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For most of this century agricultural microbiologists and microbial ecologists have been interested in the ability of some bacteria to efficiently dissolve "insoluble" calcium phosphates. This phenotype has been termed mineral phosphate solubilizing (Mps). This minireview discusses recent work suggesting that the direct oxidation pathway for glucose dissimilation forms the metabolic basis for the strongly Mps+ phenotype observed in many species of gram negative bacteria. In the past, the physiological significance of the direct oxidation pathway for a given species was often considered obscure, giving rise to the term "dissimilatory bypass" to describe the apparent inefficient utilization of reducing power. The Mps function may provide a reason for the expression of this pathway in many bacteria whose bioenergetic needs are satisfied by the Embden-Meyerhof, Entner-Doudoroff or pentose phosphate pathways. The direct oxidation of glucose results in the production of gluconic acid and (often) 2-ketogluconic acid in the periplasmic space which, in turn, results in acidification of the region adjacent to the cell or colony. Calcium phosphates in the medium (soil) are dissolved by these acidic protons. It is proposed that extracellular acidification via the direct oxidation pathway may play an ecological role in some environments including soils where calcium phosphates provide a significant pool of unavailable mineral P. Specific strategies for testing this hypothesis including the use of DNA and antibody probes are discussed.

Maximum crop yields require sufficient phosphorus fertilization. Only phosphate in a soluble ionic form (Pi) is effective as a mineral nutrient. Current fertilizer technology supplies the soil solution with Pi via the application of large amounts of phosphate salts. Problems with this technology include energy-intensive production processes, the need for large scale mechanical application with associated environmental consequences, and reprecipitation of the phosphate into insoluble mineral complexes. It has been estimated that in some soils up to 75% of applied phosphate fertilizer may be lost to the plant because of mineral phase reprecipitation. Many approaches, ranging from cultural practices to biological inoculants such as mycorrhizal fungi, are being employed to enhance P-use efficiency. One area that is currently under-investigated is the ability of certain types of bacteria to solubilize mineral and organic phosphates. A review of the literature in the area of bacterial phosphate solubilization confirms that this trait is displayed by a wide range of bacteria. The phosphate starvation inducible (PSI) organic phosphate-solubilizing capability of E. coli is a component of a coordinately regulated gene system: the pho regulon. It has long been known that bacteria are also capable of solubilizing mineral phosphates such as hydroxyapatite. To date there has been no systematic study of the genetics of this phenomenon. Data from my laboratory indicate that the bacterial mineral phosphate-solubilizing (MPS) trait is regulated by the external level of Pi This conclusion is supported by results obtained from several types of molecular genetic studies. It is proposed that bacteria have mineral phosphate solubilizing (mps) genes. The potential agronomic applications of bacterial mineral and organic P solubilizing systems are discussed.

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Escherichia coli is capable of synthesizing the apo-glucose dehydrogenase enzyme (GDH) but not the cofactor pyrroloquinoline quinone (PQQ), which is essential for formation of the holoenzyme. Therefore, in the absence of exogenous PQQ, E. coli does not produce gluconic acid. Evidence is presented to show that the expression of an Erwinia herbicola gene in E. coli HB101(pMCG898) resulted in the production of gluconic acid, which, in turn, implied PQQ biosynthesis. Transposon mutagenesis showed that the essential gene or locus was within a 1.8-kb region of a 4.5-kb insert of the plasmid pMCG898. This 1. (J. Bacteriol. 171:447-455, 1989). In minicell analysis, pMCG898 encoded a protein with an Mr of 41,000. These data indicate that E. coli HB101 (pMCG898) produced the GDH-PQQ holoenzyme, which, in turn, catalyzed the oxidation of glucose to gluconic acid in the periplasmic space. As a result of the gluconic acid production, E. coli HB101(pMCG898) showed an enhanced mineral phosphate-solubilizing phenotype due to acid dissolution of the hydroxyapatite substrate.Quinoproteins play a major role in the regulation of bioenergetic processes in many gram-negative bacteria, including Erwinia spp. (2). For many species of Erwinia, the nonphosphorylating oxidation pathway is the primary mechanism for aldose sugar utilization (2). The quinoprotein glucose dehydrogenase (GDH) controls the unique step in this metabolic pathway. As such, GDH plays a direct role in the generation of the transmembrane proton motive force via the oxidation of aldose sugars (5). It is now generally accepted that, in gram-negative bacteria, the membranebound quinoprotein GDH is present on the periplasmic side of the cytoplasmic membrane. GDH is a member of the largest group of quinoproteins, those that use the cofactor 2,7,9-tricarboxyl-lH-pyrrolo[2,3-flquinoline-4,5-dione (PQQ) (5).Escherichia coli does not synthesize PQQ, but it does synthesize apo-GDH and is therefore dependent on uptake of PQQ from the environment or culture medium (9). Binding of the cofactor is presumably simplified by the location of the apoenzyme on the outer face of the cytoplasmic membrane. The GDH holoenzyme may be formed in E. coli when functional genes for PQQ biosynthesis are introduced. This system has been elegantly exploited by Goosen et al. (9) in order to identify and isolate PQQ synthase genes from Acinetobacter calcoaceticus on the basis of their expression on plasmids cloned into E. coli. The expression of PQQ * Corresponding authors.synthase genes in E. coli resulted in GDH activity in the absence of exogenous PQQ. We have used a slight modification of this system to identify a PQQ synthase gene from Erwinia herbicola EHO10 (18). Our experimental approach included a unique phenotypic screening system based on our interest in elucidation of the metabolic basis for the mineral phosphate solubilization (Mps+) phenotype in gram-negative bacteria.Poorly soluble mineral phosphates such as hydroxyapatite (HAP) are dissolved via acidification (7,8). The bacterial ...