Diamine Fungal Inducers of Secondary Metabolism: 1,3-Diaminopropane and Spermidine Trigger Enzymes Involved in β-Alanine and Pantothenic Acid Biosynthesis, Precursors of Phosphopantetheine in the Activation of Multidomain Enzymes

The biosynthesis of antibiotics and other secondary metabolites (also named special metabolites) is regulated by multiple regulatory networks and cascades that act by binding transcriptional factors to the promoter regions of different biosynthetic gene clusters. The binding affinity of transcriptional factors is frequently modulated by their interaction with specific ligand molecules. In the last decades, it was found that the biosynthesis of penicillin is induced by two different molecules, 1,3-diaminopropane and spermidine, but not by putrescine (1,4-diaminobutane) or spermine. 1,3-diaminopropane and spermidine induce the expression of penicillin biosynthetic genes in Penicillium chrysogenum. Proteomic studies clearly identified two different proteins that respond to the addition to cultures of these inducers and are involved in β-alanine and pantothenic acid biosynthesis. These compounds are intermediates in the biosynthesis of phosphopantetheine that is required for the activation of non-ribosomal peptide synthetases, polyketide synthases, and fatty acid synthases. These large-size multidomain enzymes are inactive in the "apo" form and are activated by covalent addition of the phosphopantetheine prosthetic group by phosphopantetheinyl transferases. Both 1,3-diaminopropane and spermidine have a similar effect on the biosynthesis of cephalosporin by Acremonium chrysogenum and lovastatin by Aspergillus terreus, suggesting that this is a common regulatory mechanism in the biosynthesis of bioactive secondary metabolites/natural products.

Involvement of LaeA and Velvet Proteins in Regulating the Production of Mycotoxins and Other Fungal Secondary Metabolites

Fungi are rich sources of secondary metabolites of agrochemical, pharmaceutical, and food importance, such as mycotoxins, antibiotics, and antitumor agents. Secondary metabolites play vital roles in fungal pathogenesis, growth and development, oxidative status modulation, and adaptation/resistance to various environmental stresses. LaeA contains an S-adenosylmethionine binding site and displays methyltransferase activity. The members of velvet proteins include VeA, VelB, VelC, VelD and VosA for each member with a velvet domain. LaeA and velvet proteins can form multimeric complexes such as VosA-VelB and VelB-VeA-LaeA. They belong to global regulators and are mainly impacted by light. One of their most important functions is to regulate gene expressions that are responsible for secondary metabolite biosynthesis. The aim of this mini-review is to represent the newest cognition of the biosynthetic regulation of mycotoxins and other fungal secondary metabolites by LaeA and velvet proteins. In most cases, LaeA and velvet proteins positively regulate production of fungal secondary metabolites. The regulated fungal species mainly belong to the toxigenic fungi from the genera of Alternaria, Aspergillus, Botrytis, Fusarium, Magnaporthe, Monascus, and Penicillium for the production of mycotoxins. We can control secondary metabolite production to inhibit the production of harmful mycotoxins while promoting the production of useful metabolites by global regulation of LaeA and velvet proteins in fungi. Furthermore, the regulation by LaeA and velvet proteins should be a practical strategy in activating silent biosynthetic gene clusters (BGCs) in fungi to obtain previously undiscovered metabolites.

New Biocides Based on N4-Alkylcytidines: Effects on Microorganisms and Application for the Protection of Cultural Heritage Objects of Painting

The rapid increase in the antibiotic resistance of microorganisms, capable of causing diseases in humans as destroying cultural heritage sites, is a great challenge for modern science. In this regard, it is necessary to develop fundamentally novel and highly active compounds. In this study, a series of N4-alkylcytidines, including 5- and 6-methylcytidine derivatives, with extended alkyl substituents, were obtained in order to develop a new generation of antibacterial and antifungal biocides based on nucleoside derivatives. It has been shown that N4-alkyl 5- or 6-methylcytidines effectively inhibit the growth of molds, isolated from the paintings in the halls of the Ancient Russian Paintings of the State Tretyakov Gallery, Russia, Moscow. The novel compounds showed activity similar to antiseptics commonly used to protect works of art, such as benzalkonium chloride, to which a number of microorganisms have acquired resistance. It was also shown that the activity of N4-alkylcytidines is comparable to that of some antibiotics used in medicine to fight Gram-positive bacteria, including resistant strains of Staphylococcus aureus and Mycobacterium smegmatis. N4-dodecyl-5- and 6-methylcytidines turned out to be the best. This compound seems promising for expanding the palette of antiseptics used in painting, since quite often the destruction of painting materials is caused by joint fungi and bacteria infection.

