A rare leucine codon in adpA is implicated in the morphological defect of bldA mutants of Streptomyces coelicolor

The Best of Both Worlds-Streptomyces coelicolor and Streptomyces venezuelae as Model Species for Studying Antibiotic Production and Bacterial Multicellular Development

Streptomyces bacteria have been studied for more than 80 years thanks to their ability to produce an incredible array of antibiotics and other specialized metabolites and their unusual fungal-like development. Their antibiotic production capabilities have ensured continual interest from both academic and industrial sectors, while their developmental life cycle has provided investigators with unique opportunities to address fundamental questions relating to bacterial multicellular growth. Much of our understanding of the biology and metabolism of these fascinating bacteria, and many of the tools we use to manipulate these organisms, have stemmed from investigations using the model species Streptomyces coelicolor and Streptomyces venezuelae. Here, we explore the pioneering work in S. coelicolor that established foundational genetic principles relating to specialized metabolism and development, alongside the genomic and cell biology developments that led to the emergence of S. venezuelae as a new model system. We highlight key discoveries that have stemmed from studies of these two systems and discuss opportunities for future investigations that leverage the power and understanding provided by S. coelicolor and S. venezuelae.

Global Effects of the Developmental Regulator BldB in Streptomyces venezuelae

In Streptomyces, the Bld (Bald) regulators control formation of the reproductive aerial hyphae. The functions of some of these regulators have been well characterized, but BldB has remained enigmatic. In addition to the bldB gene itself, Streptomyces venezuelae has 10 paralogs of bldB that sit next to paralogs of whiJ and abaA. Transcriptome sequencing (RNA-seq) revealed that loss of BldB function causes the dramatic transcriptional upregulation of the abaA paralogs and a novel inhibitor of sporulation, iosA, and that cooverexpression of just two of these genes, iosA and abaA6, was sufficient to recapitulate the bldB mutant phenotype. Further RNA-seq analysis showed that the transcription factor WhiJ9 is required for the activation of iosA seen in the bldB mutant, and biochemical studies showed that WhiJ9 mediates the activation of iosA expression by binding to direct repeats in the iosA-whiJ9 intergenic region. BldB and BldB9 hetero-oligomerize, providing a potential link between BldB and the iosA-whiJ9-bldB9 locus. This work greatly expands our overall understanding of the global effects of the BldB developmental regulator. IMPORTANCE To reproduce and disperse, the filamentous bacterium Streptomyces develops specialized reproductive structures called aerial hyphae. The formation of these structures is controlled by the bld (bald) genes, many of which encode transcription factors whose functions have been characterized. An exception is BldB, a protein whose biochemical function is unknown. In this study, we gain insight into the global effects of BldB function by examining the genome-wide transcriptional effects of deleting bldB. We identify a small set of genes that are dramatically upregulated in the absence of BldB. We show that their overexpression causes the bldB phenotype and characterize a transcription factor that mediates the upregulation of one of these target genes. Our results provide new insight into how BldB influences Streptomyces development.

Streptomyces polyketides mediate bacteria-fungi interactions across soil environments

Although the interaction between prokaryotic and eukaryotic microorganisms is crucial for the functioning of ecosystems, information about the processes driving microbial interactions within communities remains scarce. Here we show that arginine-derived polyketides (arginoketides) produced by Streptomyces species mediate cross-kingdom microbial interactions with fungi of the genera Aspergillus and Penicillium, and trigger the production of natural products. Arginoketides can be cyclic or linear, and a prominent example is azalomycin F produced by Streptomyces iranensis, which induces the cryptic orsellinic acid gene cluster in Aspergillus nidulans. Bacteria that synthesize arginoketides and fungi that decode and respond to this signal were co-isolated from the same soil sample. Genome analyses and a literature search indicate that arginoketide producers are found worldwide. Because, in addition to their direct impact, arginoketides induce a secondary wave of fungal natural products, they probably contribute to the wider structure and functioning of entire soil microbial communities.

Streptomyces RNases - Function and impact on antibiotic synthesis

Streptomyces are soil dwelling bacteria that are notable for their ability to sporulate and to produce antibiotics and other secondary metabolites. Antibiotic biosynthesis is controlled by a variety of complex regulatory networks, involving activators, repressors, signaling molecules and other regulatory elements. One group of enzymes that affects antibiotic synthesis in Streptomyces is the ribonucleases. In this review, the function of five ribonucleases, RNase E, RNase J, polynucleotide phosphorylase, RNase III and oligoribonuclease, and their impact on antibiotic production will be discussed. Mechanisms for the effects of RNase action on antibiotic synthesis are proposed.

Sulfane Sulfur Posttranslationally Modifies the Global Regulator AdpA to Influence Actinorhodin Production and Morphological Differentiation of Streptomyces coelicolor

The transcription factor AdpA is a key regulator controlling both secondary metabolism and morphological differentiation in Streptomyces. Due to its critical functions, its expression undergoes multilevel regulations at transcriptional, posttranscriptional, and translational levels, yet no posttranslational regulation has been reported. Sulfane sulfur, such as hydro polysulfide (HS_nH, n ≥ 2) and organic polysulfide (RS_nH, n ≥ 2), is common inside microorganisms, but its physiological functions are largely unclear. Here, we discovered that sulfane sulfur posttranslationally modifies AdpA in Streptomyces coelicolor via specifically reacting with Cys⁶² of AdpA to form a persulfide (Cys⁶²-SSH). This modification decreases the affinity of AdpA to its self-promoter P_adpA, allowing increased expression of adpA, further promoting the expression of its target genes actII-4 and wblA. ActII-4 activates actinorhodin biosynthesis, and WblA regulates morphological development. Bioinformatics analyses indicated that AdpA-Cys⁶² is highly conserved in Streptomyces, suggesting the prevalence of such modification in this genus. Thus, our study unveils a new type of regulation on the AdpA activity and sheds a light on how sulfane sulfur stimulates the production of antibiotics in Streptomyces.

