N Ahmed, S Shahab
N Ahmed, S Shahab. Phosphate Solubilization: Their Mechanism Genetics And Application. The Internet Journal of Microbiology. 2009 Volume 9 Number 1.
The global necessity to increase agricultural production from a steadily decreasing and degrading land resource base has placed considerable strain on agro ecosystems (Tilak, 2005). Current strategy is to maintain and improve agricultural productivity exclusively via the use of chemical fertilizers. Although the use of chemical fertilizers is credited with nearly fifty percent increase in agricultural production but they are closely associated with environmental pollution and health hazards (Gaur and Gaind, 1999). Many synthetic fertilizers contain acids, such as sulfuric acid and hydrochloric acid, which tend to increase the acidity of the soil, reduce the soil's beneficial organism population and interfere with plant growth. Generally, healthy soil contains enough nitrogen-fixing bacteria to fix sufficient atmospheric nitrogen to supply the needs of growing plants. However, continued use of chemical fertilizers may destroy these nitrogen-fixing bacteria. Furthermore, chemical fertilizers may affect plant health. For example, citrus trees tend to yield fruits that are lower in vitamin C when treated with synthetic fertilizer. Lack of trace elements in soil regularly dosed with chemical fertilizers is not uncommon. This lack of vital micronutrients can generally be attributed to the use of chemical fertilizers. On the other hand Biofertilizer adds nutrients to soil.Environmentally friendly biotechnological approaches offer alternatives to chemical fertilizers (Dobbelaere
The global necessity to increase agricultural production from a steadily decreasing and degrading land resource base has placed considerable strain on agro ecosystems (Tilak, 2005). Current strategy is to maintain and improve agricultural productivity exclusively via the use of chemical fertilizers. Although the use of chemical fertilizers is credited with nearly fifty percent increase in agricultural production but they are closely associated with environmental pollution and health hazards (Gaur and Gaind, 1999). Many synthetic fertilizers contain acids, such as sulfuric acid and hydrochloric acid, which tend to increase the acidity of the soil, reduce the soil's beneficial organism population and interfere with plant growth. Generally, healthy soil contains enough nitrogen-fixing bacteria to fix sufficient atmospheric nitrogen to supply the needs of growing plants. However, continued use of chemical fertilizers may destroy these nitrogen-fixing bacteria. Furthermore, chemical fertilizers may affect plant health. For example, citrus trees tend to yield fruits that are lower in vitamin C when treated with synthetic fertilizer. Lack of trace elements in soil regularly dosed with chemical fertilizers is not uncommon. This lack of vital micronutrients can generally be attributed to the use of chemical fertilizers. On the other hand Biofertilizer adds nutrients to soil.
Environmentally friendly biotechnological approaches offer alternatives to chemical fertilizers (Dobbelaere
It has been estimated that in some soil up to 75% of applied phosphate fertilizer may become unavailable to the plant because of mineral phase reprecipitation (Goldstein, 1986; Sundara
Phoshate Availability in Soil
Phosphorus (P) is one of the major essential macronutrients for biological growth and development (Ehrlich, 1990). It is present at levels of 400–1200 mg/kg of soil (Fernandez, 1988). The concentration of soluble P in soil is usually very low, normally at levels of 1 ppm or less then 1ppm (Goldstein, 1994). The cell might take up several P forms but the greatest part is absorbed in the forms of Phosphate (Beever and Burns, 1980).
Mineral forms of phosphorus are represented in soil by primary minerals, such as apatite, hydroxyapatite, and oxyapatite. They are found as part of the stratum rock and their principal characteristic is their insolubility. In spite of that, they constitute the biggest reservoirs of this element in soil because, under appropriate conditions, they can be solubilized and become available for plants and microorganisms. Mineral phosphate can be also found associated with the surface of hydrated oxides of Fe, Al, and Mn, which are poorly soluble and assimilable. This is characteristic of ferralitic soils, in which hydration and accumulation of hydrated oxides and hydroxides of Fe takes place, producing an increase of phosphorus fixation capacity (Fernandez, 1988).
There are two components of P in soil, organic and inorganic phosphates. A large proportion is present in insoluble forms, and therefore, not available for plant nutrition. Inorganic P occurs in soil, mostly in insoluble mineral complexes, some of these appearing after the application of chemical fertilizers. These precipitated forms cannot be absorbed by plants. Organic matter, on the other hand, is an important reservoir of immobilized P that accounts for 20–80% of soil P (Richardson, 1994).
