Distinct roles of putrescine and spermidine in the regulation of ornithine decarboxylase in Neurospora crassa

Multi-omics analysis reveals regulators of the response to nitrogen limitation in Yarrowia lipolytica

Background

Yarrowia lipolytica is an oleaginous ascomycete yeast that stores lipids in response to limitation of nitrogen. While the enzymatic pathways responsible for neutral lipid accumulation in Y. lipolytica are well characterized, regulation of these pathways has received little attention. We therefore sought to characterize the response to nitrogen limitation at system-wide levels, including the proteome, phosphoproteome and metabolome, to better understand how this organism regulates and controls lipid metabolism and to identify targets that may be manipulated to improve lipid yield.

Results

We found that ribosome structural genes are down-regulated under nitrogen limitation, during which nitrogen containing compounds (alanine, putrescine, spermidine and urea) are depleted and sugar alcohols and TCA cycle intermediates accumulate (citrate, fumarate and malate). We identified 1219 novel phosphorylation sites in Y. lipolytica, 133 of which change in their abundance during nitrogen limitation. Regulatory proteins, including kinases and DNA binding proteins, are particularly enriched for phosphorylation. Within lipid synthesis pathways, we found that ATP-citrate lyase, acetyl-CoA carboxylase and lecithin cholesterol acyl transferase are phosphorylated during nitrogen limitation while many of the proteins involved in β-oxidation are down-regulated, suggesting that storage lipid accumulation may be regulated by phosphorylation of key enzymes. Further, we identified short DNA elements that associate specific transcription factor families with up- and down-regulated genes.

Conclusions

Integration of metabolome, proteome and phosphoproteome data identifies lipid accumulation in response to nitrogen limitation as a two-fold result of increased production of acetyl-CoA from excess citrate and decreased capacity for β-oxidation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2471-2) contains supplementary material, which is available to authorized users.

Role of endocytosis in the internalization of spermidine-C(2)-BODIPY, a highly fluorescent probe of polyamine transport

The mechanism of transmembrane polyamine internalization in mammalian cells remains unknown. A novel fluorescent spermidine conjugate [Spd-C(2)-BODIPY; N-(4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene-3-propionyl)-N'-(S -[spermidine-(N(4)-ethyl)]thioacetyl)ethylenediamine] was synthesized from N(4)-(mercaptoethyl)spermidine by a simple, one-step coupling procedure. In Chinese-hamster ovary (CHO) cells, Spd-C(2)-BODIPY accumulation was inhibited by exogenous putrescine, spermidine and spermine, was subject to feedback transport inhibition and was up-regulated by prior polyamine depletion achieved with a biosynthetic inhibitor. Probe internalization was decreased by about 85% in a polyamine-transport-deficient CHO mutant cell line. Using confocal laser scanning fluorescence microscopy, internalized Spd-C(2)-BODIPY was concentrated in vesicle-like structures similar to the recycling endosomes observed with fluorescent transferrin, which partly co-localized with the polyamine probe. In yeast, Spd-C(2)-BODIPY uptake was stringently dependent on receptor-mediated endocytosis, as determined with a mutant defective in early- endosome formation. On the other hand, Spd-C(2)-BODIPY did not mimic the substrate behaviour of natural polyamines in yeast, as shown by the lack of correlation of its uptake characteristics with the phenotypes of mutants defective in either polyamine transport or biosynthesis. These data suggest that endocytosis might be an integral part of the mechanism of polyamine transport in mammalian cells, and that the mammalian and yeast transport systems use qualitatively different transport mechanisms. However, the current data do not rule out the possibility that sequestration of the probe into vesicular structures might be secondary to its prior uptake via a "classical" plasma membrane carrier. Spd-C(2)-BODIPY, a highly sensitive probe of polyamine transport with biochemical parameters qualitatively similar to those of natural polyamines in mammalian cells, should be very useful for dissecting the pathway responsible for polyamine internalization.