AdpA, a developmental regulator, promotes ε-poly-l-lysine biosynthesis in Streptomyces albulus

Background

AdpA is a global regulator of morphological differentiation and secondary metabolism in Streptomyces, but the regulatory roles of the Streptomyces AdpA family on the biosynthesis of the natural product ε-poly-l-lysine (ε-PL) remain unidentified, and few studies have focused on increasing the production of ε-PL by manipulating transcription factors in Streptomyces.

Results

In this study, we revealed the regulatory roles of different AdpA homologs in ε-PL biosynthesis and morphological differentiation and effectively promoted ε-PL production and sporulation in Streptomycesalbulus NK660 by heterologously expressing adpA from S.neyagawaensis NRRLB-3092 (adpA_Sn). First, we identified a novel AdpA homolog named AdpA_Sa in S.albulus NK660 and characterized its function as an activator of ε-PL biosynthesis and morphological differentiation. Subsequently, four heterologous AdpA homologs were selected to investigate their phylogenetic relationships and regulatory roles in S.albulus, and AdpA_Sn was demonstrated to have the strongest ability to promote both ε-PL production and sporulation among these five AdpA proteins. The ε-PL yield of S.albulus heterologously expressing adpA_Sn was approximately 3.6-fold higher than that of the control strain. Finally, we clarified the mechanism of AdpA_Sn in enhancing ε-PL biosynthesis and its effect on ε-PL polymerization degree using real-time quantitative PCR, microscale thermophoresis and MALDI-TOF–MS. AdpA_Sn was purified, and its seven direct targets, zwf, tal, pyk2, pta, ack, pepc and a transketolase gene (DC74_2409), were identified, suggesting that AdpA_Sn may cause the redistribution of metabolic flux in central metabolism pathways, which subsequently provides more carbon skeletons and ATP for ε-PL biosynthesis in S.albulus.

Conclusions

Here, we characterized the positive regulatory roles of Streptomyces AdpA homologs in ε-PL biosynthesis and their effects on morphological differentiation and reported for the first time that AdpA_Sn promotes ε-PL biosynthesis by affecting the transcription of its target genes in central metabolism pathways. These findings supply valuable insights into the regulatory roles of the Streptomyces AdpA family on ε-PL biosynthesis and morphological differentiation and suggest that AdpA_Sn may be an effective global regulator for enhanced production of ε-PL and other valuable secondary metabolites in Streptomyces.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01785-6.

Production of Epoxyketone Peptide-Based Proteasome Inhibitors by Streptomyces sp. BRA-346: Regulation and Biosynthesis

Streptomyces sp. BRA-346 is an Actinobacteria isolated from the Brazilian endemic tunicate Euherdmania sp. We have reported that this strain produces epoxyketone peptides, as dihydroeponemycin (DHE) and structurally related analogs. This cocktail of epoxyketone peptides inhibits the proteasome chymotrypsin-like activity and shows high cytotoxicity to glioma cells. However, low yields and poor reproducibility of epoxyketone peptides production by BRA-346 under laboratory cultivation have limited the isolation of epoxyketone peptides for additional studies. Here, we evaluated several cultivation methods using different culture media and chemical elicitors to increase the repertoire of peptide epoxyketone production by this bacterium. Furthermore, BRA-346 genome was sequenced, revealing its broad genetic potential, which is mostly hidden under laboratory conditions. By using specific growth conditions, we were able to evidence different classes of secondary metabolites produced by BRA-346. In addition, by combining genome mining with untargeted metabolomics, we could link the metabolites produced by BRA-346 to its genetic capacity and potential regulators. A single biosynthetic gene cluster (BGC) was related to the production of the target epoxyketone peptides by BRA-346. The candidate BGC displays conserved biosynthetic enzymes with the reported eponemycin (EPN) and TMC-86A (TMC) BGCs. The core of the putative epoxyketone peptide BGC (ORFs A-L), in which ORF A is a LuxR-like transcription factor, was cloned into a heterologous host. The recombinant organism was capable to produce TMC and EPN natural products, along with the biosynthetic intermediates DH-TMC and DHE, and additional congeners. A phylogenetic analysis of the epn/tmc BGC revealed related BGCs in public databases. Most of them carry a proteasome beta-subunit, however, lacking an assigned specialized metabolite. The retrieved BGCs also display a diversity of regulatory genes and TTA codons, indicating tight regulation of this BGC at the transcription and translational levels. These results demonstrate the plasticity of the epn/tmc BGC of BRA-346 in producing epoxyketone peptides and the feasibility of their production in a heterologous host. This work also highlights the capacity of BRA-346 to tightly regulate its secondary metabolism and shed light on how to awake silent gene clusters of Streptomyces sp. BRA-346 to allow the production of pharmacologically important biosynthetic products.

