Regulation of purine ribonucleotide synthesis by end product inhibition. I. Effect of purine nucleotides on inosine-5'-phosphate dehydrogenase, xanthosine-5'-phosphate aminase and adenylosuccinate lyase of Bacillus subtilis

Combining engineering and evolution to create novel metabolic mutualisms between species

Synthetic communities can be used as model systems for the molecular examination of species interactions. Manipulating synthetic communities to create novel beneficial interactions provides insight into the mechanisms of cooperation as well as the potential to improve the productivity of industrially relevant systems. Here, we present a general scheme for evolving a mutualism from a bacterial consortium in which one species consumes the by-products of another.

Human IMP dehydrogenase catalyzes the dehalogenation of 2-fluoro- and 2-chloroinosine 5'-monophosphate in the absence of NAD

The ability of human type II inosine monophosphate dehydrogenase (IMPDH, EC 1.1.1.205) to catalyze the formation of xanthosine 5'-monophosphate (XMP) from C2 halogen-substituted analogs of IMP has been investigated. Adenosine deaminase was used to enzymatically synthesize 2-fluoroinosine and 2-chloroinosine from the 2-fluoro- and 2-chloroadenosine nucleoside analogs. Chemical phosphorylation yielded the corresponding 5'-nucleoside monophosphate derivatives. IMPDH catalyzes the conversion of both 2-fluoro- and 2-chloroinosine 5'-monophosphate (2-F- and 2-Cl-IMP) to XMP. The dehalogenation reactions proceed without nicotinamide adenine dinucleotide (NAD), the hydride acceptor required for the oxidation of IMP, the normal substrate of the enzyme. Formation of XMP from the 2-halo-IMPs was verified by UV absorption spectroscopy and by HPLC. Formation of XMP from 2-F-IMP yielded stoichiometric amounts of fluoride anion. IMP and XMP were competitive inhibitors toward 2-Cl-IMP in the dehalogenation reaction. Neither 2-F-IMP nor 2-Cl-IMP irreversibly inactivate IMPDH. Kinetic constants for the dehalogenation reactions have been determined and compared to the dehydrogenation reaction at 25 degrees C. (For 2-F-IMP: kcat = 0.058 s-1, Km = 62 microM. For 2-Cl-IMP: kcat = 0.049 s-1, Km = 48 microM. For the IMP dehydrogenation reaction: kcat = 0.25 s-1, Km [IMP] = 4.1 microM, Km [NAD] = 29 microM). Hydrolytic dehalogenation of 2-halo-IMPs without a requirement for NAD demonstrates the formation of a tetrahedral intermediate in the catalytic mechanism of IMP dehydrogenase.

Nucleotide sequence of the guaB locus encoding IMP dehydrogenase of Escherichia coli K12

IMP dehydrogenase, the product of the guaB locus in Escherichia coli K12, catalyzes the synthesis of XMP by the NAD+ dependent oxidation of IMP. The guaB locus has been subcloned from the Clarke and Carbon plasmid pLC34-10. The sequence of the guaB structural gene and surrounding DNA was determined by the dideoxy chain termination method of Sanger. The 1.533 kb guaB gene encodes an IMP dehydrogenase subunit of molecular weight 54,512. S1 nuclease mapping placed the site of guaBA mRNA initiation approximately 188 bp from the start of the guaB structural gene. The -10 and -35 regions that define the guaBA promoter were located upstream of the start of the guaBA transcription initiation site. The control region of approximately 188 bp does not show any obvious potential for secondary structure. A secondary lambda att site has been identified 42 bp distal to the guaB start codon.

Purification and properties of inosine monophosphate oxidoreductase from nitrogen-fixing nodules of cowpea (Vigna unguiculata L. Walp)

Using ammonium sulfate precipitation, gel filtration, and affinity chromatography, inosine monophosphate (IMP) oxidoreductase (EC 1.2.1.14) was isolated from the soluble proteins of the plant cell fraction of nitrogen-fixing nodules of cowpea (Vigna unguiculata L. Walp). The enzyme, purified more than 140-fold with a yield of 11%, was stabilized with glycerol and required a sulfydryl-reducing agent for maximum activity. Gel filtration indicated a molecular weight of 200,000, and sodium dodecyl sulfate-gel electrophoresis a single subunit of 50,000 Da. The final specific activity ranged from 1.1 to 1.5 mumol min-1 mg protein-1. The enzyme had an alkaline pH optimum and showed a high affinity for IMP (Km = 9.1 X 10(-6) M at pH 8.8 and NAD levels above 0.25 mM) and NAD (Km = 18-35 X 10(-6) M at pH 8.8). NAD was the preferred coenzyme, with NADP reduction less than 10% of that with NAD, while molecular oxygen did not serve as an electron acceptor. Intermediates of ureide metabolism (allantoin, allantoic acid, uric acid, inosine, xanthosine, and XMP) did not affect the enzyme, while AMP, GMP, and NADH were inhibitors. GMP inhibition was competitive with a Ki = 60 X 10(-6) M. The purified enzyme was activated by K+ (Km = 1.6 X 10(-3) M) but not by NH+4. The K+ activation was competitively inhibited by Mg2+. The significance of the properties of IMP oxidoreductase for regulation of ureide biosynthesis in legume root nodules is discussed.