Polyamine regulation of ornithine decarboxylase synthesis in Neurospora crassa

Ornithine decarboxylase (ODC) of the fungus Neurospora crassa, encoded by the spe-1 gene, catalyzes an initial and rate-limiting step in polyamine biosynthesis and is highly regulated by polyamines. In N. crassa, polyamines repress the synthesis and increase the degradation of ODC protein. Changes in the rate of ODC synthesis correlate with similar changes in the abundance of spe-1 mRNA. We identify two sequence elements, one in each of the 5' and 3' regions of the spe-1 gene of N. crassa, required for this polyamine-mediated regulation. A 5' polyamine-responsive region (5' PRR) comprises DNA sequences both in the upstream untranscribed region and in the long 5' untranslated region (5'-UTR) of the gene. The 5' PRR is sufficient to confer polyamine regulation to a downstream, heterologous coding region. Use of the beta-tubulin promoter to drive the expression of various portions of the spe-1 transcribed region revealed a 3' polyamine-responsive region (3' PRR) downstream of the coding region. Neither changes in cellular polyamine status nor deletion of sequences in the 5'-UTR alters the half-life of spe-1 mRNA. Sequences in the spe-1 5'-UTR also impede the translation of a heterologous coding region, and polyamine starvation partially relieves this impediment. The results show that N. crassa uses a unique combination of polyamine-mediated transcriptional and translational control mechanisms to regulate ODC synthesis.

Enhanced production of arginine and urea by genetically engineered Escherichia coli K-12 strains

Escherichia coli strains capable of enhanced synthesis of arginine and urea were produced by derepression of the arginine regulon and simultaneous overexpression of the E. coli carAB and argI genes and the Bacillus subtilis rocF gene. Plasmids expressing carAB driven by their natural promoters were unstable. Therefore, E. coli carAB and argI genes with and without the B. subtilis rocF gene were constructed as a single operon under the regulation of the inducible promoter ptrc. Arginine operator sequences (Arg boxes) from argI were also cloned into the same plasmids for titration of the arginine repressor. Upon overexpression of these genes in E. coli strains, very high carbamyl phosphate synthetase, ornithine transcarbamylase, and arginase catalytic activities were achieved. The biosynthetic capacity of these engineered bacteria when overexpressing the arginine biosynthetic enzymes was 6- to 16-fold higher than that of controls but only if exogenous ornithine was present (ornithine was rate limiting). Overexpression of arginase in bacteria with a derepressed arginine biosynthetic pathway resulted in a 13- to 20-fold increase in urea production over that of controls with the parent vector alone; in this situation, the availability of carbamyl phosphate was rate limiting.

Polyamines in growth and dimorphism of Paracoccidioides brasiliensis

Putrescine and spermidine were the only polyamines found in Paracoccidioides brasiliensis, a dimorphic fungus pathogenic for humans. Free polyamines (putrescine > spermidine) increased during the first 24 h of yeast growth, with a second peak at 42 h, and also during the first 12 h of mycelium-to-yeast transition (spermidine > putrescine). Conjugated and bound polyamines were also quantified. 1, 4-Diamino-2-butanone decreased free putrescine and spermidine accumulation by inhibiting the activity of ornithine decarboxylase. The increase in free polyamines corresponds to bud emergence in yeast growth and to the mycelium-to-yeast transition of P. brasiliensis.

Rat antizyme inhibits the activity but does not promote the degradation of mouse ornithine decarboxylase in Trypanosoma brucei

Ornithine decarboxylase (ODC) of African trypanosomes is an important target for anti-trypanosomal chemotherapy because of its remarkable stability in vivo. The in vivo activity and stability of mammalian ODC are regulated by polyamines. Polyamines induce antizyme, which inactivates ODC by tight association and promotes degradation of ODC by the mammalian 26 S proteasome. Here we found, in contrast to mammalian cells, that polyamines caused no reduction of ODC activity in Trypanosoma brucei. Mouse ODC expressed in T. brucei was also unaffected by exogenous polyamines, suggesting that a mammalian antizyme equivalent may be absent in T. brucei. The rat antizyme expressed in T. brucei was found capable of inhibiting mouse ODC activity by the formation of rat antizyme-mouse ODC complex. However, complex formation did not lead to degradation of mouse ODC in T. brucei. Further in vitro experiments suggested the presence of an inhibitory factor(s) in trypanosome, which interferes with the degradation of mouse ODC. We also demonstrated the presence of proteasomes in T. brucei. But the mobility of the trypanosomal proteasome on native gel is different from that of the mammalian proteasome. Thus, the absence of antizyme, the presence of inhibitory factor(s), and the differences between trypanosomal and mammalian proteasome may account for the stability of mouse ODC in T. brucei cells.