AdpA Positively Regulates Morphological Differentiation and Chloramphenicol Biosynthesis in Streptomyces venezuelae

In members of genus Streptomyces, AdpA is a master transcriptional regulator that controls the expression of hundreds of genes involved in morphological differentiation, secondary metabolite biosynthesis, chromosome replication, etc. However, the function of AdpASv, an AdpA ortholog of Streptomyces venezuelae, is unknown. This bacterial species is a natural producer of chloramphenicol and has recently become a model organism for studies on Streptomyces. Here, we demonstrate that AdpASv is essential for differentiation and antibiotic biosynthesis in S. venezuelae and provide evidence suggesting that AdpASv positively regulates its own gene expression. We speculate that the different modes of AdpA-dependent transcriptional autoregulation observed in S. venezuelae and other Streptomyces species reflect the arrangement of AdpA binding sites in relation to the transcription start site. Lastly, we present preliminary data suggesting that AdpA may undergo a proteolytic processing and we speculate that this may potentially constitute a novel regulatory mechanism controlling cellular abundance of AdpA in Streptomyces. IMPORTANCEStreptomyces are well-known producers of valuable secondary metabolites which include a large variety of antibiotics and important model organisms for developmental studies in multicellular bacteria. The conserved transcriptional regulator AdpA of Streptomyces exerts a pleiotropic effect on cellular processes, including the morphological differentiation and biosynthesis of secondary metabolites. Despite extensive studies, the function of AdpA in these processes remains elusive. This work provides insights into the role of a yet unstudied AdpA ortholog of Streptomyces venezuelae, now considered a novel model organism. We found that AdpA plays essential role in morphological differentiation and biosynthesis of chloramphenicol, a broad-spectrum antibiotic. We also propose that AdpA may undergo a proteolytic processing that presumably constitutes a novel mechanism regulating cellular abundance of this master regulator.

Beyond Self-Resistance: ABCF ATPase LmrC Is a Signal-Transducing Component of an Antibiotic-Driven Signaling Cascade Accelerating the Onset of Lincomycin Biosynthesis

In natural environments, antibiotics are important means of interspecies competition. At subinhibitory concentrations, they act as cues or signals inducing antibiotic production; however, our knowledge of well-documented antibiotic-based sensing systems is limited. Here, for the soil actinobacterium Streptomyces lincolnensis, we describe a fundamentally new ribosome-mediated signaling cascade that accelerates the onset of lincomycin production in response to an external ribosome-targeting antibiotic to synchronize antibiotic production within the population. The entire cascade is encoded in the lincomycin biosynthetic gene cluster (BGC) and consists of three lincomycin resistance proteins in addition to the transcriptional regulator LmbU: a lincomycin transporter (LmrA), a 23S rRNA methyltransferase (LmrB), both of which confer high resistance, and an ATP-binding cassette family F (ABCF) ATPase, LmrC, which confers only moderate resistance but is essential for antibiotic-induced signal transduction. Specifically, antibiotic sensing occurs via ribosome-mediated attenuation, which activates LmrC production in response to lincosamide, streptogramin A, or pleuromutilin antibiotics. Then, ATPase activity of the ribosome-associated LmrC triggers the transcription of lmbU and consequently the expression of lincomycin BGC. Finally, the production of LmrC is downregulated by LmrA and LmrB, which reduces the amount of ribosome-bound antibiotic and thus fine-tunes the cascade. We propose that analogous ABCF-mediated signaling systems are relatively common because many ribosome-targeting antibiotic BGCs encode an ABCF protein accompanied by additional resistance protein(s) and transcriptional regulators. Moreover, we revealed that three of the eight coproduced ABCF proteins of S. lincolnensis are clindamycin responsive, suggesting that the ABCF-mediated antibiotic signaling may be a widely utilized tool for chemical communication. IMPORTANCE Resistance proteins are perceived as mechanisms protecting bacteria from the inhibitory effect of their produced antibiotics or antibiotics from competitors. Here, we report that antibiotic resistance proteins regulate lincomycin biosynthesis in response to subinhibitory concentrations of antibiotics. In particular, we show the dual character of the ABCF ATPase LmrC, which confers antibiotic resistance and simultaneously transduces a signal from ribosome-bound antibiotics to gene expression, where the 5' untranslated sequence upstream of its encoding gene functions as a primary antibiotic sensor. ABCF-mediated antibiotic signaling can in principle function not only in the induction of antibiotic biosynthesis but also in selective gene expression in response to any small molecules targeting the 50S ribosomal subunit, including clinically important antibiotics, to mediate intercellular antibiotic signaling and stress response induction. Moreover, the resistance-regulatory function of LmrC presented here for the first time unifies functionally inconsistent ABCF family members involving antibiotic resistance proteins and translational regulators.

Genetic Network Architecture and Environmental Cues Drive Spatial Organization of Phenotypic Division of Labor in Streptomyces coelicolor

A number of bacteria are known to differentiate into cells with distinct phenotypic traits during processes such as biofilm formation or the development of reproductive structures. These cell types, by virtue of their specialized functions, embody a division of labor. However, how bacteria build spatial patterns of differentiated cells is not well understood. Here, we examine the factors that drive spatial patterns in divisions of labor in colonies of Streptomyces coelicolor, a multicellular bacterium capable of synthesizing an array of antibiotics and forming complex reproductive structures (e.g., aerial hyphae and spores). Using fluorescent reporters, we demonstrate that the pathways for antibiotic biosynthesis and aerial hypha formation are activated in distinct waves of gene expression that radiate outwards in S. coelicolor colonies. We also show that the spatiotemporal separation of these cell types depends on a key activator in the developmental pathway, AdpA. Importantly, when we manipulated local gradients by growing competing microbes nearby, or through physical disruption, expression in these pathways could be decoupled and/or disordered, respectively. Finally, the normal spatial organization of these cell types was partially restored with the addition of a siderophore, a public good made by these organisms, to the growth medium. Together, these results indicate that spatial divisions of labor in S. coelicolor colonies are determined by a combination of physiological gradients and regulatory network architecture, key factors that also drive patterns of cellular differentiation in multicellular eukaryotic organisms.

