Spermine, spermidine and putrescine in fungal development

Piriformospora indica recruits host-derived putrescine for growth promotion in plants

Growth promotion induced by the endosymbiont Piriformospora indica has been observed in various plants; however, except growth phytohormones, specific functional metabolites involved in P. indica-mediated growth promotion are unknown. Here, we used a gas chromatography-mass spectrometry-based untargeted metabolite analysis to identify tomato (Solanum lycopersicum) metabolites whose levels were altered during P. indica-mediated growth promotion. Metabolomic multivariate analysis revealed several primary metabolites with altered levels, with putrescine (Put) induced most significantly in roots during the interaction. Further, our results indicated that P. indica modulates the arginine decarboxylase (ADC)-mediated Put biosynthesis pathway via induction of SlADC1 in tomato. Piriformospora indica did not promote growth in Sladc1-(virus-induced gene silencing of SlADC1) lines of tomato and showed less colonization. Furthermore, using LC-MS/MS we showed that Put promoted growth by elevation of auxin (indole-3-acetic acid) and gibberellin (GA4 and GA7) levels in tomato. In Arabidopsis (Arabidopsis thaliana) adc knockout mutants, P. indica colonization also decreased and showed no plant growth promotion, and this response was rescued upon exogenous application of Put. Put is also important for hyphal growth of P. indica, indicating that it is co-adapted by both host and microbe. Taken together, we conclude that Put is an essential metabolite and its biosynthesis in plants is crucial for P. indica-mediated plant growth promotion and fungal growth.

Leveraging single-cell genomics to expand the fungal tree of life

Environmental DNA surveys reveal that most fungal diversity represents uncultured species. We sequenced the genomes of eight uncultured species across the fungal tree of life using a new single-cell genomics pipeline. We show that, despite a large variation in genome and gene space recovery from each single amplified genome (SAG), ≥90% can be recovered by combining multiple SAGs. SAGs provide robust placement for early-diverging lineages and infer a diploid ancestor of fungi. Early-diverging fungi share metabolic deficiencies and show unique gene expansions correlated with parasitism and unculturability. Single-cell genomics holds great promise in exploring fungal diversity, life cycles and metabolic potential.

Preventing Fusarium head blight of wheat and cob rot of maize by inhibition of fungal deoxyhypusine synthase

Upon posttranslational activation, the eukaryotic initiation factor-5A (eIF-5A) transports a subset of mRNAs out of the nucleus to the ribosomes for translation. Activation of the protein is an evolutionary highly conserved process that is unique to eIF-5A, the conversion of a lysine to a hypusine. Instrumental for the synthesis of hypusine is the first of two enzymatic reactions mediated by deoxyhypusine synthase (DHS). We show that DHS of wheat and the pathogenic fungus Fusarium graminearum, which causes one of the most destructive crop diseases worldwide, are transcriptionally upregulated during their pathogenic interaction. Although DHS of wheat, fungus, and human can be equally inhibited by the inhibitor CNI-1493 in vitro, application during infection of wheat and maize flowers results in strong inhibition of the pathogen without interference with kernel development. Our studies provide a novel strategy to selectively inhibit fungal growth without affecting plant growth. We identified fungal DHS as a target for the development of new inhibitors, for which CNI-1493 may serve as a lead substance.

Control of the phytopathogen Botrytis cinerea using adipic acid monoethyl ester

The in vitro and in vivo antifungal activity of adipic acid monoethyl ester (AAME) on the necrotrophic pathogen Botrytis cinerea has been studied. This chemical effectively controlled this important phytopathogen, inhibited spore germination and mycelium development at non-phytotoxic concentrations. The effectiveness of AAME treatment is concentration-dependent and influenced by pH. Spore germination in the presence of AAME is stopped at a very early stage, preventing germ tube development. In addition, cytological changes such as retraction of the conidial cytoplasm in the fungus are observed. AAME was also found to act on membrane integrity, affecting permeability without exhibiting lytic activity, as described previously for other antifungal compounds. Polyamine content in the mycelium of B. cinerea was also affected in response to AAME treatment, resulting in putrescine reduction and spermine accumulation similar to a number of antifungal agents. Microscopic observation of treated conidia after inoculation on tomato leaves suggested that inhibited spores are not able to attach to and penetrate the leaf. Finally, AAME completely suppressed the grey mould disease of tomato fruits under controlled inoculation conditions, providing evidence for its efficacy in a biological context and for the potential use of this chemical as an alternative fungicide treatment.

