Separate pyrimidine-nucleotide pools for messenger-RNA and ribosomal-RNA synthesis in HeLa S3 cells.

Kinetic analyses of mRNA and 28-S RNA labeling [3H]uridine revealed distinctly different steady-state specific radioactivities finally reached for uridine in mRNA and 28-S RNA when exogenous [3H]uridine was kept constant for several cell doubling times. While the steady-state label of (total) UTP and of uridine in mRNA responded to the same extent to a suppression of pyrimidine synthesis de novo by high uridine concentrations in the culture medium, uridine in 28-S RNA was scarcely influenced. Similar findings were obtained with respect to labeling of cytidine in the various RNA species due to an equilibration of UTP with CTP [5-3H]Uridine is also incorporated into deoxycytidine of DNA, presumably via dCTP. The specific radioactivity of this nucleosidase attained the same steady-state value as UTP, uridine in mRNA and cytidine in mRNA. The data indicate the existence of two pyrimidine nucleotide pools. One is a large, general UTP pool comprising the bulk of the cellular UTP and serving nucleoplasmic nucleic acid formation (uridine and cytidine in mRNA, deoxycytidine in DNA). Its replenishment by de novo synthesis can be suppressed completely by exogenous uridine above 100 muM concentrations. A second, very small UTP (and CTP) pool with a high turnover provides most of the precursors for nucleolar RNA formation (rRNA). This pool is not subject to feedback inhibition by extracellular uridine to an appreciable extent. Determinations of (total) UTP turnover also show that the bulk of cellular RNA (rRNA) cannot be derived from the large UTP pool.     

Purine biosynthesis de novo in rat skeletal muscle.

Purine biosynthesis by the 'de novo' pathway was demonstrated in isolated rat extensor digitorum longus muscle with [1-14C]glycine, [3-14C]serine and sodium [14C]formate as nucleotide precursors. Evidence is presented which suggests that the source of glycine and serine for purine biosynthesis is extracellular rather than intracellular. The relative incorporation rates of the three precursors were formate greater than glycine greater than serine. Over 85% of the label from formate and glycine was recovered in the adenine nucleotides, principally ATP. Azaserine markedly inhibited purine biosynthesis from both formate and glycine. Cycloserine inhibited synthesis from serine, but not from formate. Adenine, hypoxanthine and adenosine markedly inhibited purine synthesis from sodium [14C]formate.     

Effect of various precursors on the synthesis of adenine and uracil nucleotides in the rat heart (author's transl)

The dynamics of cardiac adenine and uracil nucleotides, following a subcutaneous injection of isoproterenol, was studied on the rat in vivo. The effect of continuous supply of adenosine, uridine, or ribose on the level of ATP and UTP was investigated on control rats and on isoproterenol-treated animals. The precursors were administered by continuous infusion (1 ml.h-1) into the superior caval vein. 1. ATP and UTP levels were decreased within one hour after a single dose of isoproterenol (5 mg.kg-1) (Fig. 1). 2. Then, the level of ATP rose slowly toward the control value. The normal level was not reached within 48 h (Fig. 1). 3. On the contrary, the initial drop in UTP concentration was followed by a rapid restoration. The control value was reached in 3 h, and then the UTP pool was increased to 180% of the normal level, 12 h after isoproterenol application. 4. As previously shown by other authors, the restoration of ATP was accelerated by a continuous supply of adenosine (37 micromoles per hour) or ribose (170 micromoles per hour) (Fig. 2). 5. The infusion of ribose (170 micromoles per hour) or uridine (41 micromoles per hour) completely suppressed the initial decrease in UTP level caused by beta-receptor stimulation. The further enlargement of the UTP pool was greatly enhanced by ribose or uridine (Fig. 3). 6. The infusion of adenosine was also positive on UTP regeneration. On the contrary, uridine had no effect on the ATP pool (Fig. 3). 7. When supplied to non-treated animals, all precursors caused an enhancement of the UTP level. Adenosine and ribose increased the ATP pool (Fig. 2 and 3). These results contribute to the comparison of the efficiency of the various pathways of cardiac nucleotide synthesis. They show that both de novo synthesis and salvage pathways are limited by the amount of precursors. The increase in UTP synthesis caused by ribose is consistent with the theory put forward for purines (ZIMMER et GERLACH, 1974) that phosphoribosyl-pyrophosphate availability limits the efficiency of de novo synthesis of nucleotides; it demonstrates that this concept is also true for de novo synthesis of pyrimidine nucleotides.     