A second major component of soil P is organic matter. Organic forms of P may constitute 30–50% of the total phosphorus in most soils, although it may range from as low as 5% to as high as 95% (Paul and Clark, 1988). Organic P in soil is largely in the form of inositol phosphate (soil phytate). It is synthesized by microorganisms and plants and is the most stable of the organic forms of phosphorus in soil, accounting for up to 50% of the total organic P (Dalal, 1977; Anderson 1980; Harley and Smith, 1983). Other organic P compounds in soil are in the form of phosphomonoesters, phosphodiesters including phospholipids and nucleic acids, and phosphotriesters.Of the total organic phosphorus in soil, only approximately 1% can be identified as nucleic acids or their derivatives (Paul and Clark, 1988). Various studies have shown that only approximately 1–5 ppm of phospholipids phosphorus occurs in soil, although values as high as 34 ppm have been detected (Paul and Clark, 1988). Large quantities of xenobiotic phosphonates, which are used as pesticides, detergent additives, antibiotics, and flame retardants, are released into the environment. These C-P compounds are generally resistant to chemical hydrolysis and biodegradation, but several reports have documented microbial P release from these sources (Ohtake, 1996; McGrath, 1998).
Organic Phosphate Solubilization
Organic phosphate solubilization is also called mineralization of organic phosphorus, and it occurs in soil at the expense of plant and animal remains, which contain a large amount of organic phosphorus compounds. The decomposition of organic matter in soil is carried out by the action of numerous saprophytes, which produce the release of radical orthophosphate from the carbon structure of the molecule. The organophosphonates can equally suffer a process of mineralization when they are victims of biodegradation (McGrath, 1995). The microbial mineralization of organic phosphorus is strongly influenced by environmental parameters; in fact, moderate alkalinity favors the mineralization of organic phosphorus (Paul and Clark, 1988) The degradability of organic phosphorous compounds depend mainly on the physicochemical and biochemical properties of their molecules, e.g. nucleic acids, phospholipids, and sugar phosphates are easily broken down, but phytic acid, polyphosphates, and phosphonates are decomposed more slowly (Ohtake, 1996; McGrath, 1995; McGrath 1998).
Phosphorus can be released from organic compounds in soil by three groups of enzymes:
The main activity apparently corresponds to the work of acid phosphatases and phytases because of the predominant presence of their substrates in soil.
Inorganic Phosphate Mineralization
Several reports have suggested the ability of different bacterial species to solubilize insoluble inorganic phosphate compounds, such as tricalcium phosphate, dicalcium phosphate, hydroxyapatite, and rock phosphate (Goldstein, 1986). In two thirds of all arable soils, the pH is above 7.0, so that most mineral P is in the form of poorly soluble calcium phosphates (CaPs). Microorganisms must assimilate P via membrane transport, so dissolution of CaPs to Pi (H2PO4) is considered essential to the global P cycle. Evaluation of samples from soils throughout the world has shown that, in general, the direct oxidation pathway provides the biochemical basis for highly efficacious phosphate solubilization in Gram-negative bacteria via diffusion of the strong organic acids produced in the periplasm into the adjacent environment. Therefore, the quinoprotein glucose dehydrogenase (PQQGDH) may play a key role in the nutritional ecophysiology of soil bacteria. MPS bacteria may be used for industrial bioprocessing of rock phosphate ore (a substituted fluroapatite) or even for direct inoculation of soils as a ‘biofertilizer’ analogous to nitrogen-fixing bacteria. Both the agronomic and ecological aspects of the direct oxidation mediated MPS trait. (Gold stein
Among the bacterial genera with this capacity are
Mechanism of Phosphate Solubilization
A number of theories have been proposed to explain the mechanism of phosphate solubilization. Important among them are:
Acid production theory
Proton and enzyme theory
Acid Production Theory
According to this theory, the process of phosphate solubilization by PSM is due to the production of organic acids which is accompanied by the acidification of the medium (Puente
The amount and type of the organic acid produced varied with the microorganism. The organic acids released in the culture filtrates react with the insoluble phosphate. The amount of soluble phosphate released depends on the strength and type of acid. Aliphatic acids are found to be more effective in P solubilization then phenolic acids and citric acids. Fumaric acid has highest P solubilizing ability. Tribasic and dibasic acids are also more effective than monobasic acids. In the presence of tribasic acids and dibasic acids, a secondary effect appears due to ability of these acids to form unionized association compounds with calcium thereby removing calcium from the solution and increasing soluble phosphate concentration (Gaur and Gaind,1999).
