Crystal structure of adenine phosphoribosyltransferase from Leishmania tarentolae: potential implications for APRT catalytic mechanism.

The three-dimensional structure of Leishmania tarentolae adenine phosphoribosyltransferase (APRT) in complex with adenosine-5-monophosphate (AMP) and a phosphate ion has been solved. Refinement against X-ray diffraction data extending to 2.2-A resolution led to a final crystallographic R factor of 18.3%. Structural comparisons amongst this APRT enzyme and other 'type I' PRTases whose structures have been determined reveal several important features of the PRTases catalytic mechanism. Based on structural superpositions and molecular interaction potential calculations, it was possible to suggest that the PRPP is the first substrate to bind, while the AMP is the last product to leave the active site, in accordance to recent kinetic studies performed with the Leishmania donovani APRT.

Induced activity of adenine phosphoribosyltransferase (APRT) in iron-deficiency barley roots: a possible role for phytosiderophore production.

To isolate the genes involved in the response of graminaceous plants to Fe-deficient stress, a protein induced by Fe-deficiency treatment was isolated from barley (Hordeum vulgare L.) roots. Based on the partial amino acid sequence of this protein, a cDNA (HvAPT1) encoding adenine phosphoribosyltransferase (APRT: EC 2.4.2.7) was cloned from a cDNA library prepared from Fe-deficient barley roots. Southern analysis suggested that there were at least two genes encoding APRT in barley. Fe deficiency increased HvAPT1 expression in barley roots and resupplying Fe to the Fe-deficient plants rapidly negated the increase in HvAPT1 mRNA. Analysis of localization of HvAPT1-sGFP fusion proteins in tobacco BY-2 cells indicated that the protein from HvAPT1 was localized in the cytoplasm of cells. Consistent with the results of Northern analysis, the enzymatic activity of APRT in barley roots was remarkably increased by Fe deficiency. This induction of APRT activity by Fe deficiency was also observed in roots of other graminaceous plants such as rye, maize, and rice. In contrast, the induction was not observed to occur in the roots of a non-graminaceous plant, tobacco. Graminaceous plants generally synthesize the mugineic acid family phytosiderophores (MAs) in roots under Fe-deficient conditions. In this paper, a possible role of HvAPT1 in the biosynthesis of MAs related to adenine salvage in the methionine cycle is discussed.

X-Inactivation and histone H4 acetylation in embryonic stem cells.

In female mammalian cells, dosage compensation for X-linked genes is achieved by the transcriptional silencing, early in development, of many genes on just one of the two X chromosomes. Several properties distinguish the inactive X (Xi) from its active counterpart (Xa). These include expression of Xist, a gene located in the X-inactivation center (Xic), late replication, differential methylation of selected CpG islands and underacetylation of histone H4. The relationship between these properties and transcriptional silencing remains unclear. Female mouse embryonic stem (ES) cells have two active X chromosomes, one of which is inactivated as cells differentiate in culture. We describe here the use of these cells in studying the sequence of events leading to X-inactivation. By immunofluorescent labeling of metaphase chromosome spreads from ES cells with antibodies to acetylated H4, we show that an underacetylated X chromosome appears only after 4 days of differentiation, and only in female cells. The frequency of cells with an underacetylated X reaches a maximum by Day 6. In undifferentiated cells, H4 in centric heterochromatin is acetylated to the same extent as that in euchromatin but has become relatively underacetylated, as in adult cells, by Day 4 of differentiation (i.e. , when deacetylation of Xi is first seen). The overall deacetylation of Xi follows Xist expression and the first appearance of a single, late-replicating X, both of which occur on Day 2. It also follows the silencing of X-linked genes. Levels of mRNA from four such genes, Hprt, G6pd, Rps4, and Pgk-1, had all fallen by approximately 50% (relative to the autosomal gene Aprt) by Days 2-4. The results show that properties that characterize Xi are put in place in a set order over several days. H4 deacetylation occupies a defined place within this sequence, suggesting that it is an intrinsic part of the X-inactivation process. The stage at which a completely deacetylated Xi is first seen suggests that deacetylation may be necessary for the maintenance of silencing but is not required for its initiation. Nor is it required for, or an immediate consequence of, late replication. However, we note that selective deacetylation of H4 on specific genes would not be detected by the microscopical approach we have used and that such selective deacetylation may still be part of the silencing process.

A second form of adenine phosphoribosyltransferase in Arabidopsis thaliana with relative specificity towards cytokinins.

