Deoxyhypusine Modification of Eukaryotic Translation Initiation Factor 5A (eIF5A) Is Essential for Trypanosoma brucei Growth and for Expression of Polyprolyl-containing Proteins

Domain function and predicted structure of three heterodimeric endonuclease subunits of RNA editing catalytic complexes in Trypanosoma brucei

Each of the three similar RNA Editing Catalytic Complexes (RECCs) that perform gRNA-directed uridine insertion and deletion during Trypanosoma brucei mitochondrial (mt) mRNA editing has a distinct endonuclease activity that requires two related RNase III proteins, with only one competent for catalysis. We identified multiple loss-of-function mutations in the RNase III and other motifs of the non-catalytic KREPB6, KREPB7, and KREPB8 components by random mutagenesis and screening. These mutations had various effects on growth, editing, and both the abundances and RECC associations of these RNase III protein pairs in bloodstream form (BF) and procyclic form (PF) cells. Protein structure modelling predicted that the Zinc Finger (ZnF) of each paired RNase III protein contacts RNA positioned at the heterodimeric active site which is flanked by helices of a novel RNase III-Associated Motif (RAM). The results indicate that the protein domains of the non-catalytic subunits function together in RECC integrity, substrate binding, and editing site recognition during the multistep RNA editing process. Additionally, several mutants display distinct functional consequences in different life cycle stages. These results highlight the complementary roles of protein pairs and three RECCs within the complicated T. brucei mRNA editing machinery that matures mt mRNAs differentially between developmental stages.

Role of eIF5A in Mitochondrial Function

The eukaryotic translation initiation factor 5A (eIF5A) is an evolutionarily conserved protein that binds ribosomes to facilitate the translation of peptide motifs with consecutive prolines or combinations of prolines with glycine and charged amino acids. It has also been linked to other molecular functions and cellular processes, such as nuclear mRNA export and mRNA decay, proliferation, differentiation, autophagy, and apoptosis. The growing interest in eIF5A relates to its association with the pathogenesis of several diseases, including cancer, viral infection, and diabetes. It has also been proposed as an anti-aging factor: its levels decay in aged cells, whereas increasing levels of active eIF5A result in the rejuvenation of the immune and vascular systems and improved brain cognition. Recent data have linked the role of eIF5A in some pathologies with its function in maintaining healthy mitochondria. The eukaryotic translation initiation factor 5A is upregulated under respiratory metabolism and its deficiency reduces oxygen consumption, ATP production, and the levels of several mitochondrial metabolic enzymes, as well as altering mitochondria dynamics. However, although all the accumulated data strongly link eIF5A to mitochondrial function, the precise molecular role and mechanisms involved are still unknown. In this review, we discuss the findings linking eIF5A and mitochondria, speculate about its role in regulating mitochondrial homeostasis, and highlight its potential as a target in diseases related to energy metabolism.

The eukaryotic initiation factor 5A (eIF5A1), the molecule, mechanisms and recent insights into the pathophysiological roles

Since the demonstration of its involvement in cell proliferation, the eukaryotic initiation factor 5A (eIF5A) has been studied principally in relation to the development and progression of cancers in which the isoform A2 is mainly expressed. However, an increasing number of studies report that the isoform A1, which is ubiquitously expressed in normal cells, exhibits novel molecular features that reveal its new relationships between cellular functions and organ homeostasis. At a first glance, eIF5A can be regarded, among other things, as a factor implicated in the initiation of translation. Nevertheless, at least three specificities: (1) its extreme conservation between species, including plants, throughout evolution, (2) its very special and unique post-translational modification through the activating-hypusination process, and finally (3) its close relationship with the polyamine pathway, suggest that the role of eIF5A in living beings remains to be uncovered. In fact, and beyond its involvement in facilitating the translation of proteins containing polyproline residues, eIF5A is implicated in various physiological processes including ischemic tolerance, metabolic adaptation, aging, development, and immune cell differentiation. These newly discovered physiological properties open up huge opportunities in the clinic for pathologies such as, for example, the ones in which the oxygen supply is disrupted. In this latter case, organ transplantation, myocardial infarction or stroke are concerned, and the current literature defines eIF5A as a new drug target with a high level of potential benefit for patients with these diseases or injuries. Moreover, the recent use of genomic and transcriptomic association along with metadata studies also revealed the implication of eIF5A in genetic diseases. Thus, this review provides an overview of eIF5A from its molecular mechanism of action to its physiological roles and the clinical possibilities that have been recently reported in the literature.