Eliciting the silent lucensomycin biosynthetic pathway in Streptomyces cyanogenus S136 via manipulation of the global regulatory gene adpA

Actinobacteria are among the most prolific sources of medically and agriculturally important compounds, derived from their biosynthetic gene clusters (BGCs) for specialized (secondary) pathways of metabolism. Genomics witnesses that the majority of actinobacterial BGCs are silent, most likely due to their low or zero transcription. Much effort is put into the search for approaches towards activation of silent BGCs, as this is believed to revitalize the discovery of novel natural products. We hypothesized that the global transcriptional factor AdpA, due to its highly degenerate operator sequence, could be used to upregulate the expression of silent BGCs. Using Streptomyces cyanogenus S136 as a test case, we showed that plasmids expressing either full-length adpA or its DNA-binding domain led to significant changes in the metabolome. These were evident as changes in the accumulation of colored compounds, bioactivity, as well as the emergence of a new pattern of secondary metabolites as revealed by HPLC-ESI-mass spectrometry. We further focused on the most abundant secondary metabolite and identified it as the polyene antibiotic lucensomycin. Finally, we uncovered the entire gene cluster for lucensomycin biosynthesis (lcm), that remained elusive for five decades until now, and outlined an evidence-based scenario for its adpA-mediated activation.

Systems and synthetic biology to elucidate secondary metabolite biosynthetic gene clusters encoded in Streptomyces genomes

Covering: 2010 to 2020 Over the last few decades, Streptomyces have been extensively investigated for their ability to produce diverse bioactive secondary metabolites. Recent advances in Streptomyces research have been largely supported by improvements in high-throughput technology 'omics'. From genomics, numerous secondary metabolite biosynthetic gene clusters were predicted, increasing their genomic potential for novel bioactive compound discovery. Additional omics, including transcriptomics, translatomics, interactomics, proteomics and metabolomics, have been applied to obtain a system-level understanding spanning entire bioprocesses of Streptomyces, revealing highly interconnected and multi-layered regulatory networks for secondary metabolism. The comprehensive understanding derived from this systematic information accelerates the rational engineering of Streptomyces to enhance secondary metabolite production, integrated with the exploitation of the highly efficient 'Design-Build-Test-Learn' cycle in synthetic biology. In this review, we describe the current status of omics applications in Streptomyces research to better understand the organism and exploit its genetic potential for higher production of valuable secondary metabolites and novel secondary metabolite discovery.

LeuRS can leucylate type I and type II tRNALeus in Streptomyces coelicolor

Transfer RNAs (tRNAs) are divided into two types, type I with a short variable loop and type II with a long variable loop. Aminoacylation of type I or type II tRNA^Leu is catalyzed by their cognate leucyl-tRNA synthetases (LeuRSs). However, in Streptomyces coelicolor, there are two types of tRNA^Leu and only one LeuRS (ScoLeuRS). We found that the enzyme could leucylate both types of ScotRNA^Leu, and had a higher catalytic efficiency for type II ScotRNA^Leu(UAA) than for type I ScotRNA^Leu(CAA). The results from tRNA and enzyme mutagenesis showed that ScoLeuRS did not interact with the canonical discriminator A73. The number of nucleotides, rather than the type of base of the variable loop in the two types of ScotRNA^Leus, was determined as important for aminoacylation. In vitro and in vivo assays showed that the tertiary structure formed by the D-loop and TψC-loop is more important for ScotRNA^Leu(UAA)_. We showed that the leucine-specific domain (LSD) of ScoLeuRS could help LeuRS, which originally only leucylates type II tRNA^Leu, to aminoacylate type I ScotRNA^Leu(CAA) and identified the crucial amino acid residues at the C-terminus of the LSD to recognize type I ScotRNA^Leu(CAA). Overall, our findings identified a rare recognition mechanism of LeuRS to tRNA^Leu.

The genome sequence of Streptomyces rochei 7434AN4, which carries a linear chromosome and three characteristic linear plasmids

Streptomyces rochei 7434AN4 produces two structurally unrelated polyketide antibiotics, lankacidin and lankamycin, and carries three linear plasmids, pSLA2-L (211 kb), -M (113 kb), and -S (18 kb), whose nucleotide sequences were previously reported. The complete nucleotide sequence of the S. rochei chromosome has now been determined using the long-read PacBio RS-II sequencing together with short-read Illumina Genome Analyzer IIx sequencing and Roche 454 pyrosequencing techniques. The assembled sequence revealed an 8,364,802-bp linear chromosome with a high G + C content of 71.7% and 7,568 protein-coding ORFs. Thus, the gross genome size of S. rochei 7434AN4 was confirmed to be 8,706,406 bp including the three linear plasmids. Consistent with our previous study, a tap-tpg gene pair, which is essential for the maintenance of a linear topology of Streptomyces genomes, was not found on the chromosome. Remarkably, the S. rochei chromosome contains seven ribosomal RNA (rrn) operons (16S-23S-5S), although Streptomyces species generally contain six rrn operons. Based on 2ndFind and antiSMASH platforms, the S. rochei chromosome harbors at least 35 secondary metabolite biosynthetic gene clusters, including those for the 28-membered polyene macrolide pentamycin and the azoxyalkene compound KA57-A.