Antifungal activity of some deoxyhypusine synthase inhibitors (short communication: plant mycology and crop protection)

Several inhibitors of deoxyhypusine synthase were examined for antifungal activity. All compounds, used at concentrations of 1.0 mM or less, reduced mycelial growth of the oat leaf stripe pathogen Pyrenophora avenae grown on solid media. Four of the compounds completely inhibited fungal growth, 4a doing so at a concentration of only 0.61 mM. Inhibition of fungal growth afforded by some of the compounds was more marked than was reflected by the radial growth measurements.

Polyamine metabolism during the germination of Sclerotinia sclerotiorum ascospores and its relation with host infection

• Polyamine biosynthesis inhibitors were used to study polyamine metabolism during the germination of Sclerotinia sclerotiorum ascospores, and to evaluate the potential of polyamine biosynthesis inhibition for the control of ascospore-borne diseases in plants. • The effects of inhibitors on ascospore germination, free polyamine levels, ornithine decarboxylase activity and development of disease symptoms on tobacco (Nicotiana tabacum) leaf discs inoculated with ascospores were determined. • α-Difluoromethylornithine inhibited ornithine decarboxylase and decreased free spermidine levels, but had no effect on ascospore germination. Both, the spermidine synthase inhibitor cyclohexylamine and the S-adenosyl-methionine decarboxylase inhibitor methylglyoxal bis-[guanyl hydrazone] decreased free spermidine levels, but only the latter inhibited ascospore germination, at concentrations of 5 mm or higher. Lesion development on leaf discs was reduced by cyclohexylamine and methylglyoxal bis-[guanyl hydrazone], but not by α-difluoromethylornithine. In the absence of inhibitors, dormant ascospores contained higher polyamine levels than mycelium. • Ascospore germination did not depend on ornithine decarboxylase activity and inhibitors of this enzyme will probably have a limited potential for the control of ascospore-borne plant diseases. On the contrary, spermidine synthase and S-adenosyl-methionine decarboxylase could be more suitable targets for fungicidal action. The relative insensitivity of ascospore germination to polyamine biosynthesis inhibitors may be caused by their high polyamine content.

Synthesis and antifungal activity of five classes of diamines

Examples of five classes of diamines were synthesized and tested for antifungal activity. Two classes, the bis(cyclohexylmethyl)diamines and the bis(benzyl)diamines, were most effective in reducing mycelial growth of the oat leaf stripe pathogen Pyrenophora avenae Ito & Kuribay when used at a concentration of 250microM. The bis(benzyl)diamine BBD5 and the hydroxypyridylethylamine HPE2 both reduced powdery mildew infection of barley seedlings by greater than 70% when applied as a post-inoculation spray at 250 microM. Several of the compounds examined, and especially BBD5 and HPE2, reduced the formation of spermidine but greatly increased spermine levels. These changes in P avenae treated with BBD5 and HPE2 were also accompanied by greatly elevated activity of polyamine oxidase. It is suggested that the antifungal activity of these compounds may be related to the accumulation of spermine and specifically to its toxicity.

Ornithine decarboxylase knockout in Tapesia yallundae abolishes infection plaque formation in vitro but does not reduce virulence toward wheat

A knockout strain of Tapesia yallundae lacking the single ornithine decarboxylase (ODC) allele has been created by targeted gene replacement. A central region of the ODC gene was isolated by polymerase chain reaction with degenerate oligonucleotides and used to probe a lambda genomic library. The gene was sequenced, and the encoded ODC protein sequence was shown to be similar to those from other fungi. The functionality of the T. yallundae ODC was confirmed by complementation of an Aspergillus nidulans mutant (puA) strain devoid of ODC activity, restoring growth in the absence of exogenous polyamines. Transformation-mediated gene replacement was used to create strains that were auxotrophic for putrescine and lack ODC coding sequences. ODC knockout strains were unable to differentiate infection structures after in vitro induction and showed an abnormal hyphal branching phenotype. Pathogenicity studies on these mutants showed that, surprisingly, they are not reduced in virulence compared with nondisrupted transformants. This suggests that the strains carrying an ODC disruption can obtain sufficient polyamines from the host plant for normal growth and differentiation and, therefore, that fungal ODC may not be a suitable target for fungicides.