Proteomics analysis reveals that the proto-oncogene eIF-5A indirectly influences the growth, invasion and replication of Toxoplasma gondii tachyzoite

BACKGROUND

The proliferative stage (tachyzoite) of Toxoplasma gondii (T. gondii) is critical for its transmission and pathogenesis, and a proto-oncogene eukaryotic translation initiation factor (eIF-5A) plays an important role in various cellular processes such as cell multiplication.

METHODS

We performed a proteomic study to evaluate the specific roles of eIF-5A involved in invasion and replication of T. gondii, and both in vivo and in vitro trials using eIF-5A-interfered and wild tachyzoites were performed to verify the proteomic results.

RESULTS

The results of our study showed that T. gondii eIF-5A affected tachyzoite growth and also participated in the synthesis of proteins through regulation of both ribosomal and splicing pathways. Inhibition of eIF-5A in T. gondii resulted in the downregulated expression of soluble adhesions, such as microneme protein 1 (MIC1) and MIC4, which in turn decreased the parasite population that adhered to the surface of host cells. The reduced attachment, combined with lower expression of some rhoptry proteins (ROPs) and dense granule antigens (GRAs) involved in different stages of T. gondii invasion such as ROP4 and GRA3, ultimately reduce the invasion efficiency. These processes regulated by eIF-5A eventually affect the replication of tachyzoites.

CONCLUSIONS

Our findings showed that eIF-5A influenced tachyzoite survival and was also involved in the process of parasite invasion and replication. These results will provide new clues for further development of targeted drugs to control T. gondii infection.

Structural features and development of an assay platform of the parasite target deoxyhypusine synthase of Brugia malayi and Leishmania major

Deoxyhypusine synthase (DHS) catalyzes the first step of the post-translational modification of eukaryotic translation factor 5A (eIF5A), which is the only known protein containing the amino acid hypusine. Both proteins are essential for eukaryotic cell viability, and DHS has been suggested as a good candidate target for small molecule-based therapies against eukaryotic pathogens. In this work, we focused on the DHS enzymes from Brugia malayi and Leishmania major, the causative agents of lymphatic filariasis and cutaneous leishmaniasis, respectively. To enable B. malayi (Bm)DHS for future target-based drug discovery programs, we determined its crystal structure bound to cofactor NAD⁺. We also reported an in vitro biochemical assay for this enzyme that is amenable to a high-throughput screening format. The L. major genome encodes two DHS paralogs, and attempts to produce them recombinantly in bacterial cells were not successful. Nevertheless, we showed that ectopic expression of both LmDHS paralogs can rescue yeast cells lacking the endogenous DHS-encoding gene (dys1). Thus, functionally complemented dys1Δ yeast mutants can be used to screen for new inhibitors of the L. major enzyme. We used the known human DHS inhibitor GC7 to validate both in vitro and yeast-based DHS assays. Our results show that BmDHS is a homotetrameric enzyme that shares many features with its human homologue, whereas LmDHS paralogs are likely to form a heterotetrameric complex and have a distinct regulatory mechanism. We expect our work to facilitate the identification and development of new DHS inhibitors that can be used to validate these enzymes as vulnerable targets for therapeutic interventions against B. malayi and L. major infections.