Gene miaA for post-transcriptional modification of tRNAXXA is important for morphological and metabolic differentiation in Streptomyces

Members of actinobacterial genus Streptomyces possess a sophisticated life cycle and are the deepest source of bioactive secondary metabolites. Although morphogenesis and secondary metabolism are subject to transcriptional co-regulation, streptomycetes employ an additional mechanism to initiate the aforementioned processes. This mechanism is based on delayed translation of rare leucyl codon UUA by the only cognate tRNA^Leu _UAA (encoded by bldA). The bldA-based genetic switch is an extensively documented example of translational regulation in Streptomyces. Yet, after five decades since the discovery of bldA, factors that shape its function and peculiar conditionality remained elusive. Here we address the hypothesis that post-transcriptional tRNA modifications play a role in tRNA-based mechanisms of translational control in Streptomyces. Particularly, we studied two Streptomyces albus J1074 genes, XNR_1074 (miaA) and XNR_1078 (miaB), encoding tRNA (adenosine(37)-N6)-dimethylallyltransferase and tRNA (N6-isopentenyl adenosine(37)-C2)-methylthiotransferase respectively. These enzymes produce, in a sequential manner, a hypermodified ms² i⁶ A37 residue in most of the A36-A37-containing tRNAs. We show that miaB and especially miaA null mutant of S. albus possess altered morphogenesis and secondary metabolism. We provide genetic evidence that miaA deficiency impacts translational level of gene expression, most likely through impaired decoding of codons UXX and UUA in particular.

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.

Genome mining reveals the origin of a bald phenotype and a cryptic nucleocidin gene cluster in Streptomyces asterosporus DSM 41452

Streptomyces asterosporus DSM 41452 is a producer of the polyketide annimycin and the non-ribosomal depsipeptide WS9326A. This strain is also notable for exhibiting a bald phenotype that is devoid of spores and aerial mycelium when grown on solid media. Based on the similarity of the 16S rRNA sequence to Streptomyces calvus, the only known producer of the fluorometabolite nucleocidin, the genome of S. asterosporus DSM 41452 was sequenced and analyzed. Twenty-nine natural product gene clusters were detected in the genome, including a gene cluster predicted to encode the fluorometabolite nucleocidin. Through genome analysis and gene complementation experiments, we demonstrate that the bald phenotype arises from a transposon gene inserted within the promoter sequence for the pleiotropic regulator adpA. Complementation of S. asterosporus DSM 41452 with a functional adpA sequence restored morphological differentiation and promoted the production of nucleocidin.

Heterologous AdpA transcription factors enhance landomycin production in Streptomyces cyanogenus S136 under a broad range of growth conditions

Streptomyces cyanogenus S136 is the only known producer of landomycin A (LaA), one of the largest glycosylated angucycline antibiotics possessing strong antiproliferative properties. There is rising interest in elucidation of mechanisms of action of landomycins, which, in turn, requires access to large quantities of the pure compounds. Overproduction of LaA has been achieved in the past through manipulation of cluster-situated regulatory genes. However, other components of the LaA biosynthetic regulatory network remain unknown. To fill this gap, we elucidated the contribution of AdpA family pleiotropic regulators in landomycin production via expression of adpA genes of different origins in S. cyanogenus S136. Overexpression of the native S. cyanogenus S136 adpA ortholog had no effect on landomycin titers. In the same time, expression of several heterologous adpA genes led to significantly increased landomycin production under different cultivation conditions. Hence, heterologous adpA genes are a useful tool to enhance or activate landomycin production by S. cyanogenus. Our ongoing research effort is focused on identification of mutations that render S. cyanogenus AdpA nonfunctional.

Global regulator BldA regulates morphological differentiation and lincomycin production in Streptomyces lincolnensis

Global regulator BldA, the only tRNA for a rare leucine codon UUA, is best known for its ability to affect morphological differentiation and secondary metabolism in the genus Streptomyces. In this study, we confirmed the regulatory function of the bldA gene (Genbank accession no. EU124663.1) in Streptomyces lincolnensis. Disruption of bldA hinders the sporulation and lincomycin production, that can recur when complemented with a functional bldA gene. Western blotting assays demonstrate that translation of the lmbB2 gene which encodes a L-tyrosine hydroxylase is absolutely dependent on BldA; however, mistranslation of the lmbU gene which encodes a cluster-situated regulator (CSR) is observed in a bldA mutant. Intriguingly, when the preferential cognate codon CTG was used, the expression level of LmbU was not the highest compared to the usage of rare codon TTA or CTA, indicating the rare codon in this position is significant for the regulation of lmbU expression. Moreover, replacement of TTA codons in both genes with another leucin codon in the bldA mutant did not restore lincomycin production. Thus, we believe that the bldA gene regulates lincomycin production via controlling the translation of not only lmbB2 and lmbU, but also the other TTA-containing genes. In conclusion, the present study demonstrated the importance of the bldA gene in morphological differentiation and lincomycin production in S. lincolnensis.