Synthesis of six novel N,N-dialkyl derivatives of spermidine and effects on growth of the fungal plant pathogen Pyrenophora avenae

Six novel N,N-dialkyl derivatives of spermidine were synthesised and examined for activity against the oat stripe pathogen Pyrenophora avenae. Two of these spermidine analogues, N,N-dimethyl-N1-(3-aminopropyl)-1,3-diaminopropane trihydrochloride (27) and N,N-dimethyl-N1-(3-aminopropyl)-1,4-diaminobutane trihydrochloride (28), reduced radial extension of P. avenae on plates when used at 2 mM, and caused more substantial reductions in fungal growth in liquid culture when used at 1 mM. Preliminary data suggest that neither compound affected polyamine biosynthesis, determined by following the incorporation of label from ornithine into polyamines and examining intracellular polyamine concentrations in fungal tissue.

Polyamine metabolism in the thermotolerant mesophilic fungus Aspergillus fumigatus

Biomass production by Aspergillus fumigatus was greatest at 40-45 degrees C and was associated with an increase in concentration of the diamine putrescine and activity of its biosynthetic enzyme ornithine decarboxylase. Concentrations of the other amines, cadaverine, spermidine and spermine were considerably lower than putrescine concentration and did not change significantly over the temperature range 20-50 degrees C. This is surprising in view of the greatly increased flux of label from ornithine through to spermidine at 45 and 50 degrees C, indicating an increased formation of this triamine. It is suggested that there was increased formation of spermidine derivatives at these temperatures. Interestingly, there was greatly increased formation of the higher homologues of cadaverine, aminopropylcadaverine and N,N'-bis(3-aminopropyl)cadaverine, in A. fumigatus at 45 and 50 degrees C.

Synthesis and antifungal activity of two novel spermidine analogues

Two spermidine analogues were synthesised and examined for antifungal activity. Both compounds used as 1 mM post-inoculation sprays reduced infection of barley seedlings by the powdery mildew fungus, Erysiphe graminis f.sp. hordei, infection of broad bean seedlings by the rust fungus, Uromyces viciae-fabae, and infection of apple seedlings by the powdery mildew fungus, Podosphaera leucotricha. Since these fungal pathogens cannot be cultured axenically, the effects of the two spermidine analogues on mycelial growth in vitro, as well as preliminary investigations on polyamine biosynthesis, were undertaken using the oat stripe pathogen, Pyrenophora avenae. Although neither compound affected radial growth of the fungus on plates, both analogues reduced fungal biomass in liquid culture substantially. The two spermidine analogues, used at a concentration of 1 mM, had no significant effect on the conversion of labelled ornithine into polyamines in P. avenae.

Polyamines as modulators of microcycle conidiation in Aspergillus flavus

Since polyamines (PAs) play a potential role in the regulation of growth and developmental processes in a wide variety of organisms, we have examined the influence of the PAs putrescine (Put) and spermidine (Spd) and the PA biosynthetic inhibitors alpha-difluoromethylornithine (DFMO), alpha-difluoromethylarginine (DFMA), methylglyoxal bis-(guanylhydrazone) (MGBG) and cyclohexylamine (CHA), singly and in combinations on microcycle conidiation (MC) in Aspergillus flavus. The exogenous application of the diamine Put (concentrations ranging from 0.1 to 5 mM) caused a sharp decline of MC in a dose-dependent fashion, but induced vegetative growth. However, the triamine Spd (0.1-5 mM) had a minimal effect on MC and induced a shift from MC to normal condition. PA inhibitors, especially DFMO, MGBG and CHA, produced greater inhibition of MC and complete inhibition of MC was observed at 5 mM of these inhibitors. DFMA even at 5 mM had only a weak inhibitory effect on MC. DFMO also inhibited conidial germination and germ tube growth. MGBG and CHA, while having an inhibitory effect on MC, induced vegetative growth. The inhibitory effect of PA inhibitors was partially reversed by exogenous Put or Spd, with Spd being more effective than Put. The analysis of free PA levels during various phases of MC revealed that undifferentiated spores contained a high Put/Spd ratio and there was a dramatic decrease in Put/Spd ratio before and during microcycle conidiophore maturity. The change in spermine titres could not be detected. These observations imply that Put is essential for vegetative growth, while Spd is involved in MC, and that a low Put/Spd ratio seems to be important for spore differentiation to MC.