Target-based drug discovery strategies hold the promise to discover safer and more effective treatments for Neglected Tropical Diseases (NTDs). Genetic manipulation techniques have been used to successfully identify essential genes in eukaryotic parasites. Unfortunately, the fact that a gene is essential under controlled laboratory conditions does not automatically make the corresponding gene-product vulnerable to pharmacological intervention in a clinical setting within the human host. To allow the discovery and development of small molecule tool compounds that can be used to validate pharmacologically vulnerable targets, one must first establish compound screening assays and obtain structural information for the candidate target. Eukaryotic cells lacking deoxyhypusine synthase (DHS) function are not viable. DHS catalyzes the first step in a post-translational modification that is critical for the function of eIF5A. Presence of mature eIF5A is also essential for eukaryotic cell viability. Here we reported compound screening assays (yeast-based for Brugia malayi and Leishmania major; in vitro for B. malayi only) and provided further regulatory and structural insights we hope will aid in the identification and development of inhibitors for the DHS enzymes from two NTD-causing organisms—B. malayi, the causative agent of lymphatic filariasis and L. major, the causative agent of cutaneous leishmaniasis.

Klf5 down-regulation induces vascular senescence through eIF5a depletion and mitochondrial fission

Although dysregulation of mitochondrial dynamics has been linked to cellular senescence, which contributes to advanced age-related disorders, it is unclear how Krüppel-like factor 5 (Klf5), an essential transcriptional factor of cardiovascular remodeling, mediates the link between mitochondrial dynamics and vascular smooth muscle cell (VSMC) senescence. Here, we show that Klf5 down-regulation in VSMCs is correlated with rupture of abdominal aortic aneurysm (AAA), an age-related vascular disease. Mice lacking Klf5 in VSMCs exacerbate vascular senescence and progression of angiotensin II (Ang II)-induced AAA by facilitating reactive oxygen species (ROS) formation. Klf5 knockdown enhances, while Klf5 overexpression suppresses mitochondrial fission. Mechanistically, Klf5 activates eukaryotic translation initiation factor 5a (eIF5a) transcription through binding to the promoter of eIF5a, which in turn preserves mitochondrial integrity by interacting with mitofusin 1 (Mfn1). Accordingly, decreased expression of eIF5a elicited by Klf5 down-regulation leads to mitochondrial fission and excessive ROS production. Inhibition of mitochondrial fission decreases ROS production and VSMC senescence. Our studies provide a potential therapeutic target for age-related vascular disorders.

Editosome RNase III domain interactions are essential for editing and differ between life cycle stages in Trypanosoma brucei

Multiprotein editosomes catalyze gRNA-specified insertion and deletion of uridines to create functional mitochondrial mRNAs in Trypanosoma brucei Three functionally distinct editosomes are distinguished by their single KREN1, KREN2, or KREN3 RNase III endonuclease and, respectively, KREPB8, KREPB7, and KREPB6 partner proteins. These endonucleases perform the first catalytic step of editing, cleaving mRNA in diverse mRNA/gRNA heteroduplex substrates. We identified divergent and likely noncatalytic RNase III domains in KREPB4, KREPB5, KREPB6, KREPB7, KREPB8, KREPB9, and KREPB10 editosome proteins. Because known RNase III endonuclease functional domains are dimeric, the editing endonucleases may form heterodimers with one or more of these divergent RNase III proteins. We show here using conditional null cell lines that KREPB6, KREPB7, and KREPB8 are essential in both procyclic form (PF) and bloodstream (BF) cells. Loss of these proteins results in growth defects and loss of editing in vivo, as does mutation of their RNase III domain that is predicted to prevent dimerization. Loss of KREPB6, KREPB7, or KREPB8 also dramatically reduces cognate endonuclease abundance, as does the RNase III mutation, indicating that RNase III interactions with their partner proteins stabilize the endonucleases. The phenotypic consequences of repression are more severe in BF than in PF, indicating differences in endonuclease function between developmental stages that could impact regulation of editing. These results suggest that KREPB6, KREPB7, and KREPB8 form heterodimers with their respective endonucleases to perform mRNA cleavage. We also present a model wherein editosome proteins with divergent RNase III domains function in substrate selection via enzyme-pseudoenzyme interactions.