Organic acids contribute to the lowering of solution pH as they dissociate in a pH dependent equilibrium, into their respective anion(s) and proton(s). Organic acids buffer solution pH and will continue to dissociate as protons are consumed by the dissolution reaction (Welch
Besides organic acids, inorganic acids such as nitric and sulphuric acids are also produced by the nitrifying bacteria and thiobacillus during the oxidation of nitrogenous or inorganic compounds of sulphur which react with calcium phosphate and convert them into soluble forms (Gaur and Gaind, 1999).
The most efficient mineral phosphate solubilization (MPS) phenotype in Gram negative bacteria results from extracellular oxidation of glucose via the quinoprotein glucose dehydrogenase to gluconic acid (Kpomblekou
Gluconic acid biosynthesis is carried out by the glucose dehydrogenase (GDH) enzyme and the co-factor, pyrroloquinoline quinone (PQQ). Goldstein and Liu (1987) cloned a gene from
Gluconic acid is the principal organic acid produced by
Goebel and Krieg (1984) showed that gluconic acid was not formed during growth of either
Glucose is the precursor for synthesis of gluconic acid (Rodrigues etal 2004). This has suggested that Phosphate solubilization in these strains is mediated by glucose or gluconic acid metabolism. As solubilization of phosphate preceded detection of gluconic acid in the medium, perhaps even low levels of the acid (below the detection level of HPLC) started to dissolve the sparingly soluble phosphate. Alternatively, consumption of gluconic acid by growing cells could also take place. In
The latter may result from production of gluconic acid and NH4 + uptake, which may release protons to the medium. In the faster growing
The P-solubilizing capability of gluconic acid was much higher as compared to 2-keto-gluconic acid in the filtrate from strain
Protons can be pumped into the external medium by various membrane associated pumps which set up ionic gradients for the acquisition of nutrients (Jones and Gadd, 1990; Sigler, and Hofer, 1991; Gadd, 1993). In addition, protons arise from produced organic acids which also possess an organic acid anion which is usually capable of forming a complex with metal cation (Burgstaller,. and Schinner, 1993; Hughes and Poole, 1991).
The production of citric or gluconic acid and the extrusion of H+ result from membrane transport mechanisms was described as possible mechanism for dissolving rock-phosphate from hydroxy apatite, iron phosphate, and aluminum phosphate by
The nature and type of acid production is mainly dependent on the carbon source (Reyes
Proton and Enzyme Theory
Esterase type enzymes are known to be involved in liberating phosphorus from organic phosphatic compounds. PSMs (phosphate solubilizing microorganisms) are also known to produce phosphatase enzyme along with acids which cause the solubilization of P in aquatic environment (Alghazali
Solubilization without acid production is due to the release of protons accompanying respiration or ammonium assimilation (Taha
More solubilization occurs with ammonium salts than with nitrate salts as the nitrogen source in the media (Gaur and Gaind, 1999).
Besides these two mechanisms the production of chelating substances
Dissolution of phosphate in soil is a very important process for plant growth. Several studies have shown that the phosphate uptake by plants can be markedly increased by either mycorrhizal fungi (Azcon-Aguilar
Phosphate –Plant Interaction
Phosphorus is one of one of the major plant nutrient limiting plant growth. It plays a key role in nutrition of plants as it promotes development of deeper roots. The average soil is rich in phosphorus as it contains about 0.05% (w/w) phosphorus (Barber, 1984) but only one tenth of this is available to plants approximately 95–99% is present in the form of insoluble phosphates and hence cannot be utilized by the plants and due to its poor solubility and chemical fixation in the soil (Gaurand Gaind, 1999) causing a low efficiency of soluble P fertilizers.
To increase the availability of phosphorus for plants, large amounts of fertilizer is used on a regular basis. But after application, a large proportion of fertilizer phosphorus is quickly transferred to the insoluble forms. Therefore, very little percentage of the applied phosphorus is used, making continuous application necessary. (Abd Alla, 1994).