Polyamines, DNA methylation, and fungal differentiation

Mucorales constitute a group of fungi that, because of their growth characteristics, have been used extensively in the study of cell differentiation, cell morphogenesis, and stimuli perception. We have studied the role of polyamine metabolism in the development of different Mucorales, with emphasis on Mucor and Phycomyces species. It has been observed that previous to each differentiative step, the cellular levels of the most regulated enzyme of the pathway, ornithine decarboxylase (ODC), and polyamines suffer a noticeable increase. Addition of diaminobutanone (DAB), a competitive inhibitor of ODC, blocks all the corresponding differentiative phenomena. In its presence, germinating spores fail to produce germ tubes and keep growing isodiametrically; mycelia do not sporulate but continue their vegetative growth, and yeast cells are unable to engage in a dimorphic transition without alterations in their growth rate. This differential effect of the ODC inhibitor in growth and development is apparently due to the location of ODC in at least two different cell compartments, one of which is impermeable to the drug. Inhibition of development is counteracted by putrescine and more noticeably by 5-azacytidine (5AC), a strong inhibitor of DNA methylation. Methylation levels of DNA are high in spores, and they become reduced after germination. Demethylation is inhibited by hydroxyurea, which blocks DNA replication, and by DAB. The effect of the latter is reversed by 5AC. These results suggest a relationship between polyamines and DNA methylation. Analysis of metallothioneine gene (CUP) behavior and expression during spore germination has confirmed this hypothesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Ornithine decarboxylase activity in Entamoeba invadens

Growth of E. invadens was paralleled by a concomitant increase in ornithine decarboxylase activity which peaked after 5 days of cultivation in TYI-S-33 medium. Over this period, enzyme activity increased about nine-fold with respect to that present at the start of incubation. Thereafter and coinciding with the onset of the stationary growth phase, enzyme activity started to decline reaching trace levels after 8 days of cultivation. Most of the enzyme remained soluble following centrifugation of amoeba homogenates at 105,000 g. alpha-Difluoromethylornithine failed to affect ornithine decarboxylase activity in vitro and amoeba growth. The enzyme was markedly inhibited by polyamines (putrescine, spermidine and spermine) and 1,4-diamino-2-butanone, a putrescine-analog. The latter arrested proliferation of cells, an effect that could not be reversed by polyamines which by themselves also inhibited growth to a low but significant extent. Our results indicate that polyamine biosynthesis from ornithine is required for growth of E. invadens and that this function is rapidly abolished following entry into the stationary growth phase.

Effect of polyamine biosynthesis inhibitors on mycelial growth and concentrations of polyamines in Ophiostoma ulmi (Buism.) Nannf

Several inhibitors of polyamine biosynthesis reduced mycelial growth of the fungus Ophiostoma ulmi (Buism.) Nannf. cultured on malt extract-agar medium. Growth inhibition was particularly evident when expressed in terms of fresh weight rather than colony diameter. Of the different drugs tested, the most effective was difluoromethylarginine (DFMA), while difluoromethylornithine (DFMO) and methylglyoxal(bis)guanyl-hydrazone (MGBG) plus cyclohexylamine (CHA) reduced growth only by 75 and 62% respectively. The growth inhibition by DFMO + DFMA, measured as colony diameter, was apparently reversed by putrescine (PUT), but not when expressed in terms of fresh weight. Spermidine (SPD) was the most abundant polyamine in control cultures, followed by PUT and spermine (SPM). PUT was no longer detectable 8 d after inoculation. On day 10, DFMO and DFMA, alone and in combination, caused a significant reduction in cellular SPD concentrations, while exogenously supplied PUT restored the levels of this polyamine to control values. MGBG + CHA caused a conspicuous accumulation of PUT and an approximately 50% reduction in SPD titres. DFMA, alone or in combination with DFMO and with or without PUT, led to increased cellular levels of SPM. The latter polyamine, but not PUT or SPD, strongly retarded growth when added to the growth medium. As suggested by the effectiveness of DFMA in inhibiting growth, arginine decarboxylase activity was shown to be prevalent over ornithine decarboxylase activity in this fungus.

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.

Regulation of S-adenosylmethionine decarboxylase during the germination of sporangiospores of Mucor rouxii

General properties of S-adenosylmethionine decarboxylase (SAMDC) from Mucor rouxii were studied. Dormant spores of the fungus did not contain detectable levels of the enzyme, but it started to be synthesized at early stages of spore germination. Kinetics of synthesis changed before emergence of the germ tube, with a corresponding increase in a second SAMDC activity which, in contrast to the one originally synthesized, was not activated by putrescine. Development of the second enzyme activity required de novo protein synthesis. Neither enzymic activity was stimulated by Mg2+. Addition of the SAMDC inhibitor methylglyoxal bis-(guanylhydrazone) (MGBG) stopped fungal development in growth phase Ia: cells became spherical and showed ultrastructural alterations. Although MGBG inhibited polyamine formation, it barely inhibited protein and RNA biosynthesis during the first hour of incubation. However, at later periods, biosynthesis of both macromolecules was strongly decreased. When MGBG was added to growth media 3 h after inoculation of spores, it did not affect spore germination and outgrowth. A hypothesis for two different roles of spermidine and putrescine in spore germination is discussed.

