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 ...