Soils microorganisms are involved in a range of processes that affect Phosphate transformation and thus influence the subsequent availability of phosphate to plant roots (Richardson, 2001). Free living phosphate solubilizing microorganisms (PSM) are always present in soils. The populations of inorganic Phosphate solubilizing microorganisms are sometimes very low, less than 102 CFU g-1 of soil as observed in a soil in Northern Spain (Peix
Plant Growth Promoting Bacteria
Although plant growth promoting bacteria occur in soil, usually their numbers are not high enough to compete with other bacteria commonly established in the rhizosphere. Therefore, for agronomic utility, inoculation of plants by target microorganisms at a much higher concentration than those normally found in soil is necessary to take advantage of their beneficial properties for plant yield enhancement. (Igual, 2001)
In recent years, interest in soil microorganisms that can promote plant growth has been increased considerably. The use of PGPRs to control soil borne pathogens is a practice with a promising future, because the Montreal Protocol (an international treaty to protect the earth from the detrimental effects) proposes the elimination of toxic chemicals. This has forced the plant scientists to look for new alternatives to replace fertilizers. A number of different bacteria have been reported to promote plant growth, including
Mechanism of Plant Growth Promotion
PGPR use one or more of direct or indirect mechanisms of action to improve plant growth and health. These mechanisms can probably be active simultaneously or sequentially at different stages of plant growth. They include
Biological nitrogen fixation
Biological control of plant pathogens
Improvement of other plant nutrients uptake
Phytohormone production like indole-3-acetic acid and indole butyric acid
The Phosphate solubilization effect seems to be the most important mechanism of plant growth promotion in moderately to fertile soils (Chabot
Rhizobia have well known beneficial symbiotic atmospheric nitrogen fixing symbiosis with legumes, have an excellent potential to be used as PGPR with non legumes (Antoun
Biological control of plant pathogens and deleterious microbes, through the production of antibiotics, lytic enzymes, hydrogen cyanide, and siderophores. Induction of the systemic resistance against many pathogens, insect and nematodes is also a recent indirect mechanism of action of PGPR (Ramamoorthy
They assist competition for nutrients and space which significantly improve plant health and promote growth as evidenced by increases in seedling emergence, vigor and yield (Antoun and Kloepper, 2001).
Some PGPR have the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase, which hydrolyses ACC, the immediate precursor of ethylene in plants (Glick
PGPR can promote mycorrhizal functioning. Recently for example, Villegas and Fortin (2001) showed an interesting specific synergistic interaction between the Phosphate solubilizing bacterium
All these traits that can be present in PGPR, illustrate how it is complex and difficult to associate the promotion of plant growth with Phosphate solubilization, and they explain in part the reason of obtaining better responses from plant inoculated with a mixture of PGPR. (Hodge, 2000) This has also forced plant scientist to look for PGPRs having more than one above-mentioned PGPRs trait in it.
The amelioration of phosphate deficiency by the application of costly and environmentally hazardous phosphate fertilizers is not an ideal solution and has generated serious issues about the continued viability of current agriculture practice. This has led to a search for more environmentally friendly and economically feasible strategies to improve crop production in low phosphorus soils. In an ideal manner, such strategies should enable the efficient use of phosphate solubilizing microorganisms. Several scientists have reported the ability of different bacterial species to solubilize insoluble inorganic phosphate compounds, such as tricalcium phosphate and dicalcium phosphate (Gaur and Gaind, 1999).
Role of Phosphate Solubilizing Bacteria as Biofertilizer
Biofertilizers are organisms that enrich the nutrient quality of soil. The main sources of biofertilizers are bacteria, fungi, and cyanobacteria (blue-green algae). Plants have a number of relationships with fungi, bacteria, and algae. After the introduction of chemical fertilizers during the last century, farmers were happy of getting increased yield in agriculture. But slowly chemical fertilizers started displaying their ill-effects such as leaching out, polluting water basins, destroying flora and fauna including friendly organisms, making the crop more susceptible to the attack of diseases, reducing the soil fertility and thus causing irreparable damage to the eco system (Rodrigues and Fraga 1999).