Rapid and regulated degradation of ornithine decarboxylase

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Sequestered end products and enzyme regulation: the case of ornithine decarboxylase

The polyamines (putrescine, spermidine, and spermine) are synthesized by almost all organisms and are universally required for normal growth. Ornithine decarboxylase (ODC), an initial enzyme of polyamine synthesis, is one of the most highly regulated enzymes of eucaryotic organisms. Unusual mechanisms have evolved to control ODC, including rapid, polyamine-mediated turnover of the enzyme and control of the synthetic rate of the protein without change of its mRNA level. The high amplitude of regulation and the rapid variation in the level of the protein led biochemists to infer that polyamines had special cellular roles and that cells maintained polyamine concentrations within narrow limits. This view was sustained in part because of our continuing uncertainty about the actual biochemical roles of polyamines. In this article, we challenge the view that ODC regulation is related to precise adjustment of polyamine levels. In no organism does ODC display allosteric feedback inhibition, and in three types of organism, bacteria, fungi, and mammals, the size of polyamine pools may vary radically without having a profound effect on growth. We suggest that the apparent stability of polyamine pools in unstressed cells is due to their being largely bound to cellular polyanions. We further speculate that allosteric feedback inhibition, if it existed, would be inappropriately responsive to changes in the small, freely diffusible polyamine pool. Instead, mechanisms that control the amount of the ODC protein have appeared in most organisms, and even these are triggered inappropriately by variation of the binding of polyamines to ionic binding sites. In fact, feedback inhibition of ODC might be maladaptive during hypoosmotic stress or at the onset of growth, when organisms appear to require rapid increases in the size of their cellular polyamine pools.

Ornithine decarboxylase from Crithidia fasciculata is metabolically unstable and resistant to polyamine down-regulation

Ornithine decarboxylase (ODC) of Crithidia fasciculata extracts shows maximal activity during exponential growth of the parasite and decreases markedly in the stationary phase. The inhibition of protein synthesis by cycloheximide evoked a rapid loss of enzyme activity with a half-life of about 30 min. Upon removal of DFMO from Crithidia cultures treated with the drug for 24 h, the ODC activity increased at the same rate as total protein synthesis. The addition of putrescine at high concentrations to parasites cultivated in a synthetic medium showed that Crithidia ODC levels were not reduced by polyamines.

Ornithine decarboxylase gene of Neurospora crassa: isolation, sequence, and polyamine-mediated regulation of its mRNA

Ornithine decarboxylase (ODC), which initiates the biosynthesis of the polyamines putrescine, spermidine, and spermine, is encoded by the spe-1 gene of the fungus Neurospora crassa. This gene and its cDNA have been cloned and sequenced. The gene has a single 70-nucleotide intron in the coding sequence. The cDNA, comprising the entire coding region, recognizes a single 2.4-kb mRNA in Northern (RNA) blots. The mRNA transcript, defined by S1 mapping, has an extremely long, 535-base leader without strong secondary-structure features or an upstream reading frame. The translational start of the protein is ambiguous: a Met-Val-Met sequence precedes the Pro known to be the N terminus of the ODC polypeptide. The polypeptide encoded by the N. crassa spe-1 gene (484 amino acids) has 46% amino acid identity with that of Saccharomyces cerevisiae (466 amino acids) and 42% with that of mouse (461 amino acids). Alignment of the longer N. crassa sequence with S. cerevisiae and mouse sequences creates gaps in different sites in the S. cerevisiae and mouse sequences, suggesting that N. crassa ODC is closer to an ancestral form of the enzyme than that of either yeast or mouse ODC. N. crassa ODC, which turns over rapidly in vivo in the presence of polyamines, has two PEST sequences, found in most ODCs and other proteins with rapid turnover. In striking contrast to other eucaryotic organisms, the variation in the rate of ODC synthesis in response to polyamines in N. crassa is largely correlated with proportional changes in the abundance of ODC mRNA. Spermidine is the main effector of repression, while putrescine has a weaker effect. However, putrescine accumulation appears to increase the amount of active ODC that is made from a given amount of ODC mRNA, possibly by improving its translatability. Conversely, prolonged starvation for both putrescine and spermidine leads to the differentially impaired translation of ODC mRNA.

