Functional cloning of a Dictyostelium discoideum cDNA encoding GMP synthetase

Nonsynonymous Polymorphism in Guanine Monophosphate Synthetase Is a Risk Factor for Unfavorable Thiopurine Metabolite Ratios in Patients With Inflammatory Bowel Disease

BACKGROUND

Up to 20% of patients with inflammatory bowel disease (IBD) who are refractory to thiopurine therapy preferentially produce 6-methylmercaptopurine (6-MMP) at the expense of 6-thioguanine nucleotides (6-TGN), resulting in a high 6-MMP:6-TGN ratio (>20). The objective of this study was to evaluate whether genetic variability in guanine monophosphate synthetase (GMPS) contributes to preferential 6-MMP metabolizer phenotype.

METHODS

Exome sequencing was performed in a cohort of IBD patients with 6-MMP:6-TGN ratios of >100 to identify nonsynonymous single nucleotide polymorphisms (nsSNPs). In vitro assays were performed to measure GMPS activity associated with these nsSNPs. Frequency of the nsSNPs was measured in a cohort of 530 Caucasian IBD patients.

RESULTS

Two nsSNPs in GMPS (rs747629729, rs61750370) were detected in 11 patients with very high 6-MMP:6-TGN ratios. The 2 nsSNPs were predicted to be damaging by in silico analysis. In vitro assays demonstrated that both nsSNPs resulted in a significant reduction in GMPS activity (P < 0.05). The SNP rs61750370 was significantly associated with 6-MMP:6-TGN ratios ≥100 (odds ratio, 5.64; 95% confidence interval, 1.01-25.12; P < 0.031) in a subset of 264 Caucasian IBD patients.

CONCLUSIONS

The GMPS SNP rs61750370 may be a reliable risk factor for extreme 6MMP preferential metabolism.

GuaA and GuaB are essential for Borrelia burgdorferi survival in the tick-mouse infection cycle

Pathogens lacking the enzymatic pathways for de novo purine biosynthesis are required to salvage purines and pyrimidines from the host environment for synthesis of DNA and RNA. Two key enzymes in purine salvage pathways are IMP dehydrogenase (GuaB) and GMP synthase (GuaA), encoded by the guaB and guaA genes, respectively. While these genes are typically found on the chromosome in most bacterial pathogens, the guaAB operon of Borrelia burgdorferi is present on plasmid cp26, which also harbors a number of genes critical for B. burgdorferi viability. Using molecular genetics and an experimental model of the tick-mouse infection cycle, we demonstrate that the enzymatic activities encoded by the guaAB operon are essential for B. burgdorferi mouse infectivity and provide a growth advantage to spirochetes in the tick. These data indicate that the GuaA and GuaB proteins are critical for the survival of B. burgdorferi in the infection cycle and highlight a potential difference in the requirements for purine salvage in the disparate mammalian and tick environments.

Plasmodium falciparum: isolation and characterisation of a gene encoding protozoan GMP synthase

The final step in guanylate nucleotide biosynthesis is catalysed by GMP synthase. This paper presents the first isolation of a gene encoding a protozoan GMP synthase. The deduced amino acid sequence from Plasmodium falciparum shares 40% identity with yeast GMP synthase and contains motifs conserved in catalysis. Expression of the gene is regulated through the parasite's development in human red blood cells with maximal expression during the point of DNA replication. Psicofuranine, which inhibits GMP synthase, interrupts parasite growth, supporting the role of this enzyme. These findings will aid development of inhibitors of purine salvage in malaria parasites.

The crystal structure of GMP synthetase reveals a novel catalytic triad and is a structural paradigm for two enzyme families

The crystal structure of GMP synthetase serves as a prototype for two families of metabolic enzymes. The Class I glutamine amidotransferase domain of GMP synthetase is found in related enzymes of the purine, pyrimidine, tryptophan, arginine, histidine and folic acid biosynthetic pathways. This domain includes a conserved Cys-His-Glu triad and is representative of a new family of enzymes that use a catalytic triad for enzymatic hydrolysis. The structure and conserved sequence fingerprint of the nucleotide-binding site in a second domain of GMP synthetase are common to a family of ATP pyrophosphatases, including NAD synthetase, asparagine synthetase and argininosuccinate synthetase.

Plasmid location of Borrelia purine biosynthesis gene homologs

The Lyme disease spirochete Borrelia burgdorferi must survive in both its tick vector and its mammalian host to be maintained in nature. We have identified the B. burgdorferi guaA gene encoding GMP synthetase, an enzyme involved in de novo purine biosynthesis that is important for the survival of bacteria in mammalian blood. This gene encodes a functional product that will complement an Escherichia coli GMP synthetase mutant. The gene is located on a 26-kb circular plasmid, adjacent to and divergent from the gene encoding the outer surface protein C (OspC). The guaB gene homolog encoding IMP dehydrogenase, another enzyme in the purine biosynthetic pathway, is adjacent to guaA. In Borrelia hermsii, a tick-borne relapsing fever spirochete, the guaA and guaB genes are located on a linear plasmid. These are the first genes encoding proteins of known function to be mapped to a borrelial plasmid and the only example of genes encoding enzymes involved in the de novo purine biosynthesis pathway to be mapped to a plasmid in any organism. The unique plasmid location of these and perhaps other housekeeping genes may be a consequence of the segmented genomes in borreliae and reflect the need to adapt to both the arthropod and mammalian environments.

Cloning and sequencing of the GMP synthetase-encoding gene of Saccharomyces cerevisiae

We have localised, within a Saccharomyces cerevisiae genomic fragment, the GUA1 gene whose amplification leads to the accumulation of several polypeptides on the two-dimensional (2-D) map of yeast proteins. Comparison of the sequence of the putative GUA1 protein with a data library shows a strong similarity with Escherichia coli, Bacillus subtilis and Dictyostelium discoideum GMP synthetases (GMPS) and other glutamine amidotransferases. The fact that disruption of the chromosomal copy of the gene leads to guanine auxotrophy, that the gual::URA3 disruption does not complement an independently obtained gual-3 mutation deficient in GMPS and that GUA1 complements this latter mutation, confirms the identification of the cloned gene as GUA1 encoding the S. cerevisiae GMPS. Finally, using microsequencing, we have identified one of the polypeptides, which is overproduced in response to GUA1 amplification, as corresponding to GUA1.

Cloning and sequence of Bacillus subtilis purA and guaA, involved in the conversion of IMP to AMP and GMP

Bacillus subtilis genes purA, encoding adenylosuccinate synthetase, and guaA, coding for GMP synthetase, appear to be lethal when cloned in multicopy plasmids in Escherichia coli. The nucleotide sequences of purA and guaA were determined from a series of gene fragments isolated by polymerase chain reaction amplification, library screening, and plasmid rescue techniques. Identifications were based on amino acid sequence alignments with enzymes from other organisms. Comparison of the 5'-flanking regions of purA and guaA with the pur operon suggests similarities in mechanisms for gene regulation. Nucleotide sequences are now available for all genes involved in the 14-step pathway for de novo purine nucleotide synthesis in B. subtilis.