The principle behind this strategy is that microbes have various abilities which could be exploited for better farming practices. Some of them help in combat diseases while some have the ability to degrade soil complex compounds into simpler forms which are utilized by plants for their growth. They are extremely beneficial in enriching the soil by producing organic nutrients for the soil. To convert insoluble phosphates to a form accessible to the plants, like orthophosphate, is an important trait for a PGPB for increasing plant yields (Hilda
According to Statistics, the worldwide transaction amount of fertilizer is roughly US$40 billion. Out of this, 135 million metric tons of chemical fertilizer is applied each year, with sales volume of about US$30 billion. Although there are no clear application statistics for biofertilizer, however, its sales volume is estimated to be as much as US$3 billion. Commercial biofertilizers claiming to undergo phosphate solubilization using mixed bacterial cultures have been developed. Examples of these are: ‘Phylazonit-M’ (permission at No. 9961, 1992, by the Ministry of Agriculture of Hungary), a product containing
PSBs As Plant Growth Promoters
The substantial number of bacterial species, mostly those associated with the plant rhizosphere, may exert a beneficial effect upon plant growth. This group of bacteria has been termed “plant growth promoting bacteria” or PGPB, and among them, some phosphatesolubilizing bacteria (PSB) are already used as commercial biofertilizers for agricultural improvements. (SubbaRao1993; Rodríguez and Fraga 1999)
Among PGPR, phosphate solubilizing bacteria (PSB) are considered as promising biofertilizers since they can supply plants with phosphorus (P) from sources otherwise poorly available. Beneficial effects of the inoculation with PSB to many crop plants have been described by numerous authors (Antoun
Phosphate Supply and Demand
The expected annual growth rate in world demand for phosphate fertilizers is about 2.8 percent until 2012 (Figure 2), for an increase of 5 million tonnes P2O5compared with 2006.
About 58 percent of this growth will take place in Asia; consumption growth in South Asia is projected to surpass growth in East Asia at almost 5 percent/year. Rapid growth will also occur in East Europe and Central Asia (from a low base) and in Latin America. Phosphate fertilizer consumption will continue to decline marginally in West Europe and Central Europe.
World phosphoric acid capacity is expected to increase by 18 percent (8.1 million tonnes) by 2012. The largest increase is expected towards the end of the projection period in West Asia, when significant additional supply capacity should become operational in Saudi Arabia. Other significant expansions are scheduled in Africa (Morocco), China, and Latin America. In all other regions, phosphoric acid capacity is expected to remain almost constant or increase marginally during the period.(www.cfo)
Production of Growth Stimulating Phytohormones
Plant hormones also known as plant growth substances are naturally occurring chemicals that control plant growth and development. They regulate the rate at which individual part of a plant growth, integrate growth of those parts to form the whole organism and control reproduction. Since 1937, gibberllin, ethylene, cytokinin and abscisic acid have joined auxin as phytohormones and regarded as”classical five”(Ranjan2003).
The primary auxin in plants is indole- 3- acetic acid. Although other compounds with auxin activity such as indole butyric acid, phenyl acetic acid are also present in plants but little is known about their physiological role (Normanly
At the molecular level, auxins influence cell division, cell elongation and cell differentiation (Davies, 1995).At the macroscopic level, auxins direct vascular development, promotes apical dominance and lateral root formation, and mediate gravitropism and phototropism (Davies, 1995). Indole -3-acetic acid (IAA) and indole-3-butyric acid (IBA) are two endogenous auxins that can be interconverted (Ludwig-Miuller, 2000; Baartel
IAA is the main auxin in plants, controlling many important physiological processes including cell enlargement and division, tissue differentiation, and responses to light and gravity (Taiz and Zeiger, 1998). Bacterial IAA producers (BIPs) have the potential to interfere with any of these processes by input of IAA into the plant’s auxin pool. The consequence for the plant is usually a function of (i) the amount of IAA that is produced and (ii) the sensitivity of the plant tissue to changes in IAA concentration. A root, for instance, is one of the plant’s organs that is most sensitive to fluctuations in IAA, and its response to increasing amounts of exogenous IAA extends elongation of the primary root, formation of lateral and adventitious roots, (Davies, 1995).
IBA was found to increase root development in the propagation of stem cuttings (Ranjan,
Genetic Basis of PSBs (Phosphate Solubilizing Bacteria)
The genetic basis of phosphate solubilization is not well understood. The production of organic acids is considered to be the principal mechanism for mineral phosphate solubilization.
Several acid phosphatase genes from Gram negative bacteria have been isolated and characterized (Rossolini
For bacteria whose genes express in
Genetic Basis of Inorganic Phosphate Solubilization
The genetic basis of mineral phosphate solubilization i.e. the MPS phenotype is not well understood (Goldstein and Liu, 1987). Because the production of organic acids is considered to be the principal mechanism for mineral phosphate solubilization, it could be assumed that any gene involved in organic acid synthesis might have an effect on this character.
