Molecular and functional analysis of hypoxanthine-guanine phosphoribosyltransferase from Arabidopsis thaliana

Phosphoribosyltransferases and Their Roles in Plant Development and Abiotic Stress Response

Glycosylation is a widespread glycosyl modification that regulates gene expression and metabolite bioactivity in all life processes of plants. Phosphoribosylation is a special glycosyl modification catalyzed by phosphoribosyltransferase (PRTase), which functions as a key step in the biosynthesis pathway of purine and pyrimidine nucleotides, histidine, tryptophan, and coenzyme NAD(P)⁺ to control the production of these essential metabolites. Studies in the past decades have reported that PRTases are indispensable for plant survival and thriving, whereas the complicated physiological role of PRTases in plant life and their crosstalk is not well understood. Here, we comprehensively overview and critically discuss the recent findings on PRTases, including their classification, as well as the function and crosstalk in regulating plant development, abiotic stress response, and the balance of growth and stress responses. This review aims to increase the understanding of the role of plant PRTase and also contribute to future research on the trade-off between plant growth and stress response.

Identification of Biosynthetic and Metabolic Genes of 2-Azahypoxanthine in Lepista sordida Based on Transcriptomic Analysis

2-Azahypoxanthine was isolated from the fairy ring-forming fungus Lepista sordida as a fairy ring-inducing compound. 2-Azahypoxanthine has an unprecedented 1,2,3-triazine moiety, and its biosynthetic pathway is unknown. The biosynthetic genes for 2-azahypoxanthine formation in L. sordida were predicted by a differential gene expression analysis using MiSeq. The results revealed that several genes in the purine and histidine metabolic pathways and the arginine biosynthetic pathway are involved in the biosynthesis of 2-azahypoxanthine. Furthermore, nitric oxide (NO) was produced by recombinant NO synthase 5 (rNOS5), suggesting that NOS5 can be the enzyme involved in the formation of 1,2,3-triazine. The gene encoding hypoxanthine-guanine phosphoribosyltransferase (HGPRT), one of the major phosphoribosyltransferases of purine metabolism, increased when 2-azahypoxanthine content was the highest. Therefore, we hypothesized that HGPRT might catalyze a reversible reaction between 2-azahypoxanthine and 2-azahypoxanthine-ribonucleotide. We proved the endogenous existence of 2-azahypoxanthine-ribonucleotide in L. sordida mycelia by LC-MS/MS for the first time. Furthermore, it was shown that recombinant HGPRT catalyzed reversible interconversion between 2-azahypoxanthine and 2-azahypoxanthine-ribonucleotide. These findings demonstrate that HGPRT can be involved in the biosynthesis of 2-azahypoxanthine via 2-azahypoxanthine-ribonucleotide generated by NOS5.

A plastid nucleoside kinase is involved in inosine salvage and control of purine nucleotide biosynthesis

In nucleotide metabolism, nucleoside kinases recycle nucleosides into nucleotides-a process called nucleoside salvage. Nucleoside kinases for adenosine, uridine, and cytidine have been characterized from many organisms, but kinases for inosine and guanosine salvage are not yet known in eukaryotes and only a few such enzymes have been described from bacteria. Here we identified Arabidopsis thaliana PLASTID NUCLEOSIDE KINASE 1 (PNK1), an enzyme highly conserved in plants and green algae belonging to the Phosphofructokinase B family. We demonstrate that PNK1 from A. thaliana is located in plastids and catalyzes the phosphorylation of inosine, 5-aminoimidazole-4-carboxamide-1-β-d-ribose (AICA ribonucleoside), and uridine but not guanosine in vitro, and is involved in inosine salvage in vivo. PNK1 mutation leads to increased flux into purine nucleotide catabolism and, especially in the context of defective uridine degradation, to over-accumulation of uridine and UTP as well as growth depression. The data suggest that PNK1 is involved in feedback regulation of purine nucleotide biosynthesis and possibly also pyrimidine nucleotide biosynthesis. We additionally report that cold stress leads to accumulation of purine nucleotides, probably by inducing nucleotide biosynthesis, but that this adjustment of nucleotide homeostasis to environmental conditions is not controlled by PNK1.

