Mutants of Escherichia coli defective in ribonucleoside and deoxyribonucleoside catabolism

Presence of a novel phosphopentomutase and a 2-deoxyribose 5-phosphate aldolase reveals a metabolic link between pentoses and central carbon metabolism in the hyperthermophilic archaeon Thermococcus kodakaraensis

Numerous bacteria and mammalian cells harbor two enzymes, phosphopentomutase (PPM) and 2-deoxyribose 5-phosphate aldolase (DERA), involved in the interconversion between nucleosides and central carbon metabolism. In this study, we have examined the presence of this metabolic link in the hyperthermophilic archaeon, Thermococcus kodakaraensis KOD1. A search of the genome sequence of this strain revealed the presence of a closely related orthologue (TK2104) of bacterial DERA genes while no orthologue related to previously characterized PPM genes could be detected. Expression, purification, and characterization of the TK2104 protein product revealed that this gene actually encoded a DERA, catalyzing the reaction through a class I aldolase mechanism. As PPM activity was detected in T. kodakaraensis cells, we partially purified the protein to examine its N-terminal amino acid sequence. The sequence corresponded to a gene (TK1777) similar to phosphomannomutases within COG1109 but not COG1015, which includes all previously identified PPMs. Heterologous gene expression of TK1777 and characterization of the purified recombinant protein clearly revealed that the gene indeed encoded a PPM. Both enzyme activities could be observed in T. kodakaraensis cells under glycolytic and gluconeogenic growth conditions, whereas the addition of ribose, 2-deoxyribose, and 2'-deoxynucleosides in the medium did not lead to a significant induction of these activities. Our results clearly indicate the presence of a metabolic link between pentoses and central carbon metabolism in T. kodakaraensis, providing an alternative route for pentose biosynthesis through the functions of DERA and a structurally novel PPM.

Isolation and characterization of EMS induced splicing defective point mutations within the intron of the nrdB gene of bacteriophage T4

The nrdB gene of bacteriophage T4 codes for the small subunit of ribonucleotide reductase and contains a 598-base-pair self-splicing intron which is closely related to other group I introns of T4 and eukaryotes. The screening, isolation, and mapping of 31 nrdB intron mutations were conducted by the strategic usage of the white halo phenotype exhibited by T4 mutants defective in dyhydrofolate reductase or thymidylate synthase. These intron mutations cluster towards the ends, mainly the 3' end, and show a defect in self-splicing. These mutations map in regions of conserved structural elements, thus supporting secondary structure predictions. A distinct pattern of clustering is observed with the highest number of mutations mapping within three of the smaller regions (A, C, and D) of the nrdB intron and no mutations mapping in the largest (B) region. The highest density of mutations mapped in the smallest region (C) of the intron, containing only 96 bases, thus showing a distinct pattern of clustering within the catalytic core.

Distribution and characterization of mutations induced by nitrous acid or hydroxylamine in the intron-containing thymidylate synthase gene of bacteriophage T4

The detailed distribution and characterization of 51 hydroxylamine (HA)-induced and 59 nitrous acid (NA)-induced mutations in the intron-containing bacteriophage T4 thymidylate synthase (td) gene is reported here. Mutations were mapped in 10 regions of the td gene by recombinational marker rescue using plasmid or M13 subclones of the td gene. Phage crosses using deletion mutants with known breakpoints in the 3' end of the td intron subdivided HA and NA mutations which mapped in this region. At least 31 of the mutations map within the 1-kb group I self-splicing intron. Intron mutations mapped only in the 5' and 3' ends of the intron sequence, in accordance with the hypothesis that the 5' and 3' domains of the T4 td intron are essential for correct RNA splicing. RNA sequence analysis of a number of mapped td mutations has identified two intron nucleotides and one exon nucleotide where both HA- and NA-induced mutations commonly occur. These three loci are characterized by a GC dinucleotide, with the mutations occurring at the cytosine residue. Thus, these data indicate at least three potential sites of both HA- and NA-induced mutagenic hotspot activity within the td gene.

