Involvement of amfC in physiological and morphological development in Streptomyces coelicolor A3(2)

TetR family regulator AbrT controls lincomycin production and morphological development in Streptomyces lincolnensis

BACKGROUND

The TetR family of transcriptional regulators (TFRs), serving as crucial regulators of diverse cellular processes, undergo conformational changes induced by small-molecule ligands, which either inhibit or activate them to modulate target gene expression. Some ligands of TFRs in actinomycetes and their regulatory effects have been identified and studied; however, regulatory mechanisms of the TetR family in the lincomycin-producing Streptomyces lincolnensis remain poorly understood.

RESULTS

In this study, we found that AbrT (SLCG_1979), a TetR family regulator, plays a pivotal role in regulating lincomycin production and morphological development in S. lincolnensis. Deletion of abrT gene resulted in increased lincomycin A (Lin-A) production, but delayed mycelium formation and sporulation on solid media. AbrT directly or indirectly repressed the expression of lincomycin biosynthetic (lin) cluster genes and activated that of the morphological developmental genes amfC, whiB, and ftsZ. We demonstrated that AbrT bound to two motifs (5'-CGCGTACTCGTA-3' and 5'-CGTACGATAGCT-3') present in the bidirectional promoter between abrT and SLCG_1980 genes. This consequently repressed abrT itself and its adjacent gene SLCG_1980 that encodes an arabinose efflux permease. D-arabinose, not naturally occurring as L-arabinose, was identified as the effector molecule of AbrT, reducing its binding affinity to abrT-SLCG_1980 intergenic region. Furthermore, based on functional analysis of the AbrT homologue in Saccharopolyspora erythraea, we inferred that the TetR family regulator AbrT may play an important role in regulating secondary metabolism in actinomycetes.

CONCLUSIONS

AbrT functions as a regulator for governing lincomycin production and morphological development of S. lincolnensis. Our findings demonstrated that D-arabinose acts as a ligand of AbrT to mediate the regulation of lincomycin biosynthesis in S. lincolnensis. Our findings provide novel insights into ligand-mediated regulation in antibiotic biosynthesis.

Deletion of a hybrid NRPS-T1PKS biosynthetic gene cluster via Latour gene knockout system in Saccharopolyspora pogona and its effect on butenyl-spinosyn biosynthesis and growth development

Butenyl-spinosyn, a promising biopesticide produced by Saccharopolyspora pogona, exhibits stronger insecticidal activity and a broader pesticidal spectrum. However, its titre in the wild-type S. pogona strain is too low to meet the industrial production requirements. Deletion of non-target natural product biosynthetic gene clusters resident in the genome of S. pogona could reduce the consumption of synthetic precursors, thereby promoting the biosynthesis of butenyl-spinosyn. However, it has always been a challenge for scientists to genetically engineer S. pogona. In this study, the Latour gene knockout system (linear DNA fragment recombineering system) was established in S. pogona. Using the Latour system, a hybrid NRPS-T1PKS cluster (˜20 kb) which was responsible for phthoxazolin biosynthesis was efficiently deleted in S. pogona. The resultant mutant S. pogona-Δura4-Δc14 exhibited an extended logarithmic phase, increased biomass and a lower glucose consumption rate. Importantly, the production of butenyl-spinosyn in S. pogona-Δura4-Δc14 was increased by 4.72-fold compared with that in the wild-type strain. qRT-PCR analysis revealed that phthoxazolin biosynthetic gene cluster deletion could promote the expression of the butenyl-spinosyn biosynthetic gene cluster. Furthermore, a TetR family transcriptional regulatory gene that could regulate the butenyl-spinosyn biosynthesis has been identified from the phthoxazolin biosynthetic gene cluster. Because dozens of natural product biosynthetic gene clusters exist in the genome of S. pogona, the strategy reported here will be used to further promote the production of butenyl-spinosyn by deleting other secondary metabolite synthetic gene clusters.

Redox-sensitive transcriptional regulator SoxR directly controls antibiotic production, development and thiol-oxidative stress response in Streptomyces avermitilis