Nucleotide Metabolism during Pine Pollen Germination

The metabolism of purine- and pyrimidine nucleotides in pine pollen (Pinus mugo) grown in suspension cultures have been examined. In the ungerminated dehydrated pollen, the presence of ATP has been demonstrated. Incubation of the pollen in a germination medium leads to an exhaustion of the ATP pool, which is restored with the onset of oxygen uptake. By labelling pollen cultures with ³²P-orthophosphate, it has been possible to quantitate the nucleotide components of the pollen, and thereby to measure changes in the nucleotide pattern at various growth stages. The most marked changes occur during the initial phase of tube growth when a large increase in the ribonucleoside triphosphate and the sugar nucleotide pools is observed. The contents of ATP and UDP-glucose are further increased if starch synthesis is initiated by the addition of sucrose to the culture medium. In order to determine whether nucleotides in pine pollen are synthesized from de novo pathways or via reutilization pathways, from breakdown products of nucleic acids, pollen was incubated with ¹⁴C-labelled precursors of both the de novo and the reutilization pathways. Incorporation experiments established de novo synthesis of ATP and GTP from glycine, and de novo synthesis of CTP and UTP from orotic acid. The operation of pathways for the utilization of exogenous nucleosides was also demonstrated. While uridine, cytidine and adenosine are incorporated into nucleoside triphosphate to a great extent, only minor incorporation of inosine and guanosine is observed. These reutilization pathways might be of importance for the synthesis of nucleotides during tube growth in situ. Addition of inhibitors of glycolysis and oxidative phosphorylation drastically reduces the level of ribonucleoside triphosphates, indicating a rapid turnover of the nucleotide pool.     

Uridine uptake and RNA synthesis in the brain of torpid and awakened ground squirrels.

1. Uptake of [3H]uridine into the nucleotide precursor pool after intraventricular injection occurs with the same intensity in the brain of torpid and normothermic awakened ground squirrels. This indicates that the membrane uridine transporters and uridine kinases operate in the hibernator's brain in a hypothermia-tolerant way. 2. Utilization of the [3H]uridine pool for synthesis of the rapidly labelled RNA in the brain of torpid ground squirrels falls more than eight times against RNA labelling in the brain of the active animals between bouts of hibernation. 3. Two hours from the beginning of the artificially provoked awakening, RNA uridine incorporation in the brain of ground squirrels has risen 6.5 times. 4. Drastic changes in [3H]uridine RNA labelling under the stable uridine uptake exclude the precursors and energy supply as the main factors determining changes in intensity of the brain RNA synthesis in the different stages of hibernation.     

Laboratory calibrations of the [h]adenine technique for measuring rates of RNA and DNA synthesis in marine microorganisms

The nucleic acid synthesis rates of several marine phytoplankton and bacteria grown in chemostat and batch cultures were measured by using [H]adenine. The [H]adenine synthesis rates showed excellent agreement with the known rates of synthesis estimated from chemical RNA and DNA data. Under certain conditions, RNA turnover and ATP pool compartmentalization produce inaccuracies in synthesis measurements made with [H]adenine. However, accurate measurements of the rates of microbial RNA and DNA synthesis can be made in any environmental situation provided a few simple precautions are observed. First, time course experiments are recommended. Second, experiments should be conducted for periods long enough to avoid problems arising from disequilibria of internal ATP pools. Finally, exogenous [H]adenine should remain in the medium over the length of the time course.     

Nutritional effects on precursor uptake and compartmentalization of intracellular pools in relation to RNA synthesis.