Ornithine decarboxylase gene of Neurospora crassa: isolation, sequence, and polyamine-mediated regulation of its mRNA

Ornithine decarboxylase (ODC), which initiates the biosynthesis of the polyamines putrescine, spermidine, and spermine, is encoded by the spe-1 gene of the fungus Neurospora crassa. This gene and its cDNA have been cloned and sequenced. The gene has a single 70-nucleotide intron in the coding sequence. The cDNA, comprising the entire coding region, recognizes a single 2.4-kb mRNA in Northern (RNA) blots. The mRNA transcript, defined by S1 mapping, has an extremely long, 535-base leader without strong secondary-structure features or an upstream reading frame. The translational start of the protein is ambiguous: a Met-Val-Met sequence precedes the Pro known to be the N terminus of the ODC polypeptide. The polypeptide encoded by the N. crassa spe-1 gene (484 amino acids) has 46% amino acid identity with that of Saccharomyces cerevisiae (466 amino acids) and 42% with that of mouse (461 amino acids). Alignment of the longer N. crassa sequence with S. cerevisiae and mouse sequences creates gaps in different sites in the S. cerevisiae and mouse sequences, suggesting that N. crassa ODC is closer to an ancestral form of the enzyme than that of either yeast or mouse ODC. N. crassa ODC, which turns over rapidly in vivo in the presence of polyamines, has two PEST sequences, found in most ODCs and other proteins with rapid turnover. In striking contrast to other eucaryotic organisms, the variation in the rate of ODC synthesis in response to polyamines in N. crassa is largely correlated with proportional changes in the abundance of ODC mRNA. Spermidine is the main effector of repression, while putrescine has a weaker effect. However, putrescine accumulation appears to increase the amount of active ODC that is made from a given amount of ODC mRNA, possibly by improving its translatability. Conversely, prolonged starvation for both putrescine and spermidine leads to the differentially impaired translation of ODC mRNA.

The level of substrate ornithine can alter polyamine-dependent DNA synthesis following phorbolester stimulation of cultured hepatoma cells

Although the precise intracellular function(s) of the polyamines remain incompletely defined, a myraid of evidence now shows that the polyamines must accumulate or be maintained at a specific intracellular concentration in order for all mammalian cells to grow or divide. The initial step in polyamine biosynthesis normally involves the decarboxylation of ornithine by the enzyme ornithine decarboxylase (ODCase E.C. 4.1.1.17) to yield putrescine. Increases in the steady-state level of intracellular ornithine have been reported to markedly alter the accumulation of the polyamines following stimulation of Reuber H35 Hepatoma cells with 12-O-tetradecanoylphorbol-beta-acetate (TPA) in the presence of serum (Wu and Byus: (Biochem. Biophys. Acta 804:89-99, 1984); Wu et al.: (Cancer Res. 41:3384-3391, 1981). We wished to determine whether or not incubation of H35 hepatoma cells with exogenous ornithine would result in a stimulation of DNA synthesis following treatment with the mitogens TPA and insulin. For these studies, H35 cells were maintained under serum-free conditions for 2-3 days in order to obtain synchronous cultures suitable for analysis of the level of DNA synthesis. Cultures treated in this manner were highly viable, maintained similar growth rates, and possessed the equivalent levels of intracellular ornithine and polyamines as the serum-containing cultures. Arginine levels, however, were approximately twofold higher following culture under serum-restricted conditions for 3 days. The addition of exogenous ornithine (0.5 mM) was accompanied by a 4-5-fold increase in intracellular steady-state ornithine levels and by a 6-8-fold increase in the presence of TPA and ornithine. In a manner identical to the serum-containing cultures (Wu and Byus (1984] the addition of TPA and exogenous ornithine to the serum-free cells caused a dose-dependent increase in intracellular putrescine (up to 5-fold) and a concomitant decrease in ODC activity in comparison to stimulation with TPA alone. The addition of TPA led to a 3-5-fold increase in the incorporation of tritiated thymidine into DNA. In the presence of exogenous ornithine, TPA-induced DNA synthesis was further stimulated more than twofold in a dose-dependent manner. Insulin (10(-10)-10(-8) M) proved to be more efficacious as a mitogen in the H35 cells and led to greater stimulation of DNA synthesis than TPA. Insulin alone also resulted in a higher steady-state level of ornithine and putrescine in comparison with TPA alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Early developmental profile of ornithine decarboxylase in the frog, Microhyla ornata and its regulation by polyamines