A proflavin-induced frameshift hotspot in the thymidylate synthase gene of bacteriophage T4

Twenty-one independent thymidylate synthase deficient (td) mutants were isolated after proflavin mutagenesis of T4D0 phage. A strikingly high proportion of these mutations (17 of 21; 80%) mapped in a small 122 nucleotide (nt) region which spans the 5' splice site of this intron-containing gene. This region comprises only 14% of the total td exon sequence. RNA sequence analysis of these mutants identified a series of frameshift insertion/deletion mutations and indicated a hotspot for proflavin-induced mutations in the 3' end of exon I of the td gene. The mutant sequences at the hotspot site fully support a previously proposed mutagenic mechanism for proflavin-induced mutations in which frameshifts are produced as a consequence of exonuclease or DNA polymerase activity at the 3' ends of nicks in the DNA produced by perturbation of the T4-encoded type II topoisomerase activity by the acridine. Sixteen of the seventeen DNA mutations in the hotspot region can be explained by the model as a consequence of enzymatic processing of nicks at two phosphodiester bonds staggered by 4 base pairs (bp) and located on opposite strands of the DNA. Thus, these mutants exhibit precisely the symmetry expected of topoisomerase-mediated mutagenesis. The DNA sequences of the td hotspot mutants, when considered with the sequences of proflavin-induced mutants in the T4 rIIB and lysozyme genes, confirm the view that proflavin-induced mutations in diverse bacteriophage T4 DNA sequences are all produced by the topoisomerase-dependent mechanisms and do not support the view that classical misalignments in DNA repeats are hotspots for proflavin-induced mutagenesis in T4.

Purine nucleoside phosphorylase: purification using an ether-linked formycin B/sepharose 6B resin with unusual properties

Formycin B [9-deazainosine] was reacted with epoxy-activated Sepharose 6B to form an affinity resin for purine nucleoside phosphorylase (PNPase). This resin had a large capacity (7,600 units/ml) for the enzyme from Escherichia coli. Enzyme retention was dependent on high ionic strength. Although this property is reminiscent of hydrophobic interaction chromatography, analogous resins prepared with pseudouridine or monoethanolamine instead of with formycin B, did not retain the enzyme even at high ionic strength. Furthermore, hypoxanthine facilitatted elution of the enzyme from the resin. It appeared, therefore, that the enzyme was not bound simply by hydrophobic interactions. A simple two-step purification procedure for PNPase from Escherichia coli was devised using this resin. Overall recovery was 50%, and purity of the final preparation was greater than 95%. This resin was also useful in the purification of PNPase from human erythrocytes. The ether linkage between formycin B and Sepharose 6B, together with the carbon-to-carbon linkage between the pentose and heterocyclic moieties of formycin B, provided stability to both chemical and enzymatic degradation. After 5 years of use and exposure to a variety of biological preparations, the resin showed no detectable decrease in its ability to bind PNPase.

Two domains for splicing in the intron of the phage T4 thymidylate synthase (td) gene established by nondirected mutagenesis

Of 97 nondirected T4 thymidylate synthase-defective (td) mutations, 27 were mapped to the intron of the split td gene. Clustering of these intron mutations defined two domains that are functional in splicing, each within approximately 220 residues of the respective splice sites. Two selected mutations, tdN57 and tdN47, fell within phylogenetically conserved pairings, with tdN57 disrupting the exon I-internal guide pairing (P1) in the 5' domain and tdN47 destabilizing the P9 helix in the 3' domain. A splicing assay with synthetic oligonucleotides complementary to RNA junction sequences revealed processing defects for T4tdN57 and T4tdN47, both of which are impaired in cleavage at the 5' and 3' splice sites. Thus prokaryotic genetics facilitates association of specific residue changes with their consequences to splicing.