Fungal BGCs for Production of Secondary Metabolites: Main Types, Central Roles in Strain Improvement, and Regulation According to the Piano Principle

Filamentous fungi are one of the most important producers of secondary metabolites. Some of them can have a toxic effect on the human body, leading to diseases. On the other hand, they are widely used as pharmaceutically significant drugs, such as antibiotics, statins, and immunosuppressants. A single fungus species in response to various signals can produce 100 or more secondary metabolites. Such signaling is possible due to the coordinated regulation of several dozen biosynthetic gene clusters (BGCs), which are mosaically localized in different regions of fungal chromosomes. Their regulation includes several levels, from pathway-specific regulators, whose genes are localized inside BGCs, to global regulators of the cell (taking into account changes in pH, carbon consumption, etc.) and global regulators of secondary metabolism (affecting epigenetic changes driven by velvet family proteins, LaeA, etc.). In addition, various low-molecular-weight substances can have a mediating effect on such regulatory processes. This review is devoted to a critical analysis of the available data on the "turning on" and "off" of the biosynthesis of secondary metabolites in response to signals in filamentous fungi. To describe the ongoing processes, the model of "piano regulation" is proposed, whereby pressing a certain key (signal) leads to the extraction of a certain sound from the "musical instrument of the fungus cell", which is expressed in the production of a specific secondary metabolite.

Molecular regulation of fungal secondary metabolism

Many bioactive secondary metabolites synthesized by fungi have important applications in many fields, such as agriculture, food, medical and others. The biosynthesis of secondary metabolites is a complex process involving a variety of enzymes and transcription factors, which are regulated at different levels. In this review, we describe our current understanding on molecular regulation of fungal secondary metabolite biosynthesis, such as environmental signal regulation, transcriptional regulation and epigenetic regulation. The effects of transcription factors on the secondary metabolites produced by fungi were mainly introduced. It was also discussed that new secondary metabolites could be found in fungi and the production of secondary metabolites could be improved. We also highlight the importance of understanding the molecular regulation mechanisms to activate silent secondary metabolites and uncover their physiological and ecological functions. By comprehensively understanding the regulatory mechanisms involved in secondary metabolite biosynthesis, we can develop strategies to improve the production of these compounds and maximize their potential benefits.

Spermidine and 1,3-Diaminopropane Have Opposite Effects on the Final Stage of Cephalosporin C Biosynthesis in High-Yielding Acremonium chrysogenum Strain

The addition of exogenous polyamines increases the production of antibiotic cephalosporin C (CPC) in Acremonium chrysogenum high-yielding (HY) strain during fermentation on a complex medium. However, the molecular basis of this phenomenon is still unknown. In the current study, we developed a special synthetic medium on which we revealed the opposite effect of polyamines. The addition of 1,3-diaminopropane resulted in an increase in the yield of CPC by 12-15%. However, the addition of spermidine resulted in a decrease in the yield of CPC by 14-15% and accumulation of its metabolic pathway precursor, deacetylcephalosporin C (DAC); the total amount of cephems (DAC and CPC) was the same as after the addition of DAP. This indicates that spermidine, but not 1,3-diaminopropane, affects the final stage of CPC biosynthesis, associated with the acetylation of its precursor. In both cases, upregulation of biosynthetic genes from beta-lactam BGCs occurred at the same level as compared to the control; expression of transport genes was at the control level. The opposite effect may be due to the fact that N1-acetylation is much more efficient during spermidine catabolism than for 1,3-diaminopropane. The addition of spermidine, but not 1,3-diaminopropane, depleted the pool of acetyl coenzyme A by more than two-fold compared to control, which could lead to the accumulation of DAC.

Spermidine Regulates Mitochondrial Function by Enhancing eIF5A Hypusination and Contributes to Reactive Oxygen Species Production and Ganoderic Acid Biosynthesis in Ganoderma lucidum