Ornithine decarboxylase (ODC) activity and polyamine levels were measured during early development of the frog, Microhyla ornata. ODC activity was found to be high and it showed three major peaks during the first 60 hr of development. Putrescine and spermidine levels increased gradually during the above period with little change in spermine. Treatment of developing embryos with exogenous putrescine and spermidine prevented the normal increase in ODC activity. Spermine did not have any significant effect. Addition of ornithine also prevented the increase in ODC activity. Experiment using exogenous ornithine and alpha-methylornithine revealed that formation of putrescine and/or spermidine from ornithine is necessary for the suppression of ODC to occur. Suppression of ODC takes place even if conversion of putrescine to spermidine is blocked, indicating that putrescine, independent of its conversion to spermidine, also plays a role in ODC regulation.

Polyamine--DNA nexus: structural ramifications and biological implications

Polyamines at physiological concentration can condense DNA, chromatin and promote B to Z DNA transitions. These properties of polyamines are crucial to the molecular organization and functional control of DNA and thus have very significant implications in the control of cellular functions. The structure of polyamines plays an important role in the binding of DNA and chromatin and it is not merely the charge, but a specific chain length of methylene (-CH2) groups that is required. Acetylation of polyamines seems to be an important mode of regulating polyamine-chromatin interaction. Purified histone acetyltransferase also possesses polyamine acetylation activity, thus histones and polyamine acetylation may occur in tandem to alter the structure/function of the nucleosome thereby regulating DNA replication and transcription. Acetylation as a means to diminish the number of charges on polyamine molecules serves as an ordered mechanism to control DNA replication and transcription in vivo. The results on the involvement of polyamines and their analogs in condensation of DNA and B to Z DNA transition correlate well with the conclusions drawn from experiments designed to observe the in vivo effects of polyamines and their analogs on the growth of prokaryotic and eukaryotic cells. For example, any change in the hydrogen bonding capacity of polyamines leads to a marked reduction in protein synthesis and the growth rate of polyamine depleted cells. A minimal level of polyamines is required for cells to move from G1 through S phase and these amines are directly involved in the DNA synthetic phase of the cell cycle. A nexus between polyamines and nucleic acids appears crucial to the cellular function(s) of polyamines.

Management of polyamine pools and the regulation of ornithine decarboxylase

The management of polyamine synthesis and polyamine pools differs fundamentally from that of most other small molecular-weight endproducts. The polyamines are vital to growth and important cellular functions, but they are toxic in excess. I argue here that their multivalent cationic character, leading to binding to cell constituents, precludes fluent feedback inhibition of synthesis. This has led to the development of elaborate alternative regulatory mechanisms controlling ornithine decarboxylase, the key initial enzyme of the pathway. Poorly regulated polyamine synthesis and the toxicity of polyamines impose upon cells a need to control uptake and to dispose of excess polyamines. Recent data on polyamine transport suggest unorthodox mechanisms of accomplishing these functions.

Effect of inhibitors of S-adenosylmethionine decarboxylase on the contents of ornithine decarboxylase and S-adenosylmethionine decarboxylase in L1210 cells

Treatment of L1210 cells with either of two inhibitors of S-adenosylmethionine decarboxylase (AdoMetDC), namely 5'-deoxy-5'-[N-methyl-N-[2-(amino-oxy)ethyl])aminoadenosine or 5'-deoxy-5'-[N-methyl-N-(3-hydrazinopropyl)]aminoadenosine, produced a large increase in the amount of ornithine decarboxylase (ODC) protein. The increased enzyme content was due to a decreased rate of degradation of the protein and to an increased rate of synthesis, but there was no change in its mRNA content. The inhibitors led to a substantial decline in the amounts of intracellular spermidine and spermine, but to a big increase in the amount of putrescine. These results indicate that the content of ODC is negatively regulated by spermidine and spermine at the levels of protein translation and turnover, but that putrescine is much less effective in bringing about this repression. Addition of either spermidine or spermine to the cells treated with the AdoMetDC inhibitors led to a decrease in ODC activity, indicating that either polyamine can bring about this effect, but spermidine produced effects at concentrations similar to those found in the control cells and appears to be the physiologically important regulator. The content of AdoMetDC protein (measured by radioimmunoassay) was also increased by these inhibitors, and a small increase in its mRNA content was observed, but this was insufficient to account for the increase in protein. A substantial stabilization of AdoMetDC occurred in these cells, contributing to the increased enzyme content, but an increase in the rate of translation cannot be ruled out.