Role of ribonucleic acid synthesis in conjugational transfer of chromosomal and plasmid deoxyribonucleic acids

A strain of Escherichia coli K-12 containing mutations that allow for the experimental control of RNA and DNA syntheses was constructed to investigate the role that RNA synthesis plays in conjugational DNA transfer when DNA replication is inhibited. The mutations possessed by this strain and its donor derivatives include: (i) thyA, which blocks synthesis of dTMP, causing a requirement for thymine; (ii) deoC, which blocks breakdown of deoxyribose 5-phosphate, permitting growth with low levels of thymine; (iii) pyrF, which blocks synthesis of UMP from OMP, imposing a requirement for uridine; (iv) cdd and pyrG, which block the deamination of cytidine to uridine and the synthesis of CTP from UTP, respectively, causing a requirement for cytidine; (v) codA and codB, which block the deamination of cytosine to uracil and cytosine transport, respectively, preventing the substitution of cytosine for cytidine; and (vi) dnaB, which blocks vegetative but not conjugational DNA replication at 42 degrees C. DNA synthesis can be blocked in the donor strains by the addition of excess uridine when exogenous thymine is not present. We found that RNA synthesis can also be blocked by addition of excess uridine when exogenous cytidine is not present. Blocking RNA synthesis prior to mating, under conditions in which DNA synthesis either is or is not inhibited, depresses DNA transfer. However, under conditions in which DNA synthesis is inhibited, the blocking of RNA synthesis immediately after mating has commenced had no effect on continued conjugational transfer of DNA. Thus, RNA synthesis is needed to initiate but not to continue conjugational DNA transfer.

Fusion of the lac genes to the promotor for the cytidine deaminase gene of Escherichia coli K-12

Phage Mu has been inserted into the structural gene for cytidine deaminase (cdd). By the use of phage lambda (lac, Mu) the promoter for the cdd gene has been fused to lacZ. In these strains lacZ expression is regulated by the cytR repressor protein and is therefore induced by cytidine. The fusion strains were used for the isolation of cddo mutants. Plaque forming lambda phages carrying the different cdd-lacZ fusions were isolated. Studies of the cdd-Mu strains showed that the cdd gene is transcribed clockwise with respect to the Escherichia coli map.

[Cytidine and deoxycytidine aminohydrolase activities from Zea mays L. aerial parts: probable existence of two isozymes both of which possess the two activities]

Enzymatic proteins with deoxycytidine and cytidine aminohydrolase activities were partially purified from Zea mays L. aerial parts by using ammonium sulfate fractionation, adsorption on calcium phosphate gel and chromatography on DEAE-cellulose.

Metabolism of RNA-ribose by Bdellovibrio bacteriovorus during intraperiplasmic growth on Escherichia coli

During intraperiplasmic growth of Bdellovibrio bacteriovorus 109J on Escherichia coli some 30 to 60% of the initial E. coli RNA-ribose disappeared as cell-associated orcinol-positive material. The levels of RNA-ribose in the suspending buffer after growth together with the RNA-ribose used for bdellovibrio DNA synthesis accounted for 50% or less of the missing RNA-ribose. With intraperiplasmic growth in the presence of added U-14C-labeled CMP, GMP, or UMP, radioactivity was found both in the respired CO2 and incorporated into the bdellovibrio cell components. The addition of exogenous unlabeled ribonucleotides markedly reduced the amounts of both the 14CO2 and 14C incorporated into the progeny bdellovibrios. During intraperiplasmic growth of B. bacteriovorus on [U-14C]ribose-labeled E. coli BJ565, ca. 74% and ca. 19% of the initial 14C was incorporated into the progeny bdellovibrios and respired CO2, respectively. Under similar growth conditions, the addition of glutamate substantially reduced only the 14CO2; however, added ribonucleotides reduced both the 14CO2 and the 14C incorporated into the progeny bdellovibrios. No similar effects were found with added ribose-5-phosphate. The distribution of 14C in the major cell components was similar in progeny bdellovibrios whether obtained from growth on [U-14C]ribose-labeled E. coli BJ565 or from E. coli plus added U-14C-labeled ribonucleotides. After intraperiplasmic growth of B. bacteriovorus on [5,6-3H-]uracil-[U-14C]ribose-labeled E. coli BJ565 (normal or heat treated), the whole-cell 14C/3H ratio of the progeny bdellovibrios was some 50% greater and reflected the higher 14C/3H ratios found in the cell fractions. B. bacteriovorus and E. coli cell extracts both contained 5'-nucleotidase, uridine phosphorylase, purine phosphorylase, deoxyribose-5-phosphate aldolase, transketolase, thymidine phosphorylase, phosphodeoxyribomutase, and transaldolase enzyme activities. The latter three enzyme activities were either absent or very low in cell extracts prepared from heat-treated E. coli cells. It is concluded that during intraperiplasmic growth B. bacteriovorus degrades some 20 to 40% of the ribonucleotides derived from the initial E. coli RNA into the base and ribose-1-phosphate moieties. The ribose-1-phosphate is further metabolized by B. bacteriovorus both for energy production and for biosynthesis, of non-nucleic acid cell material. In addition, the data indicate that during intraperiplasmic growth B. bacteriovorus can metabolize ribose only if this compound is available to it as the ribonucleoside monophosphate.