A dual regulatory circuit consisting of S-adenosylmethionine decarboxylase protein and its reaction product controls expression of the paralogous activator prozyme in Trypanosoma brucei

Polyamines are essential for cell growth of eukaryotes including the etiologic agent of human African trypanosomiasis (HAT), Trypanosoma brucei. In trypanosomatids, a key enzyme in the polyamine biosynthetic pathway, S-adenosylmethionine decarboxylase (TbAdoMetDC) heterodimerizes with a unique catalytically-dead paralog called prozyme to form the active enzyme complex. In higher eukaryotes, polyamine metabolism is subject to tight feedback regulation by spermidine-dependent mechanisms that are absent in trypanosomatids. Instead, in T. brucei an alternative regulatory strategy based on TbAdoMetDC prozyme has evolved. We previously demonstrated that prozyme protein levels increase in response to loss of TbAdoMetDC activity. Herein, we show that prozyme levels are under translational control by monitoring incorporation of deuterated leucine into nascent prozyme protein. We furthermore identify pathway factors that regulate prozyme mRNA translation. We find evidence for a regulatory feedback mechanism in which TbAdoMetDC protein and decarboxylated AdoMet (dcAdoMet) act as suppressors of prozyme translation. In TbAdoMetDC null cells expressing the human AdoMetDC enzyme, prozyme levels are constitutively upregulated. Wild-type prozyme levels are restored by complementation with either TbAdoMetDC or an active site mutant, suggesting that TbAdoMetDC possesses an enzyme activity-independent function that inhibits prozyme translation. Depletion of dcAdoMet pools by three independent strategies: inhibition/knockdown of TbAdoMetDC, knockdown of AdoMet synthase, or methionine starvation, each cause prozyme upregulation, providing independent evidence that dcAdoMet functions as a metabolic signal for regulation of the polyamine pathway in T. brucei. These findings highlight a potential regulatory paradigm employing enzymes and pseudoenzymes that may have broad implications in biology.

Trypanosoma brucei is a single-celled eukaryotic pathogen and the causative agent of human African trypanosomiasis (HAT). Polyamines are organic polycations that are essential for growth in T. brucei to facilitate protein translation and to maintain redox homeostasis. The pathway is the target of eflornithine, a current frontline therapy for treatment of HAT. Polyamine biosynthetic enzymes are regulated at multiple levels in mammals (e.g. transcription, translation and protein turnover), but in contrast, T. brucei lacks these mechanisms. Instead in T. brucei a central enzyme in polyamine metabolism called AdoMetDC must form a complex with a sister protein (termed a pseudoenzyme) to be active. Herein, we show that cellular levels of this sister protein we call prozyme are in turn feedback regulated by both AdoMetDC and by its reaction product in response to cell treatments that reduce pathway output. This regulatory paradigm highlights how pseudoenzymes can evolve to play an important role in metabolic pathway regulation and in organismal fitness.

Polyamines in protozoan pathogens

Polyamines are polycationic organic amines that are required for all eukaryotic life, exemplified by the polyamine spermidine, which plays an essential role in translation. They also play more specialized roles that differ across species, and their chemical versatility has been fully exploited during the evolution of protozoan pathogens. These eukaryotic pathogens, which cause some of the most globally widespread infectious diseases, have acquired species-specific polyamine-derived metabolites with essential cellular functions and have evolved unique mechanisms that regulate their core polyamine biosynthetic pathways. Many of these parasitic species have lost enzymes and or transporters from the polyamine metabolic pathway that are found in the human host. These pathway differences have prompted drug discovery efforts to target the parasite polyamine pathways, and indeed, the only clinically approved drug targeting the polyamine biosynthetic pathway is used to manage human African trypanosomiasis. This Minireview will primarily focus on polyamine metabolism and function in Trypanosoma, Leishmania, and Plasmodium species, which are the causative agents of human African trypanosomiasis (HAT) and Chagas disease, Leishmaniasis, and malaria, respectively. Aspects of polyamine metabolism across a diverse group of protozoan pathogens will also be explored.