Spermidine, a kind of polycation and one important member of the polyamine family, is essential for survival in many kinds of organisms and participates in the regulation of cell growth and metabolism. To explore the mechanism by which spermidine regulates ganoderic acid (GA) biosynthesis in Ganoderma lucidum, the effects of spermidine on GA and reactive oxygen species (ROS) contents were examined. Our data suggested that spermidine promoted the production of mitochondrial ROS and positively regulated GA biosynthesis. Further research revealed that spermidine promoted the translation of mitochondrial complexes I and II and subsequently influenced their activity. With a reduction in eukaryotic translation initiation factor 5A (eIF5A) hypusination by over 50% in spermidine synthase gene (spds) knockdown strains, the activities of mitochondrial complexes I and II were reduced by nearly 60% and 80%, respectively, and the protein contents were reduced by over 50%, suggesting that the effect of spermidine on mitochondrial complexes I and II was mediated through its influence on eIF5A hypusination. Furthermore, after knocking down eIF5A, the deoxyhypusine synthase gene (dhs), and the deoxyhypusine hydroxylase gene (dohh), the mitochondrial ROS level was reduced by nearly 50%, and the GA content was reduced by over 40%, suggesting that eIF5A hypusination contributed to mitochondrial ROS production and GA biosynthesis. In summary, spermidine maintains mitochondrial ROS homeostasis by regulating the translation and subsequent activity of complexes I and II via eIF5A hypusination and promotes GA biosynthesis via mitochondrial ROS signaling. The present findings provide new insight into the spermidine-mediated biosynthesis of secondary metabolites. IMPORTANCE Spermidine is necessary for organism survival and is involved in the regulation of various biological processes. However, the specific mechanisms underlying the various physiological functions of spermidine are poorly understood, especially in microorganisms. In this study, we found that spermidine hypusinates eIF5A to promote the production of mitochondrial ROS and subsequently regulate secondary metabolism in microorganisms. Our study provides a better understanding of the mechanism by which spermidine regulates mitochondrial function and provides new insight into the spermidine-mediated biosynthesis of secondary metabolites.

Penicillium chrysogenum, a Vintage Model with a Cutting-Edge Profile in Biotechnology

The discovery of penicillin entailed a decisive breakthrough in medicine. No other medical advance has ever had the same impact in the clinical practise. The fungus Penicillium chrysogenum (reclassified as P. rubens) has been used for industrial production of penicillin ever since the forties of the past century; industrial biotechnology developed hand in hand with it, and currently P. chrysogenum is a thoroughly studied model for secondary metabolite production and regulation. In addition to its role as penicillin producer, recent synthetic biology advances have put P. chrysogenum on the path to become a cell factory for the production of metabolites with biotechnological interest. In this review, we tell the history of P. chrysogenum, from the discovery of penicillin and the first isolation of strains with high production capacity to the most recent research advances with the fungus. We will describe how classical strain improvement programs achieved the goal of increasing production and how the development of different molecular tools allowed further improvements. The discovery of the penicillin gene cluster, the origin of the penicillin genes, the regulation of penicillin production, and a compilation of other P. chrysogenum secondary metabolites will also be covered and updated in this work.

3′-Amino modifications enhance the antifungal properties of N4-alkyl-5-methylcytidines for potential biocides

A set of 3′-modified N4-alkyl-5-methyl-2′-deoxycytidines has been synthesized and evaluated for biological activity. The replacement of the 3′-hydroxyl group with amino, aminoethyl and dialkylamino groups significantly enhances antifungal activity.

Polyamines Upregulate Cephalosporin C Production and Expression of β-Lactam Biosynthetic Genes in High-Yielding Acremonium chrysogenum Strain

The high-yielding production of pharmaceutically significant secondary metabolites in filamentous fungi is obtained by random mutagenesis; such changes may be associated with shifts in the metabolism of polyamines. We have previously shown that, in the Acremonium chrysogenum cephalosporin C high-yielding strain (HY), the content of endogenous polyamines increased by four- to five-fold. Other studies have shown that the addition of exogenous polyamines can increase the production of target secondary metabolites in highly active fungal producers, in particular, increase the biosynthesis of β-lactams in the Penicillium chrysogenum Wis 54-1255 strain, an improved producer of penicillin G. In the current study, we demonstrate that the introduction of exogenous polyamines, such as spermidine or 1,3-diaminopropane, to A. chrysogenum wild-type (WT) and HY strains, leads to an increase in colony germination and morphological changes in a complete agar medium. The addition of 5 mM polyamines during fermentation increases the production of cephalosporin C in the A. chrysogenum HY strain by 15-20% and upregulates genes belonging to the beta-lactam biosynthetic cluster. The data obtained indicate the intersection of the metabolisms of polyamines and beta-lactams in A. chrysogenum and are important for the construction of improved producers of secondary metabolites in filamentous fungi.