Enzymes of purine metabolism in Mycoplasma mycoides subsp. mycoides

The major pathways of ribonucleotide biosynthesis in Mycoplasma mycoides subsp. mycoides were proposed previously from studies of its usage of radioactive purines and pyrimidines. To interpret more fully the pattern of purine usage, we have assayed cell-free extracts of this organism for several enzymes associated with the salvage synthesis of purine nucleotides. M. mycoides possessed phosphoribosyltransferases for adenine, guanine, and hypoxanthine, purine nucleoside phosphorylase, GMP reductase, GMP kinase, adenylosuccinate synthetase, and adenylosuccinate lyase. Purine nucleoside kinase and adenosine deaminase were not detected. Examination of kinetic properties and regulation of some of the above enzymes revealed differences between M. mycoides and Escherichia coli. Most notable of these were the greater susceptibility of the enzymes from M. mycoides to inhibition by nucleotides and the more widespread involvement of GMP as an inhibitor. Observations on enzyme activities in vitro allow an adequate explanation of the capacity of guanine to provide M. mycoides with its full requirement for purine nucleotides.

An inducible acetate transport system in Neurospora crassa conidia

Neurospora crassa conidia possess an active transport system for the uptake of acetate. This system was characterized as: (a) energy dependent; (b) taking place against a concentration gradient; (c) saturating at higher substrate concentrations and (d) competitively inhibited by propionate. Activity of the acetate transport system can be further enhanced by preincubating conidia in 1 mM acetate medium for 180 min (the inducible transport system). The conidial system and the inducible system have similar properties. The development of the inducible transport was dependent on RNA and protein synthesis. A genetic control of this system was further confirmed by isolating a mutant acp-i acetate permease, inducible) that fails to develop the inducible transport system.

Adenosine phosphorylase activity in a mutant HEp-2 cell line contaminated with Mycoplasm hyorhinis

Metabolic studies in HEp-2/MP,MIR cells (an adenosine kinase, hypoxanthine phosphoribosyltransferase negative mutant) indicated the presence of adenosine phosphorylase activity. This activity, unknown in established mammalian cell lines, resulted in the glycosidic cleavage of both adenosine and the antiviral drug arabinosyladenine. The activity was observed readily in the presence or absence of the adenosine deaminase inhibitor conformycin. Isopycnic separation of [3H] thymidine-labeled DNA species in CsCl density gradients resulted in the appearance of two distinct peaks. The heavier peak coincided with [14C]thymidine-labeled marker DNA of human origin, whereas the lighter peak was within the range associated with mycoplasmal DNA. Testing by commercial laboratories confirmed the presence of mycoplasma in HEp-2/MP,MIR cells. The contaminant was identified as Mycoplasma hyorhinis, a porcine mycoplasma. Following gamma-irradiation (3000 rads) to block cellular mitosis, the mucoplasma-contaminated HEp-2/MP,MIR cells were cocultivated with mycoplasma-free wild-type HEp-2 cells which did not exhibit adenosine phosphorylase activity. Following serial cocultivation in a medium designed to favor the survival of the wild-type cells, adenosine phosphorylase activity was found in the previously uninfected cells. Studies of this nature emphasize the need for investigators to carefully monitor their cell lines for mycoplasma.