Genomic Insights into Evolution of AdpA Family Master Regulators of Morphological Differentiation and Secondary Metabolism in Streptomyces

The AdpA protein from a streptomycin producer Streptomyces griseus is a founding member of the AdpA family of pleiotropic regulators, known to be ubiquitously present in streptomycetes. Functional genomic approaches revealed a huge number of AdpA targets, leading to the claim that the AdpA regulon is the largest one in bacteria. The expression of adpA is limited at the level of translation of the rare leucyl UUA codon. All known properties of AdpA regulators were discovered on a few streptomycete strains. There are open questions about the true abundance and diversity of AdpA across actinobacterial taxa (and beyond) and about the possible evolutionary forces that shape the AdpA orthologous group in Streptomyces. Here we show that, with respect to the TTA codon, streptomycete adpA is more diverse than has been previously thought, as the genes differ in presence/position of this codon. Reciprocal best hits to AdpA can be found in many actinobacterial orders, with a domain organization resembling that of the prototypical AdpA, but other configurations also exist. Diversifying positive selection was detected within the DNA-binding (AraC) domain in adpA of Streptomyces origin, most likely affecting residues enabling AdpA to recognize a degenerate operator. Sequence coding for putative glutamine amidotransferase (GATase-1) domain also shows signs of positive selection. The two-domain organization of AdpA most likely arose from a fusion of genes encoding separate GATase-1 and AraC domains. Indeed, we show that the AraC domain retains a biological function in the absence of the GATase-1 part. We suggest that acquisition of the regulatory role by TTA codon is a relatively recent event in the evolution of AdpA, which coincided with the rise of the Streptomycetales clade and, at present, is under relaxed selective constraints. Further experimental scrutiny of our findings is invited, which should provide new insights into the evolution and prospects for engineering of an AdpA-centered regulatory network.

Identification of truncated form of NosP as a transcription factor to regulate the biosynthesis of nosiheptide

Nosiheptide (NOS), a typical member of the thiopeptides, possesses strong activities against multidrug-resistant, gram-positive bacterial pathogens. Similar to other thiopeptides, the biosynthetic pathway of NOS belongs to a ribosomally synthesized and posttranslationally modified peptide system. Bioinformatics analysis of the NOS gene cluster suggests that nosP gene encodes a homologous protein of the Streptomyces antibiotic regulatory protein (SARP) family. In the present study, the actual initiation codon of nosP was identified by comparison of potential initiation codons GUG and AUG. In contrast to previous predictions of starting with GUG, AUG, corresponding to methionine residue as the 53rd residue in the original sequence, is actually the initiation codon of nosP, indicating that a truncated form of NosP (NosP_53-323) is a functional protein. For better understanding of the transcriptional regulation for NOS biosynthesis, the binding region was subsequently investigated with NosP_53-323, demonstrating that NosP_53-323 specifically binds the bidirectional nosL-nosM promoter region. Additionally, NosP_53-323 was confirmed to serve as a transcription factor to activate the transcription of all 15 structural genes in the gene cluster. The present study provides new insights into pathway-specific regulation of the biosynthesis of NOS, which would be beneficial to the investigation of the regulatory function of similar SARP proteins in the gene clusters of other thiopeptides.-Wu, X., Jin, L., Zhang, H., Tong, R., Ma, M., Chen, Y. Identification of truncated form of NosP as a transcription factor to regulate the biosynthesis of nosiheptide.

Cyclic Dinucleotide-Controlled Regulatory Pathways in Streptomyces Species

The cyclic dinucleotides cyclic 3′,5′-diguanylate (c-di-GMP) and cyclic 3′,5′-diadenylate (c-di-AMP) have emerged as key components of bacterial signal transduction networks. These closely related second messengers follow the classical general principles of nucleotide signaling by integrating diverse signals into regulatory pathways that control cellular responses to changing environments. They impact distinct cellular processes, with c-di-GMP having an established role in promoting bacterial adhesion and inhibiting motility and c-di-AMP being involved in cell wall metabolism, potassium homeostasis, and DNA repair. The involvement of c-dinucleotides in the physiology of the filamentous, nonmotile streptomycetes remained obscure until recent discoveries showed that c-di-GMP controls the activity of the developmental master regulator BldD and that c-di-AMP determines the level of the resuscitation-promoting factor A(RpfA) cell wall-remodelling enzyme. Here, I summarize our current knowledge of c-dinucleotide signaling in Streptomyces species and highlight the important roles of c-di-GMP and c-di-AMP in the biology of these antibiotic-producing, multicellular bacteria.

Proteomics analysis of global regulatory cascades involved in clavulanic acid production and morphological development in Streptomyces clavuligerus

The genus Streptomyces comprises bacteria that undergo a complex developmental life cycle and produce many metabolites of importance to industry and medicine. Streptomyces clavuligerus produces the β-lactamase inhibitor clavulanic acid, which is used in combination with β-lactam antibiotics to treat certain β-lactam resistant bacterial infections. Many aspects of how clavulanic acid production is globally regulated in S. clavuligerus still remains unknown. We conducted comparative proteomics analysis using the wild type strain of S. clavuligerus and two mutants (ΔbldA and ΔbldG), which are defective in global regulators and vary in their ability to produce clavulanic acid. Approximately 33.5 % of the predicted S. clavuligerus proteome was detected and 192 known or putative regulatory proteins showed statistically differential expression levels in pairwise comparisons. Interestingly, the expression of many proteins whose corresponding genes contain TTA codons (predicted to require the bldA tRNA for translation) was unaffected in the bldA mutant.

