Impact of Phosphate Availability on Membrane Lipid Content of the Model Strains, Streptomyces lividans and Streptomyces coelicolor

Phosphate availability conditions caspofungin tolerance, capsule attachment and titan cell formation in Cryptococcus neoformans

There is an urgent need for new antifungal drugs to treat invasive fungal diseases. Unfortunately, the echinocandin drugs that are fungicidal against other important fungal pathogens are ineffective against Cryptococcus neoformans, the causative agent of life-threatening meningoencephalitis in immunocompromised people. Contributing mechanisms for echinocandin tolerance are emerging with connections to calcineurin signaling, the cell wall, and membrane composition. In this context, we discovered that a defect in phosphate uptake impairs the tolerance of C. neoformans to the echinocandin caspofungin. Our previous analysis of mutants lacking three high affinity phosphate transporters revealed reduced elaboration of the polysaccharide capsule and attenuated virulence in mice. We investigated the underlying mechanisms and found that loss of the transporters and altered phosphate availability influences the cell wall and membrane composition. These changes contribute to the shedding of capsule polysaccharide thus explaining the reduced size of capsules on mutants lacking the phosphate transporters. We also found an influence of the calcineurin pathway including calcium sensitivity and an involvement of the endoplasmic reticulum in the response to phosphate limitation. Furthermore, we identified membrane and lipid composition changes consistent with the role of phosphate in phospholipid biosynthesis and with previous studies implicating membrane integrity in caspofungin tolerance. Finally, we discovered a contribution of phosphate to titan cell formation, a cell type that displays modified cell wall and capsule composition. Overall, our analysis reinforces the importance of phosphate as a regulator of cell wall and membrane composition with implications for capsule attachment and antifungal drug susceptibility.

Consequences of the deletion of the major specialized metabolite biosynthetic pathways of Streptomyces coelicolor on the metabolome and lipidome of this strain

Chassis strains, derived from Streptomyces coelicolor M145, deleted for one or more of its four main specialized metabolites biosynthetic pathways (CPK, CDA, RED and ACT), in various combinations, were constructed for the heterologous expression of specialized metabolites biosynthetic pathways of various types and origins. To determine consequences of these deletions on the metabolism of the deleted strains comparative lipidomic and metabolomic analyses of these strains and of the original strain were carried out. These studies unexpectedly revealed that the deletion of the peptidic clusters, RED and/or CDA, in a strain deleted for the ACT cluster, resulted into a great increase in the triacylglycerol (TAG) content, whereas the deletion of polyketide clusters, ACT and CPK had no impact on TAG content. Low or high TAG content of the deleted strains was correlated with abundance or paucity in amino acids, respectively, reflecting high or low activity of oxidative metabolism. Hypotheses based on what is known on the bio-activity and the nature of the precursors of these specialized metabolites are proposed to explain the unexpected consequences of the deletion of these pathways on the metabolism of the bacteria and on the efficiency of the deleted strains as chassis strains.

Impact of the Deletion of Genes of the Nitrogen Metabolism on Triacylglycerol, Cardiolipin and Actinorhodin Biosynthesis in Streptomyces coelicolor

Since nitrogen limitation is known to be an important trigger of triacylglycerol (TAG) accumulation in most microorganisms, we first assessed the global lipid content of 21 strains derived from Streptomyces coelicolor M145 deleted for genes involved in nitrogen metabolism. Seven of these strains deleted for genes encoding proteins involved in polyamine (GlnA2/SCO2241, GlnA3/SCO6962, GlnA4/SCO1613), or protein (Pup/SCO1646) degradation, in the regulation of nitrogen metabolism (GlnE/SCO2234 and GlnK/SCO5584), or the global regulator DasR/SCO5231 that controls negatively the degradation of N-acetylglucosamine, a constituent of peptidoglycan, had a higher TAG content than the original strain, whereas five of these strains (except the glnA2 and pup mutants) had a lower cardiolipin (CL) content. The production of the blue polyketide actinorhodin (ACT) was totally abolished in the dasR mutant in both Pi conditions, whereas the deletion of pup, glnA2, glnA3, and glnA4 was correlated with a significant increase in total ACT production, but mainly in Pi limitation. Unexpectedly, ACT production was strongly reduced in the glnA3 mutant in Pi proficiency. Altogether, our data suggest that high TAG and ACT biosynthesis and low CL biosynthesis might all contribute to the lowering of oxidative stress resulting from nitrogen limitation or from other causes.