Adenine phosphoribosyltransferase (APRTase) is an important enzyme for its ability to convert adenine, a byproduct of many biochemical reactions, into AMP. By functional complementation of an Escherichia coli mutant, cDNAs encoding two APRTases have been cloned from Arabidopsis thaliana. One of the cDNAs (ATapt1) has been previously identified while the second (ATapt2) is of a previously unknown type. Kinetic analysis of the two enzymes purified from E. coli expressing the two cDNAs indicates that ATapt2 has a higher affinity for cytokinin than the ATapt1. RNase protection studies indicate that the ATapt2, is not expressed in leaves. Analysis of the gene structure indicates that ATapt2 has identical intron positions to ATapt1, but neither the intron sequence nor intron size are conserved between the two genes. The implications of a second, differentially expressed APRTase with affinity for both adenine and cytokinin are discussed.

Efficient modification of the APRT gene by FLP/FRT site-specific targeting.

The FLP/FRT site-specific recombination system was established and characterized at the APRT gene in CHO cells. Targeting frequencies with FLP-stimulation were about 1 to 5 X 10(-5), which were 6-22-fold above gene targeting frequencies in the absence of FLP. Fifty two APRT+ cell lines were analyzed by Southern blotting: 56% were FLP-targeted integrants; 33% were APRT target convertants; 11% gave undefined patterns. In separate experiments we first enriched for integrants by screening for two additional markers carried on the targeting vector; 18 of 19 (95%) of the resulting cell lines were integrants. Intrachromosomal site-specific recombination was tested by reexposing integrants to FLP. Intrachromosomal popouts were stimulated over 200-fold, while homologous recombination in an adjacent interval was unchanged. The utility of this system was demonstrated by one-step FLP targeting to generate chromosomal substrates for homologous recombination, and by a two-step, FLP-and-run procedure to construct a chromosomal substrate for illegitimate recombination.

Identification of the cis-elements required for transcriptional control of the Chinese hamster ovary APRT gene.

Transcriptional regulation of the Chinese hamster ovary (CHO) adenine phosphoribosyltransferase-encoding gene (APRT) was studied. The 5' region of the CHO APRT is G + C-rich, but lacking TATA or CCAAT boxes. RNase protection assays indicate that it contains multiple transcription start points (tsp). A tsp 64 bp upstream from the translation start codon is denoted as +1. Linker-scanning (LS) mutation analysis indicates that the -33 to +19 region is important in regulating APRT transcription. Mutations in the -23 to -14 region abolish transcription initiated from the -23 downstream region. An unidentified protein complex binds to this region. Three Sp1-binding sites are found in the APRT promoter; however, mutations of the Sp1-binding sites do not reduce APRT transcription. Mutations at two putative GCF-binding sites increase levels of transcription.

Identification of aprt gene mutations induced in repair-deficient and P450-expressing CHO cells by the food-related mutagen/carcinogen, PhIP.

We investigated the specific sequence changes produced by the dietary mutagen 2-amino-1-methyl-6-phenylimidazo-[4,5-b]pyridine (PhIP) in UV5P3 cells [a Chinese hamster ovary (CHO) cell line]. Sequence analysis of the PhIP-induced mutations in the adenine phosphoribosyltransferase (aprt) gene, which is heterozygous in the UV5P3 cells, can provide insight into the mutagenic mechanism in these repair-deficient cells expressing P4501A2. Two allele-specific 20 mer oligonucleotide primer pairs were used in the polymerase chain reaction and the allele of interest was amplified. Single-base transversions occurred in 31/32 PhIP-induced mutants; of these, 6 were A.T-->T.A, 18 were C.G-->A.T and 6 were G.C-->T.A. Twenty of the 30 changes altered specific amino acid sequences and the other 10 resulted in a stop codon. On mutant had a change from C.G-->G.C at the 3' splice site of intron 4, thereby creating a new AG splice acceptor site. Another mutant had an insertion of T within a run of repeated sequences and resulted in a frameshift mutation. There were three 'hot-spots', two at the 3' end of exon 2 and one at the beginning of exon 3; 6 (19%) mutants showed a change from A.T-->T.A (exon 2, amino acid residue 57), 11 (34%) mutants from C.G-->A.T (exon 2, amino acid residue 62), and 7 (22%) mutants from C.G-->A.T (exon 3, amino acid residue 66). Consequently, 75% of the mutations were observed at these three sites. In contrast, none of the 20 spontaneous mutants had alterations at these hotspot sites. The mutations induced by PhIP in these repair-deficient CHO cells were unique and specific, and suggest that these sequences, if found in important genes controlling cell replication and survival, may be more susceptible to mutation from these food mutagens than genes not containing these sequences.