Taxonomy, Physiology, and Natural Products of Actinobacteria

Actinobacteria are Gram-positive bacteria with high G+C DNA content that constitute one of the largest bacterial phyla, and they are ubiquitously distributed in both aquatic and terrestrial ecosystems. Many Actinobacteria have a mycelial lifestyle and undergo complex morphological differentiation. They also have an extensive secondary metabolism and produce about two-thirds of all naturally derived antibiotics in current clinical use, as well as many anticancer, anthelmintic, and antifungal compounds. Consequently, these bacteria are of major importance for biotechnology, medicine, and agriculture. Actinobacteria play diverse roles in their associations with various higher organisms, since their members have adopted different lifestyles, and the phylum includes pathogens (notably, species of Corynebacterium, Mycobacterium, Nocardia, Propionibacterium, and Tropheryma), soil inhabitants (e.g., Micromonospora and Streptomyces species), plant commensals (e.g., Frankia spp.), and gastrointestinal commensals (Bifidobacterium spp.). Actinobacteria also play an important role as symbionts and as pathogens in plant-associated microbial communities. This review presents an update on the biology of this important bacterial phylum.

c-di-GMP signalling and the regulation of developmental transitions in streptomycetes

The complex life cycle of streptomycetes involves two distinct filamentous cell forms: the growing (or vegetative) hyphae and the reproductive (or aerial) hyphae, which differentiate into long chains of spores. Until recently, little was known about the signalling pathways that regulate the developmental transitions leading to sporulation. In this Review, we discuss important new insights into these pathways that have led to the emergence of a coherent regulatory network, focusing on the erection of aerial hyphae and the synchronous cell division event that produces dozens of unigenomic spores. In particular, we highlight the role of cyclic di-GMP (c-di-GMP) in controlling the initiation of development, and the role of the master regulator BldD in mediating c-di-GMP signalling.

Altered desferrioxamine-mediated iron utilization is a common trait of bald mutants of Streptomyces coelicolor

Streptomyces coelicolor is an important model organism for developmental studies of filamentous GC-rich actinobacteria. The genetic characterization of mutants of S. coelicolor blocked at the vegetative mycelium stage, the so-called bald (bld) mutants that are unable to erect spore-forming aerial hyphae, has opened the way to discovering the molecular basis of development in actinomycetes. Desferrioxamine (DFO) production and import of ferrioxamines (FO; iron-complexed DFO) are key to triggering morphogenesis of S. coelicolor and we show here that growth of S. coelicolor on the reference medium for Streptomyces developmental studies is fully dependent on DFO biosynthesis. UPLC-ESI-MS analysis revealed that all bld mutants tested are affected in DFO biosynthesis, with bldA, bldJ, and ptsH mutants severely impaired in DFO production, while bldF, bldK, crr and ptsI mutants overproduce DFO. Morphogenesis of bldK and bldJ mutants was recovered by supplying exogenous iron. Transcript analysis showed that the bldJ mutant is impaired in expression of genes involved in the uptake of FO, whereas transcription of genes involved in both DFO biosynthesis and FO uptake is increased in bldK mutants. Our study allows proposing altered DFO production and/or FO uptake as a novel phenotypic marker of many S. coelicolor bld mutants, and strengthens the role of siderophores and iron acquisition in morphological development of actinomycetes.

Transcriptome-guided identification of SprA as a pleiotropic regulator in Streptomyces chattanoogensis

Quorum sensing molecular γ-butyrolactones (GBL) are widely distributed among the genus Streptomyces. Their cognate receptors have been demonstrated to control secondary metabolism and/or morphological differentiation. ScgA is responsible for the biosynthesis of GBL in Streptomyces chattanoogensis. According to the genome-wide transcriptome analysis of the ΔscgA mutant, we found that the expression of sprA, which encodes a GBL receptor homologue, was shown to be positively regulated by ScgA. Electrophoretic mobility shift assays and DNase I footprinting assays showed that SprA bound to two specific autoregulatory element (ARE) sequences located upstream of the sprA gene, indicating that its expression is self-regulated. SprA was involved in biosynthesis of GBL by repressing the expression of scgA. An Escherichia coli-based luciferase report system demonstrated that SprA directly repressed the expression of scgR, which encodes a GBL receptor. Like deletion of scgA, the disruption of sprA resulted in decreased production of the antibiotic natamycin in liquid culture and retarded morphological differentiation on solid agar. This work indicates that SprA acts as a pleiotropic regulator of both morphogenesis and the production of natamycin.

Fifty years after the replicon hypothesis: cell-specific master regulators as new players in chromosome replication control

Numerous free-living bacteria undergo complex differentiation in response to unfavorable environmental conditions or as part of their natural cell cycle. Developmental programs require the de novo expression of several sets of genes responsible for morphological, physiological, and metabolic changes, such as spore/endospore formation, the generation of flagella, and the synthesis of antibiotics. Notably, the frequency of chromosomal replication initiation events must also be adjusted with respect to the developmental stage in order to ensure that each nascent cell receives a single copy of the chromosomal DNA. In this review, we focus on the master transcriptional factors, Spo0A, CtrA, and AdpA, which coordinate developmental program and which were recently demonstrated to control chromosome replication. We summarize the current state of knowledge on the role of these developmental regulators in synchronizing the replication with cell differentiation in Bacillus subtilis, Caulobacter crescentus, and Streptomyces coelicolor, respectively.