Cardiolipin binding enhances KcsA channel gating via both its specific and dianion-monoanion interchangeable sites

KcsA is a potassium channel with a plethora of structural and functional information, but its activity in the KcsA-producing actinomycete membranes remains elusive. To determine lipid species involved in channel-modulation, a surface plasmon resonance (SPR)-based methodology, characterized by immobilization of membrane proteins under a membrane environment, was applied. Dianionic cardiolipin (CL) showed extremely higher affinity for KcsA than monoanionic lipids. The SPR experiments further demonstrated that CL bound not only to the N-terminal M0 helix, a lipid-sensor domain, but to the M0 helix-deleted mutant. In contrast, monoanionic lipids interacted primarily with the M0 helix. This indicates the presence of an alternative CL-binding site, plausibly in the transmembrane domain. Single-channel recordings demonstrated that CL enhanced channel opening in an M0-independent manner. Taken together, the action of monoanionic lipids is exclusively mediated by the M0 helix, while CL binds both the M0 helix and its specific site, further enhancing the channel activity.

Genome Analysis of a Variant of Streptomyces coelicolor M145 with High Lipid Content and Poor Ability to Synthetize Antibiotics

Streptomyces coelicolor M145 is a model strain extensively studied to elucidate the regulation of antibiotic biosynthesis in Streptomyces species. This strain abundantly produces the blue polyketide antibiotic, actinorhodin (ACT), and has a low lipid content. In a process designed to delete the gene encoding the isocitrate lyase (sco0982) of the glyoxylate cycle, an unexpected variant of S. coelicolor was obtained besides bona fide sco0982 deletion mutants. This variant produces 7- to 15-fold less ACT and has a 3-fold higher triacylglycerol and phosphatidylethanolamine content than the original strain. The genome of this variant was sequenced and revealed that 704 genes were deleted (9% of total number of genes) through deletions of various sizes accompanied by the massive loss of mobile genetic elements. Some deletions include genes whose absence could be related to the high total lipid content of this variant such as those encoding enzymes of the TCA and glyoxylate cycles, enzymes involved in nitrogen assimilation as well as enzymes belonging to some polyketide and possibly trehalose biosynthetic pathways. The characteristics of this deleted variant of S. coelicolor are consistent with the existence of the previously reported negative correlation existing between lipid content and antibiotic production in Streptomyces species.

Metabolic adjustments in response to ATP spilling by the small DX protein in a Streptomyces strain

ATP wasting is recognized as an efficient strategy to enhance metabolic activity and productivity of specific metabolites in several microorganisms. However, such strategy has been rarely implemented in Streptomyces species whereas antibiotic production by members of this genus is known to be triggered in condition of phosphate limitation that is correlated with a low ATP content. In consequence, to assess the effects of ATP spilling on the primary and specialized metabolisms of Streptomyces, the gene encoding the small synthetic protein DX, that has high affinity for ATP and dephosphorylates ATP into ADP, was cloned in the integrative vector pOSV10 under the control of the strong ErmE promoter. This construct and the empty vector were introduced into the species Streptomyces albogriseolus/viridodiastaticus yielding A37 and A36, respectively. A37 yielded higher biomass than A36 indicating that the DX-mediated ATP degradation resulted into a stimulation of A37 metabolism, consistently with what was reported in other microorganisms. The comparative analysis of the metabolomes of A36 and A37 revealed that A37 had a lower content in glycolytic and Tricarboxylic Acid Cycle intermediates as well as in amino acids than A36, these metabolites being consumed for biomass generation in A37. In contrast, the abundance of other molecules indicative either of energetic stress (ADP, AMP, UMP, ornithine and thymine), of activation (NAD and threonic acid) or inhibition (citramalic acid, fatty acids, TAG and L-alanine) of the oxidative metabolism, was higher in A37 than in A36. Furthermore, hydroxyl-pyrimidine derivatives and polycyclic aromatic polyketide antibiotics belonging to the angucycline class and thought to have a negative impact on respiration were also more abundantly produced by A37 than by A36. This comparative analysis thus revealed the occurrence in A37 of antagonistic metabolic strategies, namely, activation or slowing down of oxidative metabolism and respiration, to maintain the cellular energetic balance. This study thus demonstrated that DX constitutes an efficient biotechnological tool to enhance the expression of the specialized metabolic pathways present in the Streptomyces genomes that may include cryptic pathways. Its use thus might lead to the discovery of novel bioactive molecules potentially useful to human health.