Hypusine, a polyamine-derived amino acid critical for eukaryotic translation

The natural amino acid hypusine (N ^ϵ-4-amino-2-hydroxybutyl(lysine)) is derived from the polyamine spermidine, and occurs only in a single family of cellular proteins, eukaryotic translation factor 5A (eIF5A) isoforms. Hypusine is formed by conjugation of the aminobutyl moiety of spermidine to a specific lysine residue of this protein. The posttranslational synthesis of hypusine involves two enzymatic steps, catalyzed by deoxyhypusine synthase (DHPS) and deoxyhypusine hydroxylase (DOHH). Hypusine is essential for eIF5A activity. Inactivation of either the eIF5A or the DHPS gene is lethal in yeast and mouse, underscoring the vital role of eIF5A hypusination in eukaryotic cell growth and animal development. The long and basic side chain of the hypusine residue promotes eIF5A-mediated translation elongation by facilitating peptide bond formation at polyproline stretches and at many other ribosome-pausing sites. It also enhances translation termination by stimulating peptide release. By promoting translation, the hypusine modification of eIF5A provides a key link between polyamines and cell growth regulation. eIF5A has been implicated in several human pathological conditions. Recent genetic data suggest that eIF5A haploinsufficiency or impaired deoxyhypusine synthase activity is associated with neurodevelopmental disorders in humans.

Trypanosomatid Deoxyhypusine Synthase Activity Is Dependent on Shared Active-Site Complementation between Pseudoenzyme Paralogs

Trypanosoma brucei is a neglected tropical disease endemic to Africa. The polyamine spermidine is essential for post-translational hypusine modification of eukaryotic initiation factor 5A (eIF5A), which is catalyzed by deoxyhypusine synthase (TbDHS). In trypanosomatids, deoxyhypusine synthase (DHS) activity is dependent on heterotetramer formation between two paralogs, DHSc and DHSp, both with minimal activity on their own due to missing catalytic residues. We determined the X-ray structure of TbDHS showing a single functional shared active site is formed at the DHSc/DHSp heterodimer interface, with deficiencies in one subunit complemented by the other. Each heterodimer contains two NAD⁺ binding sites, one housed in the functional catalytic site and the second bound in a remnant dead site that lacks key catalytic residues. Functional analysis of these sites by site-directed mutagenesis identified long-range contributions to the catalytic site from the dead site. Differences between trypanosomatid and human DHS that could be exploited for drug discovery were identified.

Translation Factor eIF5A, Modification with Hypusine and Role in Regulation of Gene Expression. eIF5A as a Target for Pharmacological Interventions

Translation factor eIF5A participates in protein synthesis at the stage of polypeptide chain elongation. Two eIF5A isoforms are known that are encoded by related genes whose expression varies significantly in different tissues. The eIF5A1 isoform is a constitutively and ubiquitously expressed gene, while the eIF5A2 isoform is expressed in few normal tissues and is an oncogene by a number of parameters. Unique feature of eIF5A isoforms is that they are the only two proteins that contain amino acid hypusine. Modification with hypusine is critical requirement for eIF5A activity. Another distinctive feature of eIF5A is its involvement in the translation of only a subset of the total population of cell mRNAs. The genes for which mRNAs translation requires eIF5A are the members of certain functional groups and are involved in cell proliferation, apoptosis, inflammatory processes, and regulation of transcription and RNA metabolism. The involvement of eIF5A is necessary for the translation of proteins containing oligoproline fragments and some other structures. Modification of eIF5A by hypusine is implemented by two highly specialized enzymes, deoxyhypusine synthase (DHS) and deoxyhypusine hydroxylase (DOHH), which are not involved in other biochemical reactions. Intracellular activity of these enzymes is closely associated with systems of protein acetylation, polyamine metabolism and other biochemical processes. Inhibition of DHS and DOHH activity provides the possibility of pharmacological control of eIF5A activity and expression of eIF5A-dependent genes.