Polyamine depletion and growth inhibition of Cryptococcus neoformans by alpha-difluoromethylornithine and cyclohexylamine

The ability of two known inhibitors of polyamine synthesis alpha-difluoromethylornithine (DFMO), an inhibitor of ornithine decarboxylase (ODC), and cyclohexylamine, an inhibitor of spermidine synthase, to inhibit the in vitro growth and polyamine synthesis of clinical isolates of Cryptococcus neoformans was examined. Treatment of C. neoformans with either DFMO or cyclohexylamine resulted in depletion of cellular polyamines and inhibition of growth. Cryptococcus neoformans was shown to lack detectable spermine and to require high concentrations of spermidine, but not putrescine, for growth. The growth inhibition by DFMO and cyclohexylamine was reversed by exogenous polyamines. These findings document the ability of cyclohexylamine and DFMO to inhibit polyamine synthesis and growth in clinically important isolates of C. neoformans.

Biosynthetic arginine decarboxylase in phytopathogenic fungi

It has been reported that while bacteria and higher plants possess two different pathways for the biosynthesis of putrescine, via ornithine decarboxylase (ODC) and arginine decarboxylase (ADC); the fungi, like animals, only use the former pathway. We found that contrary to the earlier reports, two of the phytopathogenic fungi (Ceratocystis minor and Verticillium dahliae) contain significant levels of ADC activity with very little ODC. The ADC in these fungi has high pH optimum (8.4) and low Km (0.237 mM for C. minor, 0.103 mM for V. dahliae), and is strongly inhibited by alpha-difluoromethylarginine (DFMA), putrescine and spermidine, further showing that this enzyme is probably involved in the biosynthesis of polyamines and not in the catabolism of arginine as in Escherichia coli. The growth of these fungi is strongly inhibited by DFMA while alpha-difluoromethylornithine (DFMO) has little effect.

Growth inhibition of pathogenic yeast isolates by alpha-difluoromethylornithine: an inhibition of ornithine decarboxylase

A large body of evidence exists suggesting that polyamines can play essential roles in cellular growth and differentiation. We examined the ability of alpha-difluoromethylornithine (DFMO), an irreversible inhibitor of ornithine decarboxylase, the major rate-limiting enzyme in polyamine biosynthesis, to inhibit the growth of Candida albicans, C. tropicalis, and C. parapsilosis. Substantial growth-inhibition was observed for all three species at DFMO concentrations ranging from 1 to 100 mM. C. tropicalis was significantly more susceptible to DFMO than C. albicans or C. parapsilosis. Depletion of cellular polyamine pools was seen in all 3 species following exposure to DFMO and polyamine depletion enhanced the susceptibility of the organisms to DFMO. The action of DFMO was specifically antagonized by exogenous polyamines. These data suggest that polyamines are important in the growth of Candida spp. and that inhibitors of polyamine biosynthesis may be useful as antifungal agents.

THE EFFECTS OF A POLYAMINE BIOSYNTHESIS INHIBITOR ON INFECTION OF VICIA FABA L. BY THE RUST FUNGUS, UROMYCES VICIAE-FABAE (PERS.) SCHROET

The effects of the polyamine biosynthesis inhibitor, difluoromethylomithine (DFMO), on rust infection of the broad bean have been examined. DFMO was sprayed onto leaves at 0.025, 0.1 and 0.4 mM before and after inoculation. DFMO was also applied to leaf halves in order to examine the possible translocation of the inhibitor to untreated leaf halves (systemic treatment). DFMO gave good control of rust infection, irrespective of the treatment. DFMO applied after inoculation was more effective than a pre-inoculatory treatment when used in non-systemic tests. When used as a systemic treatment on leaf halves, pre-inoculatory application of DFMO was more effective. DFMO did not affect plant growth or endogenous concentrations of poly amines. The possible use of DFMO in the control of a range of plant diseases is discussed.