Deletion of the Bcnrps1 Gene Increases the Pathogenicity of Botrytis cinerea and Reduces Its Tolerance to the Exogenous Toxic Substances Spermidine and Pyrimethanil

During the infection of grapevine (Vitis vinifera) by the fungus Botrytis cinerea, the concentration of polyamines, which are toxic substances for the phytopathogen, increases in the grape. Nine NRPS genes have been identified in the genome of B. cinerea, yet the function of five of them remains unknown. For this reason, we have studied the expression of the 9 NRPS genes by RT-qPCR in a medium supplemented with sublethal concentrations of three polyamines (1,3-diaminopropane (1,3-DAP), spermidine (SPD), and spermine (SPM)). Our results show that the presence of polyamines in the culture medium triggered the overexpression of the Bcnrps1 gene in the pathogen. Deleting Bcnrps1 did not affect mycelial growth or adaptation to osmotic stress, and we show that its expression is not essential for the cycle of infection of the B. cinerea. However, mutating the Bcnrps1 gene resulted in overexpression of the Bcnrps6 gene, which encodes for the excretion of siderophores of the coprogen family. Moreover, gene deletion has reduced the tolerance of B. cinerea B05.10 to toxic substances such as the polyamine SPD and the fungicide pyrimethanil, and its virulence has increased. Our findings provide new insights into the function of the Bcnrps1 gene and its involvement in the tolerance of B. cinerea against exogenous toxic compounds.

Discovery of novel N4-alkylcytidines as promising antimicrobial agents

The emergence of drug-resistant strains of pathogenic microorganisms necessitates the creation of new drugs. In order to find new compounds that effectively inhibit the growth of pathogenic bacteria and fungi, we synthesized a set of N⁴-derivatives of cytidine, 2'-deoxycytidine and 5-metyl-2'-deoxycytidine bearing extended N⁴-alkyl and N⁴-phenylalkyl groups. The derivatives demonstrate activity against a number of Gram-positive bacteria, including Mycobacterium smegmatis (MIC = 24-200 μM) and Staphylococcus aureus (MIC = 50-200 μM), comparable with the activities of some antibiotics in medical use. The most promising compound appeared to be N⁴-dodecyl-5-metyl-2'-deoxycytidine 4h with activities of 24 and 48 μM against M. smegmatis and S. aureus, respectively, and high inhibitory activity of 0.5 mM against filamentous fungi that can, among other things, damage works of art, such as tempera painting. Noteworthy, some of other synthesized compounds are active against fungal growth with the inhibitory concentration in the range of 0.5-3 mM.

Requirement of LaeA for sporulation, pigmentation and secondary metabolism in Chaetomium globosum

The global regulator LaeA has been confirmed to govern the production of secondary metabolites in fungi. Herein, we examined the role of LaeA in Chaetomium globosum. Similarly as observed in other filamentous, CgLaeA had a significant effect on the secondary metabolism. The ΔCglaeA mutant strain did not exhibit chaetoglobosin A, whereas its production was restored in the CglaeAC strain. In addition, CglaeA overexpression led to an increase in chaetoglobosin A production. Transcriptional examination of the mutants indicated that CgLaeA positively regulated the expression of pathway-specific transcription factor CgcheR, while another global regulator CgvelB was negatively regulated by CgLaeA. Furthermore, CgLaeA also affected the morphological phenotypes of fungi. The ΔCglaeA mutant strains exhibited decreased sporulation and pigmentation compared with the wild-type strain, whereas the phenotypes were restored in the CglaeAC strain. Moreover, OE::CglaeA exhibited increased levels of sporulation and pigmentation. Moreover, inhibition activity against phytopathogenic fungi affected by decreased mycotoxin production of the ΔCglaeA mutant strain.

High-Yielding Lovastatin Producer Aspergillus terreus Shows Increased Resistance to Inhibitors of Polyamine Biosynthesis

The biosynthesis of pharmaceutically significant secondary metabolites in filamentous fungi is a multistep process that depends on a wide range of various factors, one of which is the intracellular content of polyamines. We have previously shown that in Aspergillus terreus lovastatin high-yielding strain (HY) exogenous introduction of polyamines during fermentation can lead to an increase in the production of lovastatin by 20–45%. However, the molecular mechanisms of this phenomenon have not been elucidated. In this regard, we carried out an inhibitory analysis at the key stage of polyamine biosynthesis, the conversion of L-ornithine to putrescine by the enzyme ornithine decarboxylase (ODC). A. terreus HY strain showed upregulation of genes for biosynthesis of polyamines, 3–10-fold, and increased resistance compared to the original wild-type strain upon inhibition of ODC on synthetic medium with 5 mM α-difluoromethylornithine (DFMO), by 20–25%, and 5 mM 1-aminooxy-3-aminopropane (APA), by 40–45%. The data obtained indicate changes in the metabolism of polyamines in A. terreus HY strain. The observed phenomenon may have a universal character among fungal producers of secondary metabolites improved by classical methods, since previously the increased resistance to ODC inhibitors was also shown for Acremonium chrysogenum, a high-yielding producer of cephalosporin C.