The redox‐sensitive transcriptional regulator SoxR is conserved in bacteria. Its role in mediating protective response to various oxidative stresses in Escherichia coli and related enteric bacteria has been well established. However, functions and regulatory mechanisms of SoxR in filamentous Streptomyces, which produce half of known antibiotics, are unclear. We report here that SoxR pleiotropically regulates antibiotic production, morphological development, primary metabolism and thiol‐oxidative stress response in industrially important species Streptomycesavermitilis. SoxR stimulated avermectin production by directly activating ave structural genes. Four genes (sav_3956, sav_4018, sav_5665 and sav_7218) that are homologous to targets of S. coelicolor SoxR are targeted by S. avermitilis SoxR. A consensus 18‐nt SoxR‐binding site, 5′‐VSYCNVVMHNKVKDGMGB‐3′, was identified in promoter regions of sav_3956, sav_4018, sav_5665, sav_7218 and target ave genes, leading to prediction of the SoxR regulon and confirmation of 11 new targets involved in development (ftsH), oligomycin A biosynthesis (olmRI), primary metabolism (metB, sav_1623, plcA, nirB, thiG, ndh2), transport (smoE) and regulatory function (sig57, sav_7278). SoxR also directly activated three key developmental genes (amfC, whiB and ftsZ) and promoted resistance of S. avermitilis to thiol‐oxidative stress through activation of target trx and msh genes. Overexpression of soxR notably enhanced antibiotic production in S. avermitilis and S. coelicolor. Our findings expand our limited knowledge of SoxR and will facilitate improvement of methods for antibiotic overproduction in Streptomyces species.

Identification of a TetR family regulator and a polyketide synthase gene cluster involved in growth development and butenyl-spinosyn biosynthesis of Saccharopolyspora pogona

Butenyl-spinosyn produced by Saccharopolyspora pogona exhibits strong insecticidal activity and broad pesticidal spectrum. However, its synthetic level was low in the wild-type strain. At present, important functional genes involved in butenyl-spinosyn biosynthesis remain unknown, which leads to difficulty in efficiently editing its genome to improve the butenyl-spinosyn yield. To accelerate the genetic modification of S. pogona, we conducted comparative proteomics analysis to screen differentially expressed proteins related to butenyl-spinosyn biosynthesis. A TetR family regulatory protein was selected from the 289 differentially expressed proteins, and its encoding gene (SP_1288) was successfully deleted by CRISPR/Cas9 system. We further deleted a 32-kb polyketide synthase gene cluster (cluster 28) to reduce the competition for precursors. Phenotypic analysis revealed that the deletion of the SP_1288 and cluster 28 resulted in a 3.10-fold increase and a 35.4% decrease in the butenyl-spinosyn levels compared with the wild-type strain, respectively. The deletion of cluster 28 affected the cell growth, glucose consumption, mycelium morphology, and sporulation by controlling the expression of ptsH, ptsI, amfC, and other genes related to sporulation, whereas SP_1288 did not. These findings confirmed not only that the CRISPR/Cas9 system can be applied to the S. pogona genome editing but also that SP_1288 and cluster 28 are closely related to the butenyl-spinosyn biosynthesis and growth development of S. pogona. The strategy reported here will be useful to reveal the regulatory mechanism of butenyl-spinosyn and improve antibiotic production in other actinomycetes. KEY POINTS: • SP_1288 deletion can significantly promote the butenyl-spinosyn biosynthesis. • Cluster 28 deletion showed pleiotropic effects on S. pogona. • SP_1288 and cluster 28 were deleted by CRISPR/Cas9 system in S. pogona.

c-di-GMP Arms an Anti-σ to Control Progression of Multicellular Differentiation in Streptomyces

Streptomyces are our primary source of antibiotics, produced concomitantly with the transition from vegetative growth to sporulation in a complex developmental life cycle. We previously showed that the signaling molecule c-di-GMP binds BldD, a master repressor, to control initiation of development. Here we demonstrate that c-di-GMP also intervenes later in development to control differentiation of the reproductive hyphae into spores by arming a novel anti-σ (RsiG) to bind and sequester a sporulation-specific σ factor (σ^WhiG). We present the structure of the RsiG-(c-di-GMP)₂-σ^WhiG complex, revealing an unusual, partially intercalated c-di-GMP dimer bound at the RsiG-σ^WhiG interface. RsiG binds c-di-GMP in the absence of σ^WhiG, employing a novel E(X)₃S(X)₂R(X)₃Q(X)₃D motif repeated on each helix of a coiled coil. Further studies demonstrate that c-di-GMP is essential for RsiG to inhibit σ^WhiG. These findings reveal a newly described control mechanism for σ-anti-σ complex formation and establish c-di-GMP as the central integrator of Streptomyces development.