An overview on the two-component systems of Streptomyces coelicolor

The two-component system (TCS) found in various organisms is a regulatory system, which is involved in the response by the organism to stimuli, thereby regulating the internal behavior of the cell. It is commonly found in prokaryotes and is an important signaling system in bacteria. TCSs are involved in the regulation of physiological and morphological differentiation of the industrially important microbes from the genus Streptomyces, which produce a vast array of bioactive secondary metabolites (SMs). Genetic engineering of TCSs can substantially increase the yield of target SMs, which is valuable for industrial-scale production. Research on TCS has mainly been completed in the model strain Streptomyces coelicolor. In this review, we summarize the recent advances in the functional identification and elucidation of the regulatory mechanisms of various TCSs in S. coelicolor, with a focus on their roles in the biosynthesis of important SMs.

Two-Component Systems of Streptomyces coelicolor: An Intricate Network to Be Unraveled

Bacteria of the Streptomyces genus constitute an authentic biotech gold mine thanks to their ability to produce a myriad of compounds and enzymes of great interest at various clinical, agricultural, and industrial levels. Understanding the physiology of these organisms and revealing their regulatory mechanisms is essential for their manipulation and application. Two-component systems (TCSs) constitute the predominant signal transduction mechanism in prokaryotes, and can detect a multitude of external and internal stimuli and trigger the appropriate cellular responses for adapting to diverse environmental conditions. These global regulatory systems usually coordinate various biological processes for the maintenance of homeostasis and proper cell function. Here, we review the multiple TCSs described and characterized in Streptomyces coelicolor, one of the most studied and important model species within this bacterial group. TCSs are involved in all cellular processes; hence, unravelling the complex regulatory network they form is essential for their potential biotechnological application.

Comparative Proteomic Analysis of Transcriptional and Regulatory Proteins Abundances in S. lividans and S. coelicolor Suggests a Link between Various Stresses and Antibiotic Production

Streptomyces coelicolor and Streptomyces lividans constitute model strains to study the regulation of antibiotics biosynthesis in Streptomyces species since these closely related strains possess the same pathways directing the biosynthesis of various antibiotics but only S. coelicolor produces them. To get a better understanding of the origin of the contrasted abilities of these strains to produce bioactive specialized metabolites, these strains were grown in conditions of phosphate limitation or proficiency and a comparative analysis of their transcriptional/regulatory proteins was carried out. The abundance of the vast majority of the 355 proteins detected greatly differed between these two strains and responded differently to phosphate availability. This study confirmed, consistently with previous studies, that S. coelicolor suffers from nitrogen stress. This stress likely triggers the degradation of the nitrogen-rich peptidoglycan cell wall in order to recycle nitrogen present in its constituents, resulting in cell wall stress. When an altered cell wall is unable to fulfill its osmo-protective function, the bacteria also suffer from osmotic stress. This study thus revealed that these three stresses are intimately linked in S. coelicolor. The aggravation of these stresses leading to an increase of antibiotic biosynthesis, the connection between these stresses, and antibiotic production are discussed.

Lipid Readjustment in Yarrowia lipolytica Odd-Chain Fatty Acids Producing Strains

Yarrowia lipolytica is a promising oleaginous yeast for producing unusual lipids, such as odd-chain fatty acids (OCFA). Their diverse applications and low natural production make OCFA particularly interesting. In recent studies, inhibiting the catabolic pathway of precursor, boosting precursor pools, and optimizing substrate combination greatly improved the production of OCFA in Y. lipolytica. We explored the lipid readjustment of OCFA in engineered Y. lipolytica strains. NPLC-Corona-CAD^® evidenced a time-dependent overproduction of free fatty acids, diglycerides, and phosphatidylcholine (PC) in obese LP compared to obese L. Phosphatidylethanolamine (PE) and phosphatidylinositol, largely overproduced in obese LP at 72 h compared to obese L, vanished at 216 h. The fatty acyls (FAs) composition of glycero- and glycerophospholipids was determined by NPLC-APPI⁺-HRMS from in-source generated monoacylglycerol-like fragment ions. C18:1 and C17:1 were predominant acylglycerols in obese L and obese LP, respectively. Phosphatidic acid, PE, and PC exhibited similar FAs composition but differed in their molecular species distributions. Cardiolipin (CL) is known to contain mostly C18:2 FAs corresponding to the composition in obese L, 50% of C18:2, and 35% of C18:1. In obese LP, both FAs dropped to drop to 20%, and C17:1 were predominant, reaching 55%. We hypothesize that CL-modified composition in obese LPs may alter mitochondrial function and limit lipid production.