Nonsense mutations of the ornithine decarboxylase structural gene of Neurospora crassa

Ornithine decarboxylase (ODC) (EC 4.1.1.17) is an early enzyme of polyamine synthesis, and its activity rises quickly at the onset of growth and differentiation in most eucaryotes. Some have speculated that the enzyme protein may have a role in the synthesis of rRNA in addition to its role in catalyzing the decarboxylation of ornithine (G. D. Kuehn and V. J. Atmar, Fed. Proc. 41:3078-3083, 1982; D. H. Russell, Proc. Natl. Acad. Sci. USA 80:1318-1321, 1983). To test this possibility, we sought mutational evidence for the indispensability of the ODC protein for normal growth of Neurospora crassa. We found three new, ODC-deficient mutants that lacked ODC protein. Among these and by reversion analysis of an earlier set of mutants, we found that two ODC-deficient mutants carried nonsense mutations in the ODC structural gene, spe-1. Allele LV10 imparted a complete deficiency for enzyme activity (less than 0.006% of normal) and had no detectable ODC antigen. Allele PE4 imparted a weak activity to cells (0.1% of derepressed spe+ cultures) and encoded a lower-molecular-weight ODC subunit (Mr = 43,000) in comparison to that of the wild-type strain (Mr = 53,000). Strains carrying either mutation, like other spe-1 mutants, grew at a normal rate in exponential culture if the medium was supplemented with spermidine, the main end product of the polyamine pathway in N. crassa. Unless an antigenically silent, N-terminal fragment with an indispensable role persists in the LV10-bearing mutant, we conclude that the ODC protein has no role in the vegetative growth of this organism other than the synthesis of polyamines. The data extend earlier evidence that spe-1 is the structural gene for ODC in N. crassa. The activity found in mutants bearing allele PE4 suggests that the amino acids nearest the carboxy terminus do not contribute to the active site of the enzyme.

Nonsense mutations of the ornithine decarboxylase structural gene of Neurospora crassa

Ornithine decarboxylase (ODC) (EC 4.1.1.17) is an early enzyme of polyamine synthesis, and its activity rises quickly at the onset of growth and differentiation in most eucaryotes. Some have speculated that the enzyme protein may have a role in the synthesis of rRNA in addition to its role in catalyzing the decarboxylation of ornithine (G. D. Kuehn and V. J. Atmar, Fed. Proc. 41:3078-3083, 1982; D. H. Russell, Proc. Natl. Acad. Sci. USA 80:1318-1321, 1983). To test this possibility, we sought mutational evidence for the indispensability of the ODC protein for normal growth of Neurospora crassa. We found three new, ODC-deficient mutants that lacked ODC protein. Among these and by reversion analysis of an earlier set of mutants, we found that two ODC-deficient mutants carried nonsense mutations in the ODC structural gene, spe-1. Allele LV10 imparted a complete deficiency for enzyme activity (less than 0.006% of normal) and had no detectable ODC antigen. Allele PE4 imparted a weak activity to cells (0.1% of derepressed spe+ cultures) and encoded a lower-molecular-weight ODC subunit (Mr = 43,000) in comparison to that of the wild-type strain (Mr = 53,000). Strains carrying either mutation, like other spe-1 mutants, grew at a normal rate in exponential culture if the medium was supplemented with spermidine, the main end product of the polyamine pathway in N. crassa. Unless an antigenically silent, N-terminal fragment with an indispensable role persists in the LV10-bearing mutant, we conclude that the ODC protein has no role in the vegetative growth of this organism other than the synthesis of polyamines. The data extend earlier evidence that spe-1 is the structural gene for ODC in N. crassa. The activity found in mutants bearing allele PE4 suggests that the amino acids nearest the carboxy terminus do not contribute to the active site of the enzyme.