A Novel AdpA Homologue Negatively Regulates Morphological Differentiation in Streptomyces xiamenensis 318

The pleiotropic transcriptional regulator AdpA positively controls morphological differentiation and regulates secondary metabolism in most Streptomyces species. Streptomyces xiamenensis 318 has a linear chromosome 5.96 Mb in size. How AdpA affects secondary metabolism and morphological differentiation in such a naturally minimized genomic background is unknown. Here, we demonstrated that AdpA _Sx , an AdpA orthologue in S. xiamenensis, negatively regulates cell growth and sporulation and bidirectionally regulates the biosynthesis of xiamenmycin and polycyclic tetramate macrolactams (PTMs) in S. xiamenensis 318. Overexpression of the adpA_Sx gene in S. xiamenensis 318 had negative effects on morphological differentiation and resulted in reduced transcription of putative ssgA, ftsZ, ftsH, amfC, whiB, wblA1, wblA2, wblE, and a gene encoding sporulation-associated protein (sxim_29740), whereas the transcription of putative bldD and bldA genes was upregulated. Overexpression of adpA_Sx led to significantly enhanced production of xiamenmycin but had detrimental effects on the production of PTMs. As expected, the transcriptional level of the xim gene cluster was upregulated, whereas the PTM gene cluster was downregulated. Moreover, AdpA _Sx negatively regulated the transcription of its own gene. Electrophoretic mobility shift assays revealed that AdpA _Sx can bind the promoter regions of structural genes of both the xim and PTM gene clusters as well as to the promoter regions of genes potentially involved in the cell growth and differentiation of S. xiamenensis 318. We report that an AdpA homologue has negative effects on morphological differentiation in S. xiamenensis 318, a finding confirmed when AdpA _Sx was introduced into the heterologous host Streptomyces lividans TK24.IMPORTANCE AdpA is a key regulator of secondary metabolism and morphological differentiation in Streptomyces species. However, AdpA had not been reported to negatively regulate morphological differentiation. Here, we characterized the regulatory role of AdpA _Sx in Streptomyces xiamenensis 318, which has a naturally streamlined genome. In this strain, AdpA _Sx negatively regulated cell growth and morphological differentiation by directly controlling genes associated with these functions. AdpA _Sx also bidirectionally controlled the biosynthesis of xiamenmycin and PTMs by directly regulating their gene clusters rather than through other regulators. Our findings provide additional evidence for the versatility of AdpA in regulating morphological differentiation and secondary metabolism in Streptomyces.

AvaR1, a Butenolide-Type Autoregulator Receptor in Streptomyces avermitilis, Directly Represses Avenolide and Avermectin Biosynthesis and Multiple Physiological Responses

Avermectins are commercially important anthelmintic antibiotics produced by Streptomyces avermitilis. The homologous TetR-family transcriptional regulators AvaR1 and AvaR2 in this species were identified previously as receptors of avenolide, a novel butenolide-type autoregulator signal required for triggering avermectin biosynthesis. AvaR2 was found to be an important pleiotropic regulator in repression of avermectin and avenolide production and cell growth, whereas the regulatory role of AvaR1 remains unclear. Investigation of AvaR1 function in the present study showed that it had no effect on cell growth or morphological differentiation, but inhibited avenolide and avermectin production mainly through direct repression of aco (the key enzyme gene for avenolide biosynthesis) and aveR (the cluster-situated activator gene). AvaR1 also directly repressed its own gene (avaR1) and two adjacent homologous genes (avaR2 and avaR3). Binding sites of AvaR1 on these five target promoter regions completely overlapped those of AvaR2, leading to the same consensus binding motif. However, AvaR1 and AvaR2 had both common and exclusive target genes, indicating that they cross-regulate diverse physiological processes. Ten novel identified AvaR1 targets are involved in primary metabolism, stress responses, ribosomal protein synthesis, and cyclic nucleotide degration, reflecting a pleiotropic role of AvaR1. Competitive EMSAs and GST pull-down assays showed that AvaR1 and AvaR2 competed for the same binding regions, and could form a heterodimer and homodimers, suggesting that AvaR1 and AvaR2 compete and cooperate to regulate their common target genes. These findings provide a more comprehensive picture of the cellular responses mediated by AvaR1 and AvaR2 regulatory networks in S. avermitilis.

SAV742, a Novel AraC-Family Regulator from Streptomyces avermitilis, Controls Avermectin Biosynthesis, Cell Growth and Development

Avermectins are useful anthelmintic antibiotics produced by Streptomyces avermitilis. We demonstrated that a novel AraC-family transcriptional regulator in this species, SAV742, is a global regulator that negatively controls avermectin biosynthesis and cell growth, but positively controls morphological differentiation. Deletion of its gene, sav_742, increased avermectin production and dry cell weight, but caused delayed formation of aerial hyphae and spores. SAV742 directly inhibited avermectin production by repressing transcription of ave structural genes, and also directly regulated its own gene (sav_742) and adjacent gene sig8 (sav_741). The precise SAV742-binding site on its own promoter region was determined by DNase I footprinting assay coupled with site-directed DNA mutagenesis, and 5-nt inverted repeats (GCCGA-n₁₀/n₁₂-TCGGC) were found to be essential for SAV742 binding. Similar 5-nt inverted repeats separated by 3, 10 or 15 nt were found in the promoter regions of target ave genes and sig8. The SAV742 regulon was predicted based on bioinformatic analysis. Twenty-six new SAV742 targets were identified and experimentally confirmed, including genes involved in primary metabolism, secondary metabolism and development. Our findings indicate that SAV742 plays crucial roles in not only avermectin biosynthesis but also coordination of complex physiological processes in S. avermitilis.