A Proteomic Analysis Indicates That Oxidative Stress Is the Common Feature Triggering Antibiotic Production in Streptomyces coelicolor and in the pptA Mutant of Streptomyces lividans

In most Streptomyces species, antibiotic production is triggered in phosphate limitation and repressed in phosphate proficiency. However, the model strain, Streptomyces coelicolor, escapes this general rule and produces actinorhoddin (ACT), a polyketide antibiotic, even more abundantly in phosphate proficiency than in phosphate limitation. ACT was shown to bear "anti-oxidant" properties suggesting that its biosynthesis is triggered by oxidative stress. Interestingly, Streptomyces lividans, a strain closely related to S. coelicolor, does not produce ACT in any phosphate condition whereas its pptA/sco4144 mutant produces ACT but only in phosphate limitation. In order to define the potentially common features of the ACT producing strains, these three strains were grown in condition of low and high phosphate availability, and a comparative quantitative analysis of their proteomes was carried out. The abundance of proteins of numerous pathways differed greatly between S. coelicolor and the S. lividans strains, especially those of central carbon metabolism and respiration. S. coelicolor is characterized by the high abundance of the complex I of the respiratory chain thought to generate reactive oxygen/nitrogen species and by a weak glycolytic activity causing a low carbon flux through the Pentose Phosphate Pathway resulting into the low generation of NADPH, a co-factor of thioredoxin reductases necessary to combat oxidative stress. Oxidative stress is thus predicted to be high in S. coelicolor. In contrast, the S. lividans strains had rather similar proteins abundance for most pathways except for the transhydrogenases SCO7622-23, involved in the conversion of NADPH into NADH. The poor abundance of these enzymes in the pptA mutant suggested a deficit in NADPH. Indeed, PptA is an accessory protein forcing polyphosphate into a conformation allowing their efficient use by various enzymes taking polyphosphate as a donor of phosphate and energy, including the ATP/Polyphosphate-dependent NAD kinase SCO1781. In phosphate limitation, this enzyme would mainly use polyphosphate to phosphorylate NAD into NADP, but this phosphorylation would be inefficient in the pptA mutant resulting in low NADP(H) levels and thus high oxidative stress. Altogether, our results indicated that high oxidative stress is the common feature triggering ACT biosynthesis in S. coelicolor and in the pptA mutant of S. lividans.

The Phosin PptA Plays a Negative Role in the Regulation of Antibiotic Production in Streptomyces lividans

In Streptomyces, antibiotic biosynthesis is triggered in phosphate limitation that is usually correlated with energetic stress. Polyphosphates constitute an important reservoir of phosphate and energy and a better understanding of their role in the regulation of antibiotic biosynthesis is of crucial importance. We previously characterized a gene, SLI_4384/ppk, encoding a polyphosphate kinase, whose disruption greatly enhanced the weak antibiotic production of Streptomyces lividans. In the condition of energetic stress, Ppk utilizes polyP as phosphate and energy donor, to generate ATP from ADP. In this paper, we established that ppk is co-transcribed with its two downstream genes, SLI_4383, encoding a phosin called PptA possessing a CHAD domain constituting a polyphosphate binding module and SLI_4382 encoding a nudix hydrolase. The expression of the ppk/pptA/SLI_4382 operon was shown to be under the positive control of the two-component system PhoR/PhoP and thus mainly expressed in condition of phosphate limitation. However, pptA and SLI_4382 can also be transcribed alone from their own promoter. The deletion of pptA resulted into earlier and stronger actinorhodin production and lower lipid content than the disruption of ppk, whereas the deletion of SLI_4382 had no obvious phenotypical consequences. The disruption of ppk was shown to have a polar effect on the expression of pptA, suggesting that the phenotype of the ppk mutant might be linked, at least in part, to the weak expression of pptA in this strain. Interestingly, the expression of phoR/phoP and that of the genes of the pho regulon involved in phosphate supply or saving were strongly up-regulated in pptA and ppk mutants, revealing that both mutants suffer from phosphate stress. Considering the presence of a polyphosphate binding module in PptA, but absence of similarities between PptA and known exo-polyphosphatases, we proposed that PptA constitutes an accessory factor for exopolyphosphatases or general phosphatases involved in the degradation of polyphosphates into phosphate.