Biosynthesis of the Fairy Chemicals, 2-Azahypoxanthine and Imidazole-4-carboxamide, in the Fairy Ring-Forming Fungus Lepista sordida

Fairy rings resulting from a fungus-plant interaction appear worldwide. 2-Azahypoxanthine (AHX) and imidazole-4-carboxamide (ICA) were first isolated from the culture broth of one of the fairy ring-forming fungi, Lepista sordida. Afterward, a common metabolite of AHX in plants, 2-aza-8-oxohypoxanthine (AOH), was found in AHX-treated rice. The biosynthetic pathway of the three compounds that are named as fairy chemicals (FCs) in plants has been partially elucidated; however, that in mushrooms remains unknown. In this study, it was revealed that the carbon skeletons of AHX and ICA were constructed from Gly in L. sordida mycelia and the fungus metabolized 5-aminoimidazole-4-carboxamide (AICA) to both of the compounds. These results indicated that FCs were biosynthesized by a diversion of the purine metabolic pathway in L. sordida mycelia, similar to that in plants. Furthermore, we showed that recombinant adenine phosphoribosyltransferase (APRT) catalyzed reversible interconversion not only between 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranosyl 5'-monophosphate (AICAR) and AICA but also between ICA-ribotide (ICAR) and ICA. Furthermore, the presence of ICAR in L. sordida mycelia was proven for the first time by LC-MS/MS detection, and this study provided the first report that there was a novel metabolic pathway of ICA in which its ribotide was an intermediate in the fungus.

Purine salvage in plants

Purine bases and nucleosides are produced by turnover of nucleotides and nucleic acids as well as from some cellular metabolic pathways. Adenosine released from the S-adenosyl-L-methionine cycle is linked to many methyltransferase reactions, such as the biosynthesis of caffeine and glycine betaine. Adenine is produced by the methionine cycles, which is related to other biosynthesis pathways, such those for the production of ethylene, nicotianamine and polyamines. These purine compounds are recycled for nucleotide biosynthesis by so-called "salvage pathways". However, the salvage pathways are not merely supplementary routes for nucleotide biosynthesis, but have essential functions in many plant processes. In plants, the major salvage enzymes are adenine phosphoribosyltransferase (EC 2.4.2.7) and adenosine kinase (EC 2.7.1.20). AMP produced by these enzymes is converted to ATP and utilised as an energy source as well as for nucleic acid synthesis. Hypoxanthine, guanine, inosine and guanosine are salvaged to IMP and GMP by hypoxanthine/guanine phosphoribosyltransferase (EC 2.4.2.8) and inosine/guanosine kinase (EC 2.7.1.73). In contrast to de novo purine nucleotide biosynthesis, synthesis by the salvage pathways is extremely favourable, energetically, for cells. In addition, operation of the salvage pathway reduces the intracellular levels of purine bases and nucleosides which inhibit other metabolic reactions. The purine salvage enzymes also catalyse the respective formation of cytokinin ribotides, from cytokinin bases, and cytokinin ribosides. Since cytokinin bases are the active form of cytokinin hormones, these enzymes act to maintain homeostasis of cellular cytokinin bioactivity. This article summarises current knowledge of purine salvage pathways and their possible function in plants and purine salvage activities associated with various physiological phenomena are reviewed.

Structure-function relationship of a plant NCS1 member--homology modeling and mutagenesis identified residues critical for substrate specificity of PLUTO, a nucleobase transporter from Arabidopsis

Plastidic uracil salvage is essential for plant growth and development. So far, PLUTO, the plastidic nucleobase transporter from Arabidopsis thaliana is the only known uracil importer at the inner plastidic membrane which represents the permeability barrier of this organelle. We present the first homology model of PLUTO, the sole plant NCS1 member from Arabidopsis based on the crystal structure of the benzyl hydantoin transporter MHP1 from Microbacterium liquefaciens and validated by molecular dynamics simulations. Polar side chains of residues Glu-227 and backbones of Val-145, Gly-147 and Thr-425 are proposed to form the binding site for the three PLUTO substrates uracil, adenine and guanine. Mutational analysis and competition studies identified Glu-227 as an important residue for uracil and to a lesser extent for guanine transport. A differential response in substrate transport was apparent with PLUTO double mutants E227Q G147Q and E227Q T425A, both of which most strongly affected adenine transport, and in V145A G147Q, which markedly affected guanine transport. These differences could be explained by docking studies, showing that uracil and guanine exhibit a similar binding mode whereas adenine binds deep into the catalytic pocket of PLUTO. Furthermore, competition studies confirmed these results. The present study defines the molecular determinants for PLUTO substrate binding and demonstrates key differences in structure-function relations between PLUTO and other NCS1 family members.