New knowledge about the biosynthesis of lovastatin and its production by fermentation of Aspergillus terreus

Lovastatin, and its semisynthetic derivative simvastatine, has great medical and economic importance, besides great potential for other uses. In the last years, a deeper and more complex view of secondary metabolism regulation has emerged, with the incorporation of cluster-specific and global transcription factors, and their relation to signaling cascades, as well as the new level of epigenetic regulation. Recently, a new mechanism, which regulates lovastatin biosynthesis, at transcriptional level, has been discovered: reactive oxygen species (ROS) regulation; also new unexpected environmental stimuli have been identified, which induce the synthesis of lovastatin, like quorum sensing-type molecules and support stimuli. The present review describes this new panorama and uses this information, together with the knowledge on lovastatin biosynthesis and genomics, as the foundation to analyze literature on optimization of fermentation parameters and medium composition, and also to fully understand new strategies for strain genetic improvement. This new knowledge has been applied to the development of more effective culture media, with the addition of molecules like butyrolactone I, oxylipins, and spermidine, or with addition of ROS-generating molecules to increase internal ROS levels in the cell. It has also been applied to the development of new strategies to generate overproducing strains of Aspergillus terreus, including engineering of the cluster-specific transcription factor (lovE), global transcription factors like the ones implicated in ROS regulation (or even mitochondrial alternative respiration aox gen), or the global regulator LaeA. Moreover, there is potential to apply some of these findings to the development of novel unconventional production systems. KEY POINTS: • New findings in regulation of lovastatin biosynthesis, like ROS regulation. • Induction by unexpected stimuli: autoinducer molecules and support stimuli. • Recent reports on culture medium and process optimization from this stand point. • Applications to molecular genetic strain improvement methods and production systems.

The critical role of plasma membrane H+-ATPase activity in cephalosporin C biosynthesis of Acremonium chrysogenum

The filamentous fungus Acremonium chrysogenum is the main industrial producer of cephalosporin C (CPC), one of the major precursors for manufacturing of cephalosporin antibiotics. The plasma membrane H⁺-ATPase (PMA) plays a key role in numerous fungal physiological processes. Previously we observed a decrease of PMA activity in A. chrysogenum overproducing strain RNCM 408D (HY) as compared to the level the wild-type strain A. chrysogenum ATCC 11550. Here we report the relationship between PMA activity and CPC biosynthesis in A. chrysogenum strains. The elevation of PMA activity in HY strain through overexpression of PMA1 from Saccharomyces cerevisiae, under the control of the constitutive gpdA promoter from Aspergillus nidulans, results in a 1.2 to 10-fold decrease in CPC production, shift in beta-lactam intermediates content, and is accompanied by the decrease in cef genes expression in the fermentation process; the characteristic colony morphology on agar media is also changed. The level of PMA activity in A. chrysogenum HY OE::PMA1 strains has been increased by 50–100%, up to the level observed in WT strain, and was interrelated with ATP consumption; the more PMA activity is elevated, the more ATP level is depleted. The reduced PMA activity in A. chrysogenum HY strain may be one of the selected events during classical strain improvement, aimed at elevating the ATP content available for CPC production.

Hydroxylamine Analogue of Agmatine: Magic Bullet for Arginine Decarboxylase

The biogenic polyamines, spermine, spermidine (Spd) and putrescine (Put) are present at micro-millimolar concentrations in eukaryotic and prokaryotic cells (many prokaryotes have no spermine), participating in the regulation of cellular proliferation and differentiation. In mammalian cells Put is formed exclusively from L-ornithine by ornithine decarboxylase (ODC) and many potent ODC inhibitors are known. In bacteria, plants, and fungi Put is synthesized also from agmatine, which is formed from L-arginine by arginine decarboxylase (ADC). Here we demonstrate that the isosteric hydroxylamine analogue of agmatine (AO-Agm) is a new and very potent (IC50 3•10-8 M) inhibitor of E. coli ADC. It was almost two orders of magnitude less potent towards E. coli ODC. AO-Agm decreased polyamine pools and inhibited the growth of DU145 prostate cancer cells only at high concentration (1 mM). Growth inhibitory analysis of the Acremonium chrysogenum demonstrated that the wild type (WT) strain synthesized Put only from L-ornithine, while the cephalosporin C high-yielding strain, in which the polyamine pool is increased, could use both ODC and ADC to produce Put. Thus, AO-Agm is an important addition to the set of existing inhibitors of the enzymes of polyamine biosynthesis, and an important instrument for investigating polyamine biochemistry.