Prevention of a plant disease by specific inhibition of fungal polyamine biosynthesis

DL-alpha-Difluoromethylornithine (DFMO), an inhibitor of the polyamine biosynthetic enzyme ornithine decarboxylase (EC 4.1.1.17), strongly retards the growth of several species of phytopathogenic fungi in vitro. Such inhibition can be completely reversed by putrescine or spermidine, confirming the essentiality of polyamines for growth of fungal hyphae. We now show that DFMO can protect bean plants (Phaseolus vulgaris Linnaeus cv. Pinto) against infection by uredospores of the bean rust fungus, Uromyces phaseoli Linnaeus, race O. Unifoliolate leaves of 10-day-old greenhouse-grown seedlings were sprayed with 400 microliter per leaf of DFMO at various concentrations in 0.01% Tween 20 at pH 7.0 before or after inoculation with uredospores of Uromyces. After 16 hr in darkness in dew chambers to facilitate spore germination, plants were transferred to the greenhouse, arranged randomly, and examined for local lesions 7 days later. All concentrations of DFMO 0.50 mM or higher gave complete protection against the pathogen; at lower concentrations, postinoculation treatments with DFMO were generally more effective than preinoculation. The appearance of lesions on plants treated with lower concentrations of DFMO was retarded 2-6 days. DFMO also confers protection on unsprayed parts of treated plants, indicating the translocation of some protective effect from sprayed areas. DL-alpha-Difluoromethylarginine, an analogous inhibitor of arginine decarboxylase (EC 4.1.1.19), which is the rate-limiting enzyme in an alternative pathway for polyamine biosynthesis in higher plants, confers no protection even at 5 mM. This emphasizes ornithine decarboxylase as the biochemical locus of choice for the prevention of plant diseases by inhibiting polyamine metabolism.

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

We wished to identify metabolic signals governing changes in ornithine decarboxylase (L-ornithine carboxy-lyase, EC 4.1.1.17) activity in Neurospora crassa. By manipulations of the ornithine supply and by the use of inhibitors of the polyamine pathway, we found that spermidine negatively governs formation of active ornithine decarboxylase and that putrescine promotes inactivation of the enzyme. Direct addition of putrescine or spermidine to cycloheximide-treated cells confirmed the role of putrescine in enzyme inactivation and showed that spermidine had no effect on this process. Increases in ornithine decarboxylase activity caused by blocking spermidine synthesis occurred prior to a significant decrease in the spermidine pool. This is consistent with our previous finding that only 10-20% of the spermidine pool is freely diffusible within N. crassa cells. We presume that only this small fraction of the pool is active in regulation.

Distribution of polyamines in methanogenic bacteria

Members of all four families of methanogenic bacteria were analyzed for polyamine concentrations. High-performance liquid chromatography analysis of dansylated cell extracts revealed typical polyamine patterns for each family. Members of Methanobacteriaceae (family I) were characterized by very low polyamine concentrations; members of Methanococcaceae (family II) were characterized by putrescine and high spermidine concentrations; members of Methanomicrobiaceae (family III) were characterized by the presence of putrescine, spermidine, and sym-homospermidine; and members of Methanosarcinaceae (family IV) contained only high concentrations of sym-homospermidine in addition to putrescine. The highest polyamine concentration was found in Methanosarcina barkeri Jülich, with 0.35% putrescine in the dry cell material. The polyamine distribution found coincides with the dendrogram based on comparative cataloguing of 16S rRNA and offers a new, rapid chemotaxonomic method for characterizing methanogenic bacteria. Variation of the growth substrates (H2-CO2, methanol, acetate, and trimethylamine) for M. barkeri resulted in quantitative but not qualitative differences in polyamine composition.

Polyamine metabolism: a potential therapeutic target in trypanosomes

alpha-Difluoromethylornithine (RMI 71,782), a specific irreversible inhibitor of the first step in polyamine biosynthesis, that is, the formation of putrescine from ornithine by ornithine decarboxylase, cures mice infected with a virulent, rodent-passaged strain of Trypanosoma brucei brucei. This parasite is closely related to the trypanosomes that cause human sleeping sickness. The drug, which is remarkably nontoxic, was effective when administered in drinking water or by intubation. The ability of the compound to inhibit ornithine decarboxylase in vitro was demonstrated by the reduced amounts of putrescine synthesized from tritiated ornithine in Trypanosoma brucei suspensions. These observations direct attention to polyamine metabolism as a target for chemotherapy of parasitic diseases.