Pathways of nucleotide biosynthesis in Mycoplasma mycoides subsp. mycoides

By measuring the specific activity of nucleotides isolated from ribonucleic acid after the incorporation of (14)C-labeled precursors under various conditions of growth, we have defined the major pathways of ribonucleotide synthesis in Mycoplasma mycoides subsp. mycoides. M. mycoides did not possess pathways for the de novo synthesis of nucleotides but was capable of interconversion of nucleotides. Thus, uracil provided the requirement for both pyrimidine ribonucleotides. Thymine is also required, suggesting that the methylation step is unavailable. No use was made of cytosine. Uridine was rapidly degraded to uracil. Cytidine competed effectively with uracil to provide most of the cytidine nucleotide and also provided an appreciable proportion of uridine nucleotide. In keeping with these results, there was a slow deamination of cytidine to uridine with further degradation to uracil in cultures of M. mycoides. Guanine was capable of meeting the full requirement of the organism for purine nucleotide, presumably by conversion of guanosine 5'-monophosphate to adenosine 5'-monophosphate via the intermediate inosine 5'-monophosphate. When available with guanine, adenine effectively gave a complete provision of adenine nucleotide, whereas hypoxanthine gave a partial provision. Neither adenine nor hypoxanthine was able to act as a precursor for the synthesis of guanine nucleotide. Exogenous guanosine, inosine, and adenosine underwent rapid cleavage to the corresponding bases and so show a pattern of utilization similar to that of the latter.

Isolation of an Escherichia coli mutant deficient in thioredoxin reductase

A mutant of Escherichia coli defective in thioredoxin reductase has been isolated and partially characterized. This mutant has no detectable thioredoxin reductase activity in vitro and yet it exhibits no in vivo defect in reduction of ribonucleotides. Evidence is presented that indicates that, in cells permeabilized via ether treatment, ribonucleoside diphosphate reduction can utilize glutathione as an alternate reducing system.

Simultaneous selection of mutants in gluconeogenesis and nucleoside catabolism in Salmonella typhimurium

Penicillin selection in minimal thymidine medium, used to select mutants in deoxynucleoside catabolism, also yields a high percentage (37%) of mutants in fructose diphosphatase. The expression of the deo regulon is retarded in the mutants defective in the glyconeogenic pathway.

Isolation of a lambda transducing bacteriophage carrying the relA gene of Escherichia coli

In Escherichia coli the relA and pyrG loci are 99% cotransducible. On the basis of this knowledge, we have isolated lambdacI857S7dpyrG transducing bacteriophages carrying both the pyrG and relA genes. Single lysogens of this bacteriophage show basal levels of ppGpp that are 10-fold higher than normal. Stringent factor is present among the gene products synthesized by lambdadpyrG relA after infection of ultraviolet-killed cells, as analyzed by polyacrylamide gel electrophoresis. The intracellular content of stringent factor, as determined by enzymatic activity, rises 20-fold after induction of a single lysogen of lambdadpyrG relA. As measured by two-dimensional gel electrophoresis, the amount of stringent factor in an exponentially growing strain carrying a pyrG relA plasmid is at least 10-fold greater than in a normal strain. These data constitute strong evidence that stringent factor is the relA gene product.

Multiple regulation of nucleoside catabolizing enzymes: effects of a polar dra mutation on the deo enzymes

Strains with an amber, polar mutation in the dra1 gene have been isolated. The mutation was introduced into a set of isogenic strains, wild type or with concurrent regulatory mutations, and further characterized by suppression and heat inactivation experiments. The effect of the polar dra mutation on the three remaining genes of the deo operon, the tpp, drm and pup genes, was determined by estimating the enzyme levels in the various dra-mutants. The effect was found to be non-coordinate, indicating the formation in the cells of two types of transcripts: A tetracistronic unit, containing the message from all four genes, and a dicistronic unit, covering the two distal genes only.

