
Author(s):  
Wanping Bian ◽  
Cristina Barsan ◽  
Isabel Egea ◽  
Eduardo Purgatto ◽  
Christian Chervin ◽  
...  
Keyword(s):  
Biochemical Analysis ◽  
Photosynthetic Activity ◽  
Chlorophyll Biosynthesis ◽  
Tomato Fruit ◽  
Calvin Cycle ◽  
Pentose Phosphate ◽  
Molecular Events ◽  
Tomato Fruit Ripening ◽  
Genes Encoding

Chromoplasts are nonphotosynthetic plastids that accumulate carotenoids. They derive from other plastid forms, mostly chloroplasts. The biochemical events responsible for the interconversion of one plastid form into another are poorly documented. However, thanks to transcriptomics and proteomics approaches, novel information is now available. Data of proteomic and biochemical analysis revealed the importance of lipid metabolism and carotenoids biosynthetic activities. The loss of photosynthetic activity was associated with the absence of the chlorophyll biosynthesis branch and the presence of proteins involved in chlorophyll degradation. Surprisingly, the entire set of Calvin cycle and of the oxidative pentose phosphate pathway persisted after the transition from chloroplast to chromoplast. The role of plastoglobules in the formation and organisation of carotenoid-containing structures and that of the Or gene in the control of chromoplastogenesis are reviewed. Finally, using transcriptomic data, an overview is given the expression pattern of a number of genes encoding plastid-located proteins during tomato fruit ripening.

scholarly journals Design and Evaluation of Novel Primers for the Detection of Genes Encoding Diverse Enzymes of Methylotrophy and Autotrophy

2012 ◽  
Vol 61 (1) ◽  
pp. 11-22 ◽  
Author(s):  
WEI-LIAN HUNG ◽  
WILLIAM G. WADE ◽  
YIN CHEN ◽  
DONOVAN P. KELLY ◽  
ANN P. WOOD
Keyword(s):  
Target Genes ◽  
Natural Populations ◽  
Ribulose Bisphosphate Carboxylase ◽  
Bacterial Strains ◽  
Methylamine Dehydrogenase ◽  
Bisphosphate Carboxylase ◽  
Key Enzymes ◽  
Hydroxypyruvate Reductase ◽  
Genes Encoding ◽  
Test Organisms

The phylogenetic significance of the diversity of key enzymes of methylotrophic and autotrophic metabolism is discussed. Primers for these key enzymes were designed using gene sequences encoding methanol dehydrogenase (mxaF; using subsets from database sequences for 22 Bacteria), hydroxypyruvate reductase (hpr; 36 sequences), methylamine dehydrogenase (mauA; 12 sequences), methanesulfonate monooxygenase (msmA; four sequences), and the ccbL and cbbM genes of ribulose bisphosphate carboxylase (26 and 23 sequences). These were effective in amplifying the correct gene products for the target genes in reference organisms and in test organisms not previously shown to contain the genes, as well as in some methylotrophic Proteobacteria isolated from the human mouth. The availability of the new primers increases the probability of detecting diverse examples of the genes encoding these key enzymes both in natural populations and in isolated bacterial strains.

scholarly journals Complete Genome Sequence of the Marine, Chemolithoautotrophic, Ammonia-Oxidizing Bacterium Nitrosococcus oceani ATCC 19707

2006 ◽  
Vol 72 (9) ◽  
pp. 6299-6315 ◽  
Author(s):  
Martin G. Klotz ◽  
Daniel J. Arp ◽  
Patrick S. G. Chain ◽  
Amal F. El-Sheikh ◽  
Loren J. Hauser ◽  
...  
Keyword(s):  
Tricarboxylic Acid Cycle ◽  
Reducing Power ◽  
Pentose Phosphate ◽  
Type I ◽  
Circular Chromosome ◽  
Bisphosphate Carboxylase ◽  
Terminal Electron Acceptor ◽  
Genes Encoding ◽  
Pentose Phosphate Pathways ◽  
Ammonia Oxidizing Bacterium