Response of the pyrimidine pathway of Escherichia coli K 12 to exogenous adenine and uracil

The effect of exogenous adenine or uracil upon the de novo pathway for synthesis of pyrimidine nucleotides in Escherichia coli K12 was investigated. Parameters studied were levels of the enzymes carbamoyl phosphate synthase (EC 2.7.2.9), aspartate carbamoyltransferase (EC 2.1.3.2) and orotate phosphoribosyltransferase (EC 2.4.2.10) and the intermediates carbamoyl phosphate, aspartate and orotate, together with the contributions of exogenous uracil and aspartate to intracellular pyrimidine nucleotide. Taken with earlier data [Bagnara, A.S. & Finch, L. R. (1974) Eur. J. Biochem- 41, 421--430] on contents of UTP, CTP and 5-phosphoribosyl 1-diphosphate in cultures of this strain after the addition of adenine or uracil, the results obtained provide new insights into the regulatory mechanisms operating on the pathway in vivo. These insights enable evaluation of the contributions of such factors as limitation for a substrate, feed-back allosteric control by end products and enzyme repression/depression mechanisms. The evidence presented indicates that depressed levels of orotate phosphoribosyltransferase in E. coli K12 result in the wasteful ultilization of asparatate for excess synthesis of pyrimidine nucleotide precursors during balanced growth of the strain in minimal medium. Exogenous adenine increases the excessive accumulation of these precursors by lowering the intracellular content of 5-phosphoribosyl 1-diphosphate (Bagnara and Finch, 1974). This causes a decrease in the conversion of orotate to orotidine 5'-monophosphate, thus lowering the utilization or orotate and its precursors for synthesis of pyrimidine nucleotides. Further, since the contents of these nucleotide end products are thereby decreased (Bagnara nad Finch, 1974), theri feed-back on the early steps in the pathway is diminished and the production of the precursors is increased. It is postulated that growth of E. coli K12 under these conditions is limited by a compound that is metabolically related to precursors to aspartate.

Purine metabolism in neisseria meningitidis. 4. Interconversion of purine ribonucleotides

Radioactive hypoxanthine and adenine were readily incorporated into the nucleic acids (RNA + DNA) of the prototrophic strain M1 of Neisseria meningitidis with labelling of the purine bases adenine and guanine. Contrary to this, adenine was incorporated into the nucleic acids with labelling of adenine only in the purine mutant 6-1c which requires adenine and guanine for growth. Guanine was in both strains incorporated into the nucleic acids with labelling of the purine base guanine only. Crude meningococcal extracts from the prototrophic strain M1 and strain 6-1b requiring histidine, proline, adenine and guanine for growth (his pro A G) were found to have activities corresponding to adenylosuccinate synthetase (EC 6.3.4.4), adenylosuccinate lyase (EC 4.3.2.2.), inosine 5'-phosphate dehydrogenase (EC 1.2.1.14)) and guanosine 5'-phosphate synthetase (EC 6.3.5.2). This last enzyme was active also with (NH4)2SO4 as amino donor. No activity corresponding to guanosine 5'-phosphate reductase (EC 1.6.6.8) could be demonstrated in either strain. The findings show that the mutant strains 6-1b and 6-1c are both blocked between aminoimidiazolcarboxamide ribonucleotide (AICA-R) and inosine 5'-phosphate (IMP). Transformation experiments with mutants requiring adenine and guanine for growth indicate that the negative property of these mutants is due to a lesion in one genetic locus or at least in loci that are very closely linked.

Inosine 5'-monophosphate dehydrogenase of Escherichia coli K12: the nature of the inhibition by guanosine 5'-monophosphate

When IMP is the variable substrate, IMP dehydrogenase (EC 1.2.1.14) gives non-linear Lineweaver-Burk plots in the presence of GMP. At 0.1mm-GMP the Hill coefficient n=1.15 and at 0.2mm-GMP n=1.34. In the absence of GMP n=1.0. The fact that n exceeds 1.0 in the presence of GMP indicates that the enzyme possesses regulatory (allosteric) properties. The inhibition by GMP is competitive with respect to IMP and non-competitive with respect to NAD(+).