Repression of Escherichia coli carbamoylphosphate synthase: relationships with enzyme synthesis in the arginine and pyrimidine pathways

Cumulative repression of Escherichia coli carbamoylphosphate synthase (CPSase; EC 2.7.2.9) by arginine and pyrimidine was analyzed in relation to control enzyme synthesis in the arginine and pyrimidine pathways. The expression of carA and carB, the adjacent genes that specify the two subunits of the enzyme, was estimated by means of an in vitro complementation assay. The synthesis of each gene product was found to be under repression control. Coordinate expression of the two genes was observed under most conditions investigated. They might thus form an operon. The preparation of strains blocked in the degradation of cytidine and harboring leaky mutations affecting several steps of pyrimidine nucleotide synthesis made it possible to distinguish between the effects of cytidine and uridine compounds in the repression of the pyrimidine pathway enzymes. The data obtained suggest that derivatives of both cytidine and uridine participate in the repression of CPSase. In addition, repression of CPSase by arginine did not appear to occur unless pyrimidines were present at a significant intracellular concentration. This observation, together with our previous report that argR mutations impair the cumulative repression of CPSase, suggests that this control is mediated through the concerted effects of regulatory elements specific for the arginine and pyrimidine pathways.

Novel mechanism of resistance to folate analogues: ribonucleoside diphosphate reductase deficiency in bacteriophage T4

Some spontaneously occurring bacteriophage T4 mutants (far mutants) were able to form plaques in the presence of concentrations of folate analogues that completely inhibit plaque formation by wild-type phage T4. Some of these far mutants were shown to be ribonucleoside diphosphate (RDP) reductase (EC 1.17.4.1) deficient, and some independently isolated RDP reductase-deficient mutants (nrd mutants) were shown to be folate analogue resistant. The map positions of the RDP reductase-deficient far mutants were shown to be within the genes controlling the phage-induced RDP reductase activity.

Location on the chromosome of Escherichia coli of genes governing purine metabolism. Adenosine deaminase (add), guanosine kinase (gsk) and hypoxanthine phosphoribosyltransferase (hpt)

Genes coding for enzymes functioning in purine salvage pathways have been located on the chromosome of Escherichia coli. The gene add encoding adenosine deaminase was located by transduction at 31 min, the gene order was established to be man-uidA-add-aroD. A deletion covering man-uidA-add was obtained. The gene gsk encoding guanosine kinase was cotransducible with purE and shown to be located at 13 min. The gene hpt encoding hypoxanthine phosphoribosyltransferase was cotransducible with tonA indicating a location at 3 min. The location of the gene gpt encoding guanine (xanthine) phosphoribosyltransferase in the proA-proB region was confirmed.

Transport of pyrimidine nucleosides in cells of Escherichia coli K 12

1. The transport of pyrimidine mucleosides into cells of Escherichis coli has been investigated in mutant strains which cannot metabolize these nucleosides. Such cells transport and concentrate purimidine mucleosides several hindredfold. 2. The transport is inhibited by energy poisons and by sulfhydryl reagents. 3. Pyrimidine mucleosides compete mutually for transport. Adenosine is also a strong competitor while guanosine and inosine are weak competitors. 4. The rate of pyrimidine mucleoside transport is shown to be under control of the cytR and deoR gene products, which are also known to regulate the synthesis of nucleoside-catabolizing enzymes. The transport system is repressed by growth on glucose, as is the synthesis of the enzymes.

Purine nucleoside phosphorylase from Escherichia coli and Salmonella typhimurium. Purification and some properties

The purine nucleoside phosphorylases from Escherichia coli and from Salmonella typhimurium have been purified to electrophoretic homogeneity and crystallized. Comparative studies revealed that the two enzymes are very much alike. They obey simple Michaelis-Menten kinetics for their substrates with the exception of phosphate for which they show negative cooperativity. Gel filtration on Sephadex G-200 of the native enzymes revealed a molecular weight for both enzymes of 138000 plus or minus 10%. By use of dodecylsulphate gel electrophoresis a subunit molecular weight of 23700 plus or minus 5% was determined, suggesting that both enzymes consist of six subunits of equal molecular weight. When the subunits were partially crosslinked with dimethyl suberimidate before dodecylsulphate electrophoresis six protein bands were observed in agreement with the proposed oligomeric state of the enzyme, consisting of six subunits of equal molecular weight. Analysis of the amino acid composition also indicates that the subunits are identical. 6M guanidinium chloride dissociates the enzymes; association experiments with native and succinylated enzymes suggested that only the hexameric form is active. Both enzymes could be dissociated into subunits by p-chloromercuribenzoate; this dissociation is prevented by the substrates: the nucleosides, the pentose 1-phosphates, and mixtures of phosphate and purine bases.