A differential cloning procedure of complex genomic DNA fragments.

We have developed a differential cloning procedure designed for cloning of anonymous restriction DNA fragments whose molecular sizes differ between two genomic DNA preparations from higher organisms. The procedure, which was extensively revised from the original one, consists of several steps as summarized below. (i) Digestion of two DNA preparations (target and reference DNA) with the same restriction enzyme (4 base cutter). (ii) Biotinylation of target DNA and conversion of reference DNA to nonamplifiable form by terminal dephosphorylation. (iii) Electrophoresis of the two DNA preparations through a synthetic gel with a large excess of reference DNA as a competitor. (iv) In-gel alkaline dissociation of DNA, followed by reassociation (in-gel competitive reassociation). (v) Elution of DNA from the gel and PCR after adapter ligation and adsorption of DNA onto streptavidin-coated matrix. By repeating these steps, we attained substantial enrichment (approximately 10,000-fold) of DNA fragments which were originally present at one copy or less per complex mammalian genome. The details of the procedure and its unique characteristics in cloning of altered genomic DNA fragments, particularly from mammalian genome, are discussed.

Transduction of the CHO aprt gene into mouse L cells using an adeno-5/APRT recombinant virus.

An adenovirus-5 recombinant virus Adapt1 carrying the Chinese hamster ovary (CHO) adenine phosphoribosyltransferase (aprt) gene was constructed by insertion of a 2.5-kb fragment containing the complete CHO aprt structural gene linked to a Moloney murine sarcoma virus (MSV) promoter into the E3 region of adenovirus-5. The CHO aprt gene was in the opposite orientation to the adenovirus E3 promoter. Mouse Lapt- tk- (LAT) cells expressed the CHO aprt gene when infected with the virus, even at low MOI (O.1). APRT activity was detectable from approximately 20 h postinfection. At a low frequency, LAT cells were transformed to aprt+, and four stable transductants were selected in adenine, azaserine (AA) medium. Such cells expressed APRT at approximately 50% wild-type activity and the enzyme was shown to be CHO APRT by starch gel electrophoresis. DNA was isolated from the transductants and probed with CHO aprt-specific DNA and with viral DNA probes. The results indicated that the CHO aprt gene was integrated into the LAT cells at a site other than mouse aprt. Although neighboring viral sequences were integrated and maintained in the transductants, viral sequences further upstream and downstream of the aprt gene were absent.

Increased de novo purine synthesis by insulin through selective enzyme induction in primary cultured rat hepatocytes.

The proliferative effect of insulin on de novo purine synthesis and on the expression of various enzymes of purine metabolism were studied in primary cultured rat hepatocytes. Insulin greater than 1.5 x 10(-8) M increased DNA and de novo purine synthesis to 260-390 and 270-420%, respectively, 24 and 8 h after the administration. Insulin at 1.5 x 10(-7) M increased the specific activity of amidophosphoribosyltransferase (ATase) to 154-180%, hypoxanthine-guanine phosphoribosyltransferase to 129%, and adenine phosphoribosyltransferase (APRT) to 205%, in contrast to unchanged xanthine dehydrogenase at 80%. Enzyme induction was supported by the results of kinetic analysis and the inhibition of the insulin-induced increase in enzyme activities by protein synthesis inhibitors. Insulin increased ATP to 127% and decreased AMP, ADP, 5'-guanylic acid (GMP), and guanosine 5'-diphosphate (GDP), respectively, to 73, 69, 73, and 69%. Insulin increased adenylate energy charge from 0.83 to 0.90 without changing total feedback inhibitory potential on ATase. No obvious increase of 5-phosphoribosyl-1-pyrophosphate supply was suggested, although its apparent availability for purine ribonucleotide synthesis was increased to 208-245%, reflecting mainly induced APRT activity to 205%. It is concluded that hepatocyte proliferation by insulin, as evidenced by purine metabolism, is mediated by the selective gene activation of anabolic enzymes and increased ATP as the basis to activate multiple metabolic pathways without remarkable changes of substrate availability or feedback inhibition.

Expression of human genes for adenine phosphoribosyltransferase and hypoxanthine-guanine phosphoribosyltransferase after genetic transformation of mouse cells with purified human DNA.