Overexpression of eukaryotic initiation factor 5A (eIF5A) affects susceptibility to benznidazole in Trypanosoma cruzi populations

Eukaryotic initiation factor 5A (eIF5A) is a conserved protein with an essential role in translation elongation. Using one and two-dimensional western blotting, we showed that the eIF5A protein level was 2-fold lower in benznidazole (BZ)-resistant (BZR and 17LER) Trypanosoma cruzi populations than in their respective susceptible counterparts (BZS and 17WTS). To confirm the role of eIF5A in BZ resistance, we transfected BZS and 17WTS with the wild-type eIF5A or mutant eIF5A-S2A (in which serine 2 was replaced by alanine). Upon overexpressing eIF5A, both susceptible lines became approximately 3- and 5-fold more sensitive to BZ. In contrast, the eIF5A-S2A mutant did not alter its susceptibility to BZ. These data suggest that BZ resistance might arise from either decreasing the translation of proteins that require eIF5A, or as a consequence of differential levels of precursors for the hypusination reactions (e.g., spermidine and trypanothione), both of which alter BZ's effects in the parasite.

Evolutionary conserved role of eukaryotic translation factor eIF5A in the regulation of actin-nucleating formins

Elongation factor eIF5A is required for the translation of consecutive prolines, and was shown in yeast to translate polyproline-containing Bni1, an actin-nucleating formin required for polarized growth during mating. Here we show that Drosophila eIF5A can functionally replace yeast eIF5A and is required for actin-rich cable assembly during embryonic dorsal closure (DC). Furthermore, Diaphanous, the formin involved in actin dynamics during DC, is regulated by and mediates eIF5A effects. Finally, eIF5A controls cell migration and regulates Diaphanous levels also in mammalian cells. Our results uncover an evolutionary conserved role of eIF5A regulating cytoskeleton-dependent processes through translation of formins in eukaryotes.

Ornithine uptake and the modulation of drug sensitivity in Trypanosoma brucei

Trypanosoma brucei, protozoan parasites that cause human African trypanosomiasis (HAT), depend on ornithine uptake and metabolism by ornithine decarboxylase (ODC) for survival. Indeed, ODC is the target of the WHO “essential medicine” eflornithine, which is antagonistic to another anti-HAT drug, suramin. Thus, ornithine uptake has important consequences in T. brucei, but the transporters have not been identified. We describe these amino acid transporters (AATs). In a heterologous expression system, TbAAT10-1 is selective for ornithine, whereas TbAAT2-4 transports both ornithine and histidine. These AATs are also necessary to maintain intracellular ornithine and polyamine levels in T. brucei, thereby decreasing sensitivity to eflornithine and increasing sensitivity to suramin. Consistent with competition for histidine, high extracellular concentrations of this amino acid phenocopied a TbAAT2-4 genetic defect. Our findings established TbAAT10-1 and TbAAT2-4 as the parasite ornithine transporters, one of which can be modulated by histidine, but both of which affect sensitivity to important anti-HAT drugs.—Macedo, J. P., Currier, R. B., Wirdnam, C., Horn, D., Alsford, S., Rentsch, D. Ornithine uptake and the modulation of drug sensitivity in Trypanosoma brucei.