Spectrophotometric assay for ornithine decarboxylase

A rapid and sensitive spectrophotometric assay for ornithine decarboxylase is described. It is based on the observation that the product of ornithine decarboxylase, putrescine, reacts with 2,4,6-trinitrobenzenesulfonic acid to give a colored product soluble in 1-pentanol whereas ornithine does not. The amount of putrescine produced by the enzyme was determined by measuring the absorbance of the 1-pentanol extract of the reaction mixture at 420 nm, and by comparing the results to those obtained by the trapping of 14CO2 and by HPLC assays. The three assays were found to be equivalent in sensitivity, with the spectrophotometric assay having the advantages of being relatively rapid, requiring only common laboratory equipment, and not requiring the use of radioactive isotopes.

Multiple mechanisms are responsible for altered expression of ornithine decarboxylase in overproducing variant cells

We selected and characterized a series of mouse S49 cell variants that overproduce ornithine decarboxylase (ODC). Previously, we described variants that have an amplified ODC gene and produce about 500-fold more ODC than the wild-type cells of origin (L. McConlogue and P. Coffino, J. Biol. Chem. 258:12083-12086, 1983). We examined a series of independent variants that overproduce ODC to a lesser degree and found that a number of mechanisms other than gene amplification are responsible for the increased ODC activity. Variants were selected for resistance to 0.1 mM difluoromethylornithine, an inhibitor of ODC, by either a single or a multistep process. All showed increased ODC activity and increased ODC mRNA steady-state levels. The half-life of the enzyme was not increased in any of the variants. In one class of variant the increase of ODC mRNA was sufficient to account for ODC overproduction. In a second class, the rate of synthesis of ODC polypeptide per ODC mRNA was at least four- to eightfold higher than that in wild-type cells. Therefore, these variants were altered in the translatability of ODC mRNA. Southern analysis showed that gene amplification does not account for the increased ODC mRNA levels in any of the variants. In both variant and wild-type cells, ODC activity was responsive to changes in polyamine pools; activity was reduced following augmentation of pool size. This change in activity was associated with modification of the rate of synthesis and degradation of ODC but no change in the level of ODC mRNA.

Multiple mechanisms are responsible for altered expression of ornithine decarboxylase in overproducing variant cells

We selected and characterized a series of mouse S49 cell variants that overproduce ornithine decarboxylase (ODC). Previously, we described variants that have an amplified ODC gene and produce about 500-fold more ODC than the wild-type cells of origin (L. McConlogue and P. Coffino, J. Biol. Chem. 258:12083-12086, 1983). We examined a series of independent variants that overproduce ODC to a lesser degree and found that a number of mechanisms other than gene amplification are responsible for the increased ODC activity. Variants were selected for resistance to 0.1 mM difluoromethylornithine, an inhibitor of ODC, by either a single or a multistep process. All showed increased ODC activity and increased ODC mRNA steady-state levels. The half-life of the enzyme was not increased in any of the variants. In one class of variant the increase of ODC mRNA was sufficient to account for ODC overproduction. In a second class, the rate of synthesis of ODC polypeptide per ODC mRNA was at least four- to eightfold higher than that in wild-type cells. Therefore, these variants were altered in the translatability of ODC mRNA. Southern analysis showed that gene amplification does not account for the increased ODC mRNA levels in any of the variants. In both variant and wild-type cells, ODC activity was responsive to changes in polyamine pools; activity was reduced following augmentation of pool size. This change in activity was associated with modification of the rate of synthesis and degradation of ODC but no change in the level of ODC mRNA.

Effects of putrescine on synthesis and degradation of ornithine decarboxylase in primary cultured hepatocytes

Changes in both synthesis rate and degradation rate of ornithine decarboxylase (ODC) were pursued in primary cultures of adult rat hepatocytes during the process of ODC induction caused by asparagine and glucagon and also during the process of rapid ODC decay caused by putrescine. The synthesis rate of ODC was determined by [35S]methionine incorporation into the enzyme, which was separated afterwards by immunoprecipitation and sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. The degradation rate of ODC was determined by following the decay of prelabeled ODC. The enzyme induction caused by asparagine (10 mM) and glucagon (1 microM) was due both to an increase in the synthesis rate and to a decrease in the degradation rate. Addition of 10 mM putrescine caused a rapid decay of ODC activity, which was faster than ODC decay in the presence of cycloheximide. This rapid decay in ODC activity was accompanied by slightly slower decay in ODC protein, which was due both to partial suppression of ODC synthesis and to several fold acceleration of ODC degradation.