The effects of nutritional variables on the processing of exogenous precursors into RNA was examined. General nutritional deprivation, or asparagine depletion, led to significant changes in the absolute pool sizes, especially of ATP, UTP and CTP. Fluctuations were found depending on the elapsed time after the nutritional perturbations occurred, and the cell density of the cultures. Depletion of the medium by 28 h of growth, or 1 h of guinea pig asparaginase action, led to considerable inhibition of the conversion of exogenous uridine to CTP by the cells. A series of experiments indicated that in 6C3HED lymphoma cells the uridine nucleotide pool which provided the immediate precursors to RNA (denoted UTP-NA) behaves as a small compartment in rapid equilibrium with exogenously supplied nucleosides. The resemblance to the compartmentation model described by Plagemann (Plagemann, P.G.W. (1972) J. Cell Biol. 52, 131-146 and (1971) J. Cell. Physiol. 77, 241-258) for rat hepatoma cells was close. The UTP-NA pool of the 6C3HED cells constitutes no more than 5% of the cellular UTP pool and is relatively slow in equilibrating with the general cell pool. Correction of the rates of incorporation of isotope into RNA by using some function of the whole cell UTP specific activity to normalize the pool effects, was shown to be invalid.     

Mechanism of Apparent Transcription Inhibition by Methyl Mercury in Cerebellar Neurons

Abstract: We have investigated the mechanism of inhibition of RNA synthesis by methyl mercury (MeHg) in isolated neonatal rat cerebellar cells. Each of the three component steps involved in the incorporation of exogenous [³H]uridine into cellular RNA was examined separately in whole-cell and/or subcellular preparations. Nuclear RNA polymerase activity was measured in preparations containing both free nuclei and whole cells. Incorporation of [³H]UTP into nuclear RNA was found to be unimpaired at concentrations of MeHg that inhibited whole-cell incorporation of [³H]uridine by > 75%. Cellular uptake of [³H]uridine was assayed in cerebellar cells treated with KCN to deplete ATP levels and block subsequent phosphorylation reactions of transported uridine. Uptake activity under these conditions was unaffected by MeHg. Measurement of intracellular phosphorylation of [³H]uridine indicated that inhibition of this activity closely paralleled that of RNA synthesis. Quantitation of individual uridine nucleotides by polyethyleneimine-cellulose TLC revealed reduced levels of UTP and UDP whereas levels of UMP were elevated, suggesting that impairment of phosphorylation was not the result of cellular ATP depletion but, more likely, a direct effect on phosphouridine kinase enzymes. This mechanism of MeHg-induced inhibition of RNA synthesis was confirmed by assays of uridine phosphorylation using cell-free extracts in which exogenous ATP was supplied.     

Nucleotide pools of Novikoff rat hepatoma cells growing in suspension culture. I. Kinetics of incorporation of nucleosides into nucleotide pools and pool sizes during growth cycle

Results from kinetic studies on the incorporation of ³H-5-uridine and ³H-8-adenosine into the acid-soluble nucleotide poor and nucleic acids by Novikoff hepatoma cells (subline N1S1-67) in suspension culture indicate that the uridine transport reaction is saturated at about 100 μM and that for adenosine at about 10 μM nucleoside in the medium, and that above 100 μM simple diffusion becomes the predominant mode of entry of both nucleosides into the cell. The Km of the transport reactions is approximately 1.3 × 10^?5 M for uridine and 6 × 10^?6 M for adenosine. The incorporation of these nucleosides into both the nucleotide pool and into nucleic acids seems to be limited by the rate of entry of the nucleic acid synthesis from the rate of incorporation of nucleosides. Other complicating factors are a change with time of labeling in the relative proporation of nucleoside incorporated into DNA and into the individual nucleotides of RNA, the splitting of uridine to uracil by th ecells, the deamination of adenosine kto inosine and the subsequent cleavage of inosine to hypoxanthine. Various lines of evidence are presented which indicate that the overall nucleotide pools of the cells are very small under normal growth conditions. During growth in the presence of 200 μM uridine or adenosine, however, the cells continue to convert the nucleosides into intracellular nucleotides much more rapidly than required for nucleic acid synthesis. This results in an accumulation of free uridine and adenosine nucleotides in the cells, the maximum amounts of which are at least equivalent to the amount of these nucleotides in total cellular RNA.     