The Major Protein Arginine Methyltransferase in Trypanosoma brucei Functions as an Enzyme-Prozyme Complex

Prozymes are catalytically inactive enzyme paralogs that dramatically stimulate the function of weakly active enzymes through complex formation. The two prozymes described to date reside in the polyamine biosynthesis pathway of the human parasite Trypanosoma brucei, an early branching eukaryote that lacks transcriptional regulation and regulates its proteome through posttranscriptional and posttranslational means. Arginine methylation is a common posttranslational modification in eukaryotes catalyzed by protein arginine methyltransferases (PRMTs) that are typically thought to function as homodimers. We demonstrate that a major T. brucei PRMT, TbPRMT1, functions as a heterotetrameric enzyme-prozyme pair. The inactive PRMT paralog, TbPRMT1^PRO, is essential for catalytic activity of the TbPRMT1^ENZ subunit. Mutational analysis definitively demonstrates that TbPRMT1^ENZ is the cofactor-binding subunit and carries all catalytic activity of the complex. Our results are the first demonstration of an obligate heteromeric PRMT, and they suggest that enzyme-prozyme organization is expanded in trypanosomes as a posttranslational means of enzyme regulation.

Biosynthesis of polyamines and polyamine-containing molecules

Polyamines are evolutionarily ancient polycations derived from amino acids and are pervasive in all domains of life. They are essential for cell growth and proliferation in eukaryotes and are essential, important or dispensable for growth in bacteria. Polyamines present a useful scaffold to attach other moieties to, and are often incorporated into specialized metabolism. Life has evolved multiple pathways to synthesize polyamines, and structural variants of polyamines have evolved in bacteria, archaea and eukaryotes. Among the complex biosynthetic diversity, patterns of evolutionary reiteration can be distinguished, revealing evolutionary recycling of particular protein folds and enzyme chassis. The same enzyme activities have evolved from multiple protein folds, suggesting an inevitability of evolution of polyamine biosynthesis. This review discusses the different biosynthetic strategies used in life to produce diamines, triamines, tetra-amines and branched and long-chain polyamines. It also discusses the enzymes that incorporate polyamines into specialized metabolites and attempts to place polyamine biosynthesis in an evolutionary context.

The Essential Role of Spermidine in Growth of Agrobacterium tumefaciens Is Determined by the 1,3-Diaminopropane Moiety

The ubiquitous polyamine spermidine is indispensable for eukaryotic growth and cell proliferation. A conserved vital function of spermidine across eukaryotes is conferred by its aminobutyl group that is transferred to a single lysine in translation factor eIF5A to form the essential hypusine post-translational modification required for cellular translation. In direct contrast, although spermidine is absolutely essential for growth of α-proteobacterial plant pathogen Agrobacterium tumefaciens, we have found, by employing a suite of natural polyamines and synthetic methylated spermidine analogues together with spermidine biosynthetic mutants, that it is solely the 1,3-diaminopropane moiety of spermidine that is required for growth. Indeed, any polyamine containing an intact terminal 1,3-diaminopropane moiety can replace spermidine for growth, including the simple diamine 1,3-diaminopropane itself, a paradigm shift in understanding polyamine function in bacteria. We have identified for the first time a spermidine retroconversion activity in bacteria, producing diamine putrescine from triamine spermidine; however, exogenously supplied tetraamine spermine is resistant to retroconversion. When spermidine levels are pharmacologically decreased, synthesis of spermine from spermidine is induced via the same biosynthetic enzymes, carboxyspermidine dehydrogenase and carboxyspermidine decarboxylase that produce spermidine from putrescine, the first identification of a spermine biosynthetic pathway in bacteria. This also suggests that spermidine represses spermine biosynthesis, but when spermidine levels decrease, it is then converted by carboxyspermidine dehydrogenase and decarboxylase enzymes to spermine, which is resistant to retroconversion and constitutes a sequestered pool of protected 1,3-diaminopropane modules required for growth. We also identify an efficient N-acetylspermidine deacetylase activity, indicative of a sophisticated bacterial polyamine homeostasis system.