Early events after infection of Escherichia coli by bacteriophage T5. Induction of a 5'-nucleotidase activity and excretion of free bases

Thymine-containing compounds, produced degradation of Escherichia coli DNA after infection of the cells with bacteriophage T5, did not accumulate in the cell but were excreted into the medium as the DNA was degraded. The ultimate degradation product was extracellular thymine that was not reutilized when T5 DNA synthesis began. This excretion of thymine may have been due in part to the induction of 5'-nucleotidase activity within 3 min after T5 infection. The level of this activity reached a maximum between 4 to 6 min after infection and then rapidly declined to its preinfection level by 10 to 15 min after infection. Chloramphenicol added before or soon after infection prevented the appearance of the nucleotidase. The induced nucleotidase activity was active not only on dTMP but also on dAMP, dGMP, and dCMP.

Characterization of new regulatory mutants of bacteriophage T4

Plating techniques which eliminate T4 plaque formation on Escherichia coli by folate analogue inhibition of dihydrofolate (FH(2)) reductase (EC 1.5.1.3) allowed the isolation of folate analogue-resistant (far) mutants of T4. One class of far mutants overproduces the phage-induced FH(2) reductase. Deoxycytidylate deaminase (EC 3.5.4.12), thymidine kinase (EC 2.7.1.21), and deoxycytidine triphosphatase (EC 3.6.1.12) are also overproduced by 20 min after infection at 37 C. The overproduction of FH(2) reductase by these far mutants is not affected by the absence of DNA synthesis. Other types of mutations that affect the synthesis of early enzymes cause overproduction in the absence of DNA synthesis of some of the above enzymes but not of FH(2) reductase. Therefore, overproducing far mutants apparently have mutations in previously undescribed genes controlling the expression of the T4 genome. Three of four mutants under study map near gene 56, and one maps near gene 52. All of these mutants show delays in DNA synthesis, phage production, and lysis and appear to show decreased levels of RNA synthesis based on the cumulative incorporation of uridine.

Metabolism of exogenous purine bases and nucleosides by Salmonella typhimurium

Purine-requiring mutants of Salmonella typhimurium LT2 containing additional mutations in either adenosine deaminase or purine nucleoside phosphorylase have been constructed. From studies of the ability of these mutants to utilize different purine compounds as the sole source of purines, the following conclusions may be drawn. (i) S. typhimurium does not contain physiologically significant amounts of adenine deaminase and adenosine kinase activities. (ii) The presence of inosine and guanosine kinase activities in vivo was established, although the former activity appears to be of minor significance for inosine metabolism. (iii) The utilization of exogenous purine deoxyribonucleosides is entirely dependent on a functional purine nucleoside phosphorylase. (iv) The pathway by which exogenous adenine is converted to guanine nucleotides in the presence of histidine requires a functional purine nucleoside phosphorylase. Evidence is presented that this pathway involves the conversion of adenine to adenosine, followed by deamination to inosine and subsequent phosphorolysis to hypoxanthine. Hypoxanthine is then converted to inosine monophosphate by inosine monophosphate pyrophosphorylase. The rate-limiting step in this pathway is the synthesis of adenosine from adenine due to lack of endogenous ribose-l-phosphate.

Genetic regulation of ribonucleoside and deoxyribonucleoside catabolism in Salmonella typhimurium

Four enzymes involved in ribonucleoside and deoxyribonucleoside catabolism (deoxyribose-5-P aldolase, thymidine phosphorylase, phosphodeoxyribomutase, and purine nucleoside phosphorylase) are coded for by four closely linked structural genes on the Salmonella chromosome. The genetic order of these genes is (deoC-deoA-deoB-deoD)-serB-thr. Studies on polarity mutants and induction patterns indicate that the deoB and deoD genes may constitute a single operon and that the deoC and deoA genes may constitute a second closely linked operon.