Effect of plasma concentrations of uridine on pyrimidine biosynthesis in cultured L1210 cells

The concentration of uridine in the media of cultured L1210 cells was maintained within the concentration range found in plasma (1 to 10 microM) to determine if such concentrations are sufficient to satisfy the pyrimidine requirements of a population of dividing cells and to determine if cells utilize de novo and/or salvage pathways when exposed to plasma concentrations of uridine. When cells were incubated in the presence of N-(phosphonacetyl)-L-aspartate to block de novo biosynthesis, plasma concentrations of uridine maintained normal cell growth. De novo pyrimidine biosynthesis, as determined by [14C]sodium bicarbonate incorporation into uracil nucleotides, was affected by the low concentrations of uridine found in the plasma. Below 1 microM uridine, de novo biosynthesis was not affected; between 3 and 5 microM uridine, de novo biosynthesis was inhibited by approximately 50%; and above 12 microM uridine, de novo biosynthesis was inhibited by greater than 95%. Inhibition of de novo biosynthesis correlated with an increase in the uracil nucleotide pool. The de novo pathway was much more sensitive to the uracil nucleotide pool size than was the salvage pathway, such that when de novo biosynthesis was inhibited by greater than 95% the uracil nucleotide pool continued to expand and the cells continued to take up [14C]uridine. Thus, the pyrimidine requirements of cultured L1210 cells can be met by concentrations of uridine found in the plasma and, when exposed to such physiologic concentrations, L1210 cells decrease their dependency on de novo biosynthesis and utilize their salvage pathway. Circulating uridine, therefore, may be of physiologic importance and could be an important determinant in anti-pyrimidine chemotherapy.     

Effects of benzimidazole on the purine and pyrimidine metabolism of yeast.

Yeast cells inhibited by benzimidazole accumulate hypoxanthine with associated efflux of xanthine. Unlike control cells, inhibited cells contain no detectable free UMP and CMP. Benzimidazole decreases uptake of [8-14C]hypoxanthine into the intracellular pool of hypoxanthine and xanthine but causes radioactive xanthine to accumulate in the medium. In inhibited cultures there is a threefold increase in incorporation of [8-14C]hypoxanthine into the total (intracellular plus extracellular) xanthine. Uptake of [8-14C]hypoxanthine into free nucleotides and into bound adenine and guanine was inhibited by 70%. Uptake of [U-14C]glycine into IMP, AMP, GMP, DNA and RNA was also substantially decreased. Incorporation of [2-14C]uracil into the intracellular uracil pool was inhibited by 30% and into free uridine and cytidine by over 90%. Benzimidazole inhibited incorporation of [8-3H]IMP into AMP and GMP, and decreased substantially the activity of glutamine-amidophosphoribosyltransferase (EC 2.4.2.14). Yeast cultures were shown to N-ribotylate benzimidazole. Results are consistent with benzimidazole inhibiting yeast growth by competing for P-rib-PP and so depriving other ribotylation processes such as the 'salvage' pathways and de novo synthesis of purines and pyrimidines.     

Free pyrimidine nucleotide pool of Ehrlich ascites-tumour cells. Compartmentation with respect to the synthesis of heterogeneous nuclear RNA and precursors to ribosomal RNA.

The incorporation of [14C]orotate and [14C]uridine into UMP residues of hnRNA (heterogeneous nuclear RNA) and pre-rRNA (precursors to rRNA) of Eharlich ascites-tumour cells was compared: orotate was incorporated at a markedly higher rate into hnRNA. On the other hand, the rate of incorporation of uridine into pre-rRTNA was even somewhat higher than into hnRNA. The ratio of specific radioactivities of CMP to UMP residues in pre-rRNA and hnRNA was studied. At all times of labelling this ratio was similar for both RNA species independently of the precursor used. On addition of excess unlabelled uridine, the CMP/UMP labelling ratio in both pre-rRNA and hnRNA rose. However, this increase was much more pronounced with hnRNA. It is concluded that nuclear pyrimidine nucleotide pool for RNA synthesis is compartmentalized. The synthesis of hnRNa is supplied preferentially by the large and the small compartment, respectively. A detailed model for the cellular compartmentation of uridine nucleotide precursors to RNA is proposed.U     

Laboratory Calibrations of the [3H]Adenine Technique for Measuring Rates of RNA and DNA Synthesis in Marine Microorganisms

The nucleic acid synthesis rates of several marine phytoplankton and bacteria grown in chemostat and batch cultures were measured by using [³H]adenine. The [³H]adenine synthesis rates showed excellent agreement with the known rates of synthesis estimated from chemical RNA and DNA data. Under certain conditions, RNA turnover and ATP pool compartmentalization produce inaccuracies in synthesis measurements made with [³H]adenine. However, accurate measurements of the rates of microbial RNA and DNA synthesis can be made in any environmental situation provided a few simple precautions are observed. First, time course experiments are recommended. Second, experiments should be conducted for periods long enough to avoid problems arising from disequilibria of internal ATP pools. Finally, exogenous [³H]adenine should remain in the medium over the length of the time course.     