Human DNA purified from HeLa cells and from three strains of skin fibroblasts was precipitated with calcium phosphate and added to mouse cells that were deficient in adenine phosphoribosyltransferase (APRT) and hypoxanthine phosphoribosyltransferase (HPRT). Selection for cells possessing either of the phosphoribosyltransferases was imposed by blocking de novo synthesis of purine nucleotides with azaserine in a medium supplemented with adenine and hypoxanthine. The frequency of colony formation after selection was 1.7 x 10(-7)-3.3 x 10(-6). Excepting some azaserine-resistant colonies that appeared only in the first experiment and infrequent revertants expressing moust APRT, all characterized clones expressed the human forms of APRT or HPRT according to the criteria of specific immunoprecipitation and electrophoretic mobility. The frequency of transfer of the human APRT gene was much greater than that of HPRT. Transfer efficiency was not significantly reduced when HeLa DNA was sheared to 6.5-13.5 kb size or when the donor DNA was isolated from a transferent that expressed human APRT.

Starch gel electrophoresis of erythrocyte hypoxanthine-guanine phosphoribosyl transferase and adenine phosphoribosyl transferase: a population survey.

Vertical starch gel electrophoresis of hypoxanthine-guanine phosphoribosyl transferase (HGPRT) and adenine phosphoribosyl transferase (APRT) was performed in several population groups including Caucasians, Negroes and Orientals. The red cells of a total of 244 individuals were examined for HGPRT and those of 265 for APRT and no variants were detected.

[Phenotypic manifestation of mutations involving resistance to 2,6-diaminopurine (apt) in the genome of purine auxotrophs of Escherichia coli K-12]. / Fenotipicheskoe proiavlenie mutatsii ustoichivosti k 2,6-diaminopurinu (apt) v genome purinovykh auksotrofov Escherichia coli K-12

Strains of Escherichia coli K-12 containing various combinations of pur (de novo synthesis of purines), pup (purine nucleoside phosphorylase), add (adenosine deaminase) and apt (adenine phosphoribosyl transferase) mutations have been constructed. The apt mutation blocks the ability of strains of pur add and pur add pup genotype to utilize both adenine and adenosine as sole purine sources. Exogenously supplied histidine (that blocks conversion of AMP to guanine nucleotides) does not reduce the growth rate of the strain of pur apt genotype on adenosine as the sole purine source. Adenine released into the cultural medium of bacteria containing simultaneously apt and pup mutations. This data suggest that cultures of E. coli are unable to phosphorylate adenosine to AMP and that they are capable to degrade adenosine to free adenine without participation of purine nucleoside phosphorylase (gene pup).

Intrachromosomal gene mapping in man: assignment of nucleoside phosphorylase to region 14cen leads to 14q21 by interspecific hybridization of cells with a t(X;14) (p22;q21) translocation.

The structural gene for purine-nucleoside phosphorylase (NP) has been assigned to a subregion of chromosome 14 by somatic cell hybridization of male and female cells containing the balanced translocation t(X;14) (p22;q21). Peripheral lymphocytes were fused to a pseudodiploid HPRT-deficient established Chinese hamster cell line. 23 primary hybrid clones (10 derived from male and 13 from female cells) were isolated and maintained in HAT selective medium. Parallel subcultures from generations 16, 24, and 40 after clonal isolation were fully karyotyped and analyzed electrophorectically for expression of the human types of NP, HPRT, G6PD, and PGK. The human NP phenotype segregated discordantly with each human chromosome except chromosome 14 and the der(14),t(X;14) translocation chromosome. In all, 8 hybrids which had retained the der(X), t(X;14) translocation chromosome under HAT selective pressure and expressed human HPRT had lost the human NP phenotype. These results indicate localization of the NP gene in region 14pter leads to 14q21.

Gene linkage analysis in the mouse by somatic cell hybridization: assignment of adenine phosphoribosyltransferase to chromosome 8 and alpha-galactosidase to the X chromosome.

Somatic cell hybridization techniques were applied to gene linkage analysis in the laboratory mouse. Cells of an established line of Chinese hamster lung fibroblasts were fused with mouse embryo fibroblasts and with mouse peritoneal macrophages obtained from different inbred strains. From 3 hybridization experiments, 123 primary and secondary clones were isolated in HAT selective medium and 24 were back-selected in 8-azaguanine. Hybrid clones were characterized for the expression of 16 murine isozymes by starch, acrylamide, and Cellogel electrophoresis, and on the basis of segregation data, 3 syntenic associations could be made. Malate oxidoreductase decarboxylating (MOD) and mannose phosphate isomerase (MPI) segregated concordantly, confirming an established linkage relationship; adenine phosphoribosyltransferase (APRT) segregated concordantly with glutathione reductase (GR) which is known to be on chromosome 8; alpha-galactosidase was observed to be syntenic with hypoxanthine phosphoribosyltransferase (HPRT), and X-linked enzyme. All other isozymes examined segregated independently of one another.