ABSTRACT The gammaproteobacterium Nitrosococcus oceani (ATCC 19707) is a gram-negative obligate chemolithoautotroph capable of extracting energy and reducing power from the oxidation of ammonia to nitrite. Sequencing and annotation of the genome revealed a single circular chromosome (3,481,691 bp; G+C content of 50.4%) and a plasmid (40,420 bp) that contain 3,052 and 41 candidate protein-encoding genes, respectively. The genes encoding proteins necessary for the function of known modes of lithotrophy and autotrophy were identified. Contrary to betaproteobacterial nitrifier genomes, the N. oceani genome contained two complete rrn operons. In contrast, only one copy of the genes needed to synthesize functional ammonia monooxygenase and hydroxylamine oxidoreductase, as well as the proteins that relay the extracted electrons to a terminal electron acceptor, were identified. The N. oceani genome contained genes for 13 complete two-component systems. The genome also contained all the genes needed to reconstruct complete central pathways, the tricarboxylic acid cycle, and the Embden-Meyerhof-Parnass and pentose phosphate pathways. The N. oceani genome contains the genes required to store and utilize energy from glycogen inclusion bodies and sucrose. Polyphosphate and pyrophosphate appear to be integrated in this bacterium's energy metabolism, stress tolerance, and ability to assimilate carbon via gluconeogenesis. One set of genes for type I ribulose-1,5-bisphosphate carboxylase/oxygenase was identified, while genes necessary for methanotrophy and for carboxysome formation were not identified. The N. oceani genome contains two copies each of the genes or operons necessary to assemble functional complexes I and IV as well as ATP synthase (one H+-dependent F0F1 type, one Na+-dependent V type).

scholarly journals The Genome of Heliobacterium modesticaldum, a Phototrophic Representative of the Firmicutes Containing the Simplest Photosynthetic Apparatus

2008 ◽  
Vol 190 (13) ◽  
pp. 4687-4696 ◽  
Author(s):  
W. Matthew Sattley ◽  
Michael T. Madigan ◽  
Wesley D. Swingley ◽  
Patricia C. Cheung ◽  
Kate M. Clocksin ◽  
...  
Keyword(s):  
Purple Bacteria ◽  
Calvin Cycle ◽  
Photosynthetic Apparatus ◽  
Complete Sequence ◽  
Ribulose Bisphosphate Carboxylase ◽  
Circular Chromosome ◽  
Bisphosphate Carboxylase ◽  
Anoxygenic Phototrophs ◽  
Genes Encoding ◽  
Heliobacterium Modesticaldum

ABSTRACT Despite the fact that heliobacteria are the only phototrophic representatives of the bacterial phylum Firmicutes, genomic analyses of these organisms have yet to be reported. Here we describe the complete sequence and analysis of the genome of Heliobacterium modesticaldum, a thermophilic species belonging to this unique group of phototrophs. The genome is a single 3.1-Mb circular chromosome containing 3,138 open reading frames. As suspected from physiological studies of heliobacteria that have failed to show photoautotrophic growth, genes encoding enzymes for known autotrophic pathways in other phototrophic organisms, including ribulose bisphosphate carboxylase (Calvin cycle), citrate lyase (reverse citric acid cycle), and malyl coenzyme A lyase (3-hydroxypropionate pathway), are not present in the H. modesticaldum genome. Thus, heliobacteria appear to be the only known anaerobic anoxygenic phototrophs that are not capable of autotrophy. Although for some cellular activities, such as nitrogen fixation, there is a full complement of genes in H. modesticaldum, other processes, including carbon metabolism and endosporulation, are more genetically streamlined than they are in most other low-G+C gram-positive bacteria. Moreover, several genes encoding photosynthetic functions in phototrophic purple bacteria are not present in the heliobacteria. In contrast to the nutritional flexibility of many anoxygenic phototrophs, the complete genome sequence of H. modesticaldum reveals an organism with a notable degree of metabolic specialization and genomic reduction.

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