Differential metabolism of guanine nucleosides by human lymphoid cell lines

Deficiency of the enzyme purine nucleoside phosphorylase is associated with a specific depletion of T cells which is presumably mediated by its substrate, 2'-deoxyguanosine. Inhibitors of this enzyme are therefore being developed as potential immunosuppressive agents. We have compared the effects of 8-aminoguanosine, a competitive inhibitor of purine nucleoside phosphorylase, on the metabolism of 2'-deoxyguanosine by human T lymphoblasts, B lymphoblasts, and mature T-cell lines. 8-Aminoguanosine markedly potentiates the accumulation of dGTP in T lymphoblasts, but results in increased GTP levels in B lymphoblasts and mature T cells. GTP accumulation is associated with ATP depletion of a magnitude similar to that seen with an inhibitor of de novo purine biosynthesis, but does not result in inhibition of either DNA or RNA synthesis. In contrast, direct inhibition of de novo purine biosynthesis sharply decreased the incorporation of [3H]uridine into both DNA and RNA. We conclude that the mechanism of cell damage resulting from prolonged accumulation of GTP appears to involve more than inhibition of de novo purine biosynthesis and consequent ATP depletion. Perturbations in guanine nucleotide pools resulting from partial inhibition of purine nucleoside phosphorylase activity in vivo could result in cellular toxicity not limited to the target T cell population.     

Purine metabolism in Leishmania donovani and Leishmania braziliensis

We have studied purine metabolism in the culture forms of Leishmania donovani and Leishmania braziliensis. These organisms are incapable of synthesizing purines de novo from glycine, serine, or formate and require an exogenous purine for growth. This requirement is better satisfied by adenosine or hypoxanthine than by guanosine. Bothe adenine and inosine are converted to a common intermediate, hypoxanthine, before transformation to nucleotides. This is due to the activity of an adenine aminohydrolase (EC 3.5.4.2), a rather unusual finding in a eukaryotic cell. There is a preferential synthesis of adenine nucleotides, even when guanine or xanthine are used as precursors.The pathways of purine nucleotide interconversions in these Leishmania resemble those found in mammalian cells except for the absence of de novo purine biosynthesis and the presence of an adenine-deaminating activity.     

Metabolism and salvage of adenine and hypoxanthine by myocytes isolated from mature rat heart

Adenine and hypoxanthine can be utilised by cardiac muscle cells as substrates for the synthesis of ATP. A possible therapeutic advantage of these compounds as high-energy precursors is their lack of vasoactive properties. Myocytes isolated from mature rat heart have been used to establish in kinetic detail the capacity of the heart to incorporate adenine, hypoxanthine and ribose into cellular nucleotides. Maximum rates of catalysis by enzymes on the salvage pathways have been established. Whilst the rate of incorporation of adenine into the ATP pool appears to depend upon intracellular concentrations of adenine and phosphoribosylpyrophosphate, for hypoxanthine the pattern is more complex. Hypoxanthine is salvaged at a slow rate compared with adenine, and is incorporated into GTP and IMP as well as into adenine nucleotides. The rate of incorporation of hypoxanthine into both IMP and ATP is accelerated in myocytes incubated with ribose. However, the rate-limiting reaction appears to be that catalysed by adenylosuccinate synthetase, for the rate of ATP synthesis is not accelerated when hypoxanthine concentration is increased from 10 to 50 microM, while the rate of IMP synthesis is more than doubled. Adenine and hypoxanthine phosphoribosyl transferases are present in equal catalytic amounts, but rat cardiac myocytes have very little adenylosuccinate synthetase activity. Exogenous ribose is incorporated into adenine nucleotides in amounts equimolar with adenine or hypoxanthine.     