Very little is known regarding the genetic regulation governing the mineral phosphate solubilization trait. In fact, the information about the genetic or biochemical mechanisms involved in the synthesis of the GDH-PQQ halo enzyme is scant, and variations between constitutive and inducible phenotypes are observed among several bacterial species (Goldstein, 1994). Glucose, gluconate, manitol, and glycerol are among the possible inducers of the halo enzyme activity (Vanschie, 1987)
Goldstein and Liu (1987) cloned a gene from
Pyrroloquinoline Quinone (PQQ)
PQQ (4, 5-dihydro-4, 5-dioxo-1H-pyrrolo- [2, 3- ] quinoline-2, 7, 9-tricarboxylic acid) PQQ is an aromatic, tricyclic
PQQ is a prosthetic group required by several bacterial dehydrogenases, including methanol dehydrogenase (MDH) of Gram negative methylotrophs and some glucose dehydrogenases. PQQ is derived from two amino acids, tyrosine and glutamic acid (Houck, 1991,Van Kleef, 1988), but the pathway for its biosynthesis is unknown.
All carbon and nitrogen atoms of PQQ are derived from conserved tyrosine and glutamate residues of the PQQA peptide. R1 and R3 represent the N- and C-terminal portions of PQQA, respectively. R2 represent a three-amino-acid linker between Glu and Tyr.
PQQ is an important cofactor of bacterial dehydrogenases, linking the oxidation of many different compounds to the respiratory chain. PQQ was the first of the class of quinone cofactors that have been discovered in the last 18 years and make up the prosthetic group of quinoproteins (Duine, 1991; Klinman &Mu, 1994, Klinman, 1996). Although plants and animals do not produce PQQ themselves, PQQ has invoked considerable interest because of its presence in human milk and its remarkable antioxidant properties. Recently, the first potential eukaryotic PQQ dependent enzyme [(aminoadipic 6-semialdehyde-dehydrogenase (AASDH; U26)] has been identified, indicating that PQQ may function as a vitamin in mammals as well (Duine1999).
Present Status, Distribution and Significance of PQQ
PQQ was discovered in 1979 from a bacterium, and afterward it was reported to be in common foods. Because PQQ-deprived mice showed several abnormalities, such as poor development and breakable skin, PQQ has been considered as a candidate for vitamin. It was a mystery, that until 2003 it was not identified as vitamin. Since the first vitamin (now called vitamin B1) was discovered in 1910 by Dr. U. Suzuki, thirteen substances have been recognized as vitamins; the latest one was vitamin B12 found in 1948.So it takes 55 years to discover “PQQ” a previously identified substance as new vitamin ( Choi 2008; Kashara and kato2003).
After it had been established that PQQ occurs in several bacterial enzymes, a logical next question was whether it also occurs in higher organisms. Perhaps stimulated by the reports (Paz, 1988) that PQQ occurs at high levels in certain body fluids and tissues of mammalian milk, and in citrus fruits, several reports followed in which beneficial effects were ascribed to its administration, e.g. that a diet supplemented with PQQ improved the “health” of mice substantially or prevented the outbreak of certain diseases (Duine, 1999).
The quinoprotein glucose dehydrogenase (GCD) has been demonstrated in a number of microorganisms including the enteric bacteria
It has been concluded that organisms like
Biosynthetic Route of PQQ
The biosynthetic route of PQQ has not been elucidated yet, but it has been proposed that glutamate and tyrosine are precursors of PQQ (Houck
Expression of the six
Genetics of PQQ
Genes involved in PQQ synthesis have been cloned from
(Source: Felder et al., 2000)
In all four sequences, a small gene is present that encodes a peptide of 22-29 amino acids, which contains conserved tyrosine and glutamate residues. Since tyrosine and glutamate are the probable precursors for PQQ synthesis (Van Kleef and Duine, 1988; Houck
A series of experiments was carried out in which cell extracts of
(Source: Felder et al., 2000)
Genes involved in PQQ biosynthesis have been cloned from several organisms. Five
In what way could a 24-amino-acid polypeptide be involved in PQQ synthesis? Its small size makes a direct enzymatic function in the conversion of glutamate and tyrosine to PQQ unlikely. A regulatory role of the gene IV product in the expression of the other
Seven genes, called pqq genes, are required for PQQ biosynthesis in