AvaR2, a pseudo γ-butyrolactone receptor homologue from Streptomyces avermitilis, is a pleiotropic repressor of avermectin and avenolide biosynthesis and cell growth

Avermectins produced by Streptomyces avermitilis are effective anthelmintic agents. The autoregulatory signalling molecule that triggers avermectin biosynthesis is a novel butenolide-type molecule, avenolide, rather than common γ-butyrolactones (GBLs). We identified AvaR2, a pseudo GBL receptor homologue, as an important repressor of avermectin and avenolide biosynthesis and cell growth. AvaR2 directly repressed transcription of aveR (the ave cluster-situated activator gene), aco (a key gene for avenolide biosynthesis), its own gene (avaR2) and two other GBL receptor homologous genes (avaR1 and avaR3) by binding to their promoter regions. The aveR promoter had the highest affinity for AvaR2. A consensus 18 bp ARE (autoregulatory element)-like sequence was found in the AvaR2-binding regions of these five target genes. Eleven novel AvaR2 targets were identified, including genes involved in primary metabolism, ribosomal protein synthesis, and stress responses. AvaR2 bound and responded to endogenous avenolide and exogenous antibiotics jadomycin B (JadB) and aminoglycosides to modulate its DNA-binding activity. Our findings help to clarify the roles of pseudo GBL receptors as pleiotropic regulators and as receptors for new type autoregulator and exogenous antibiotic signal. A pseudo GBL receptor-mediated antibiotic signalling transduction system may be a common strategy that facilitates Streptomyces interspecies communication and survival in complex environments.

Capturing the target genes of BldD in Saccharopolyspora erythraea using improved genomic SELEX method

BldD (SACE_2077), a key developmental regulator in actinomycetes, is the first identified transcriptional factor in Saccharopolyspora erythraea positively regulating erythromycin production and morphological differentiation. Although the BldD of S. erythraea binds to the promoters of erythromycin biosynthetic genes, the interaction affinities are relatively low, implying the existence of its other target genes in S. erythraea. Through the genomic systematic evolution of ligands by exponential enrichment (SELEX) method that we herein improved, four DNA sequences of S. erythraea A226, corresponding to the promoter regions of SACE_0306 (beta-galactosidase), SACE_0811 (50S ribosomal protein L25), SACE_3410 (fumarylacetoacetate hydrolase), and SACE_6014 (aldehyde dehydrogenase), were captured with all three BldD concentrations of 0.5, 1, and 2 μM, while the previously identified intergenic regions of eryBIV-eryAI and ermE-eryCI plus the promoter region of SACE_7115, the amfC homolog for aerial mycelium formation, could be captured only when the BldD's concentration reached 2 μM. Electrophoretic mobility shift assay (EMSA) analysis indicated that BldD specifically bound to above seven DNA sequences, and quantitative real-time PCR (qRT-PCR) assay showed that the transcriptional levels of the abovementioned target genes decreased when bldD was disrupted in A226. Furthermore, SACE_7115 and SACE_0306 in A226 were individually inactivated, showing that SACE_7115 was predominantly involved in aerial mycelium formation, while SACE_0306 mainly controlled erythromycin production. This study provides valuable information for better understanding of the pleiotropic regulator BldD in S. erythraea, and the improved method may be useful for uncovering regulatory networks of other transcriptional factors.