NUCLEOTIDE METABOLISM IN RAT BRAIN

Abstract— The uptake, the conversion to nucleotides, and their incorporation into RNA for labelled glycine, aspartate, the free bases and nucleosides of purines and pyrimidines were investigated with cortical slices of rat cerebrum. At the end of a 1-hr incubation time the slice-to-medium ratio of the radioactivities for labelled aspartate, glycine, adenine and adenosine were 34, 26, 20 and 5, respectively, while the slice-to-medium ratios for hypoxanthine, inosine, guanine, guanosine, xanthine, orotate, cytidine, cytosine, uridine, and uracil ranged from 1.3:1 to 2:1. Over 99 per cent of the total radioactivity taken up by the cortical slices was present in the TCA supernatant and 86, 82, 65, 50, 34, 23, 20 and 1.6 per cent of this radioactivity was in the form of nucleotides at the end of a 1-hr incubation with labelled adenine, adenosine, hypoxanthine, inosine, uridine, orotate, cytidine, and glycine, respectively. The incorporation of various radioactive precursors into RNA of cortical slices suggests that nucleotides originating from either de novo synthesis or preformed purine derivatives enter the same nucleotide pool utilized for RNA synthesis. The supernatant fraction from homogenized cerebrum was investigated for the presence of various anabolic and catabolic enzymes associated with nucleotide metabolism. These results were correlated with the data from the RNA incorporation studies, and a possible role for AMP: pyrophosphate phosphoribosyltransferase (adenine phosphoribosyltransferase, I.U.B. 2.4.2.7) to achieve intercellular transfer of AMP is discussed.     

Enzymes of Pyrimidine Metabolism in Mycoplasma mycoides subsp. mycoides

The major pathways of ribonucleotide biosynthesis in Mycoplasma mycoides subsp. mycoides have been proposed from studies on its use of radioactive purines and pyrimidines. To interpret more fully the observed pattern of pyrimidine usage, cell extracts of this organism have been assayed for several enzymes associated with the salvage synthesis of pyrimidine nucleotides. M. mycoides possessed uracil phosphoribosyltransferase, uridine phosphorylase, uridine (cytidine) kinase, uridine 5'-monophosphate kinase, and cytidine 5'-triphosphate synthetase. No activity for phosphorolysis of cytidine was detected, and no in vitro conditions were found to give measurable deamination of cytidine. Of the two potential pathways for incorporation of uridine, our data suggest that this precursor would largely undergo initial phosphorolysis to uracil and ribose-1-phosphate. Conversely, cytidine is phosphorylated directly to cytidine 5'-monophosphate in its major utilization, although conversion of cytidine to uracil, uridine, and uridine nucleotide has been observed in vivo, at least when uracil is provided in the growth medium. Measurements of intracellular nucleotide contents and their changes on additions of pyrimidine precursors have allowed suggestions as to the operation of regulatory mechanisms on pyrimidine nucleotide biosynthesis in M. mycoides in vivo. With uracil alone or uracil plus uridine as precursors of pyrimidine ribonucleotides, the regulation of uracil phosphoribosyltransferase and cytidine 5'-triphosphate synthetase is probably most important in determining the rate of pyrimidine nucleotide synthesis. When cytidine supplements uracil in the growth medium, control of cytidine kinase activity would also be important in this regard.     

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.     

Availability of bases and nucleosides as precursors of nucleic acids in L cells and in the agent of meningopneumonitis

Tribby, Ilse I. E. (University of Chicago, Chicago, Ill.), and James W. Moulder. Availability of bases and nucleosides as precursors of nucleic acids in L cells and in the agent of meningopneumonitis. J. Bacteriol. 91:2362-2367. 1966.-Uninfected L cells and the meningopneumonitis agent propagated in L cells utilized exogenous adenine, guanine, and their ribonucleosides and deoxyribonucleosides for synthesis of both deoxyribonucleic acid (DNA) and ribonucleic acid. Cytosine, cytidine, and uridine were also incorporated into the nucleic acids of both host and parasite. L cells and the meningopneumonitis agent incorporated uracil, thymine, and deoxyuridine very poorly. L cells utilized thymidine and deoxycytidine almost exclusively for DNA synthesis, but the meningopneumonitis agent did not incorporate these nucleosides at all. Since the L cell had previously been shown to convert added thymidine to its nucleotides, mainly the triphosphate, it was concluded that the meningopneumonitis agent can utilize neither the thymidine nor the thymidine nucleotides of the L-cell pool, and that it can probably synthesize the thymidine triphosphate needed for DNA synthesis from the uridine of the L-cell pool.