SACE_0012, a TetR-family transcriptional regulator, affects the morphogenesis of Saccharopolyspora erythraea

Saccharopolyspora erythraea, a mycelium-forming actinomycete, produces a clinically important antibiotic erythromycin. Extensive investigations have provided insights into erythromycin biosynthesis in S. erythraea, but knowledge of its morphogenesis remains limited. By gene inactivation and complementation strategies, the TetR-family transcriptional regulator SACE_0012 was identified to be a negative regulator of mycelium formation of S. erythraea A226. Detected by quantitative real-time PCR, the relative transcription of SACE_7115, the amfC homolog for an aerial mycelium formation protein, was dramatically increased in SACE_0012 mutant, whereas erythromycin biosynthetic gene eryA, a pleiotropic regulatory gene bldD, and the genes SACE_2141, SACE_6464, SACE_6040, that are the homologs to the sporulation regulators WhiA, WhiB, WhiG, were not differentially expressed. SACE_0012 disruption could not restore its defect of aerial development in bldD mutant, and also did not further accelerate the mycelium formation in the mutant of SACE_7040 gene, that was previously identified to be a morphogenesis repressor. Furthermore, the transcriptional level of SACE_0012 had not markedly changed in bldD and SACE_7040 mutant over A226. Taken together, these results suggest that SACE_0012 is a negative regulator of S. erythraea morphogenesis by mainly increasing the transcription of amfC gene, independently of the BldD regulatory system.

The biosynthesis and regulation of bacterial prodiginines

The red-pigmented prodiginines are bioactive secondary metabolites produced by both Gram-negative and Gram-positive bacteria. Recently, these tripyrrole molecules have received renewed attention owing to reported immunosuppressive and anticancer properties. The enzymes involved in the biosynthetic pathways for the production of two of these molecules, prodigiosin and undecylprodigiosin, are now known. However, the biochemistry of some of the reactions is still poorly understood. The physiology and regulation of prodiginine production in Serratia and Streptomyces are now well understood, although the biological role of these pigments in the producer organisms remains unclear. However, research into the biology of pigment production will stimulate interest in the bioengineering of strains to synthesize useful prodiginine derivatives.

afsS is a target of AfsR, a transcriptional factor with ATPase activity that globally controls secondary metabolism in Streptomyces coelicolor A3(2)

AfsR is a pleiotropic, global regulator that controls the production of actinorhodin, undecylprodigiosin and calcium-dependent antibiotic in Streptomyces coelicolor A3(2). AfsR, with 993 amino acids, is phosphorylated on serine and threonine residues by a protein serine/threonine kinase AfsK and contains an OmpR-like DNA-binding fold at its N-terminal portion and A- and B-type nucleotide-binding motifs in the middle of the protein. The DNA-binding domain, in-dependently of the nucleotide-binding domain, contributed the binding of AfsR to the upstream region of afsS that locates immediately 3' to afsR and encodes a 63-amino-acid protein. No transcription of afsS in the DeltaafsR background and restoration of afsS transcription by afsR on a plasmid in the same genetic background indicated that afsR served as a transcriptional activator for afsS. Interestingly, the AfsR binding site overlapped the promoter of afsS, as determined by DNase I protection assay and high-resolution S1 nuclease mapping. The nucleotide-binding domain contributed distinct ATPase and GTPase activity. The phosphorylation of AfsR by AfsK greatly enhanced the DNA-binding activity and modulated the ATPase activity. The DNA-binding ability of AfsR was independent of the ATPase activity. However, the ATPase activity was essential for transcriptional activation of afsS, probably because the energy available from ATP hydrolysis is required for the isomerization of the closed complex between AfsR and RNA polymerase to a transcriptionally competent open complex. Thus, AfsR turns out to be a unique transcriptional factor, in that it is modular, in which DNA-binding and ATPase activities are physically separable, and the two functions are modulated by phosphorylation on serine and threonine residues.

Analysis of the prodiginine biosynthesis gene cluster of Streptomyces coelicolor A3(2): new mechanisms for chain initiation and termination in modular multienzymes

BACKGROUND

Prodiginines are a large family of pigmented oligopyrrole antibiotics with medicinal potential as immunosuppressants and antitumour agents that are produced by several actinomycetes and other eubacteria. Recently, a gene cluster in Streptomyces coelicolor encoding the biosynthesis of undecylprodiginine and butyl-meta-cycloheptylprodiginine has been sequenced.

RESULTS

Using sequence comparisons, functions have been assigned to the majority of the genes in the cluster, several of which encode homologues of enzymes involved in polyketide, non-ribosomal peptide, and fatty acid biosynthesis. Based on these assignments, a complete pathway for undecylprodiginine and butyl-meta-cycloheptylprodiginine biosynthesis in S. coelicolor has been deduced. Gene knockout experiments have confirmed the deduced roles of some of the genes in the cluster.

CONCLUSIONS

The analysis presented provides a framework for a general understanding of the genetics and biochemistry of prodiginine biosynthesis, which should stimulate rational approaches to the engineered biosynthesis of novel prodiginines with improved immunosuppressant or antitumour activities. In addition, new mechanisms for chain initiation and termination catalysed by hitherto unobserved domains in modular multienzyme systems have been deduced.