Thaxtomin A production and virulence are controlled by several bld gene global regulators in Streptomyces scabies

Streptomyces scabies is the main causative agent of common scab disease, which leads to significant annual losses to potato growers worldwide. The main virulence factor produced by S. scabies is a phytotoxic secondary metabolite called thaxtomin A, which functions as a cellulose synthesis inhibitor. Thaxtomin A production is controlled by the cluster-situated regulator TxtR, which activates expression of the thaxtomin biosynthetic genes in response to cello-oligosaccharides. Here, we demonstrate that at least five additional regulatory genes are required for wild-type levels of thaxtomin A production and plant pathogenicity in S. scabies. These regulatory genes belong to the bld gene family of global regulators that control secondary metabolism or morphological differentiation in Streptomyces spp. Quantitative reverse-transcriptase polymerase chain reaction showed that expression of the thaxtomin biosynthetic genes was significantly downregulated in all five bld mutants and, in four of these mutants, this downregulation was attributed to the reduction in expression of txtR. Furthermore, all of the mutants displayed reduced expression of other known or predicted virulence genes, suggesting that the bld genes may function as global regulators of virulence gene expression in S. scabies.

Developmental biology of Streptomyces from the perspective of 100 actinobacterial genome sequences

To illuminate the evolution and mechanisms of actinobacterial complexity, we evaluate the distribution and origins of known Streptomyces developmental genes and the developmental significance of actinobacteria-specific genes. As an aid, we developed the Actinoblast database of reciprocal blastp best hits between the Streptomyces coelicolor genome and more than 100 other actinobacterial genomes (http://streptomyces.org.uk/actinoblast/). We suggest that the emergence of morphological complexity was underpinned by special features of early actinobacteria, such as polar growth and the coupled participation of regulatory Wbl proteins and the redox-protecting thiol mycothiol in transducing a transient nitric oxide signal generated during physiologically stressful growth transitions. It seems that some cell growth and division proteins of early actinobacteria have acquired greater importance for sporulation of complex actinobacteria than for mycelial growth, in which septa are infrequent and not associated with complete cell separation. The acquisition of extracellular proteins with structural roles, a highly regulated extracellular protease cascade, and additional regulatory genes allowed early actinobacterial stationary phase processes to be redeployed in the emergence of aerial hyphae from mycelial mats and in the formation of spore chains. These extracellular proteins may have contributed to speciation. Simpler members of morphologically diverse clades have lost some developmental genes.

Identified members of the Streptomyces lividans AdpA regulon involved in differentiation and secondary metabolism

BACKGROUND

AdpA is a key transcriptional regulator involved in the complex growth cycle of Streptomyces. Streptomyces are Gram-positive bacteria well-known for their production of secondary metabolites and antibiotics. Most work on AdpA has been in S. griseus, and little is known about the pathways it controls in other Streptomyces spp. We recently discovered interplay between ClpP peptidases and AdpA in S. lividans. Here, we report the identification of genes directly regulated by AdpA in S. lividans.

RESULTS

Microarray experiments revealed that the expression of hundreds of genes was affected in a S. lividans adpA mutant during early stationary phase cultures in YEME liquid medium. We studied the expression of the S. lividans AdpA-regulated genes by quantitative real-time PCR analysis after various times of growth. In silico analysis revealed the presence of potential AdpA-binding sites upstream from these genes; electrophoretic mobility shift assays indicated that AdpA binds directly to their promoter regions. This work identifies new pathways directly controlled by AdpA and that are involved in S. lividans development (ramR, SLI7885 also known as hyaS and SLI6586), and primary (SLI0755-SLI0754 encoding CYP105D5 and Fdx4) or secondary (cchA, cchB, and hyaS) metabolism.

CONCLUSIONS

We characterised six S. lividans AdpA-dependent genes whose expression is directly activated by this pleiotropic regulator. Several of these genes are orthologous to bldA-dependent genes in S. coelicolor. Furthermore, in silico analysis suggests that over hundred genes may be directly activated or repressed by S. lividans AdpA, although few have been described as being part of any Streptomyces AdpA regulons. This study increases the number of known AdpA-regulated pathways in Streptomyces spp.

Angucyclines as signals modulate the behaviors of Streptomyces coelicolor

The angucycline antibiotic jadomycin B (JdB) produced by Streptomyces venezuelae has been found here to induce complex survival responses in Streptomyces coelicolor at subinhibitory concentration. The receptor for JdB was identified as a "pseudo" gamma-butyrolactone receptor, ScbR2, which was shown to bind two previously unidentified target promoters, those of redD (redDp) and adpA (adpAp), thus directly regulating undecylprodigiosin (Red) production and morphological differentiation, respectively. Because AdpA also directly regulates the expression of redD, ScbR2, AdpA, and RedD together form a feed-forward loop controlling both differentiation and Red production phenotypes. Different signal strengths (i.e., JdB concentrations) were shown to induce the two different phenotypes by modulating the relative transcription levels of adpA vs. redD. The induction of morphological differentiation and endogenous antibiotic production by exogenous antibiotic exemplifies an important survival strategy more sophisticated than the induction of antibiotic resistance.

The gamma-butyrolactone receptors BulR1 and BulR2 of Streptomyces tsukubaensis: tacrolimus (FK506) and butyrolactone synthetases production control

Streptomyces tsukubaensis is a well-established industrial tacrolimus producer strain, but its molecular genetics is very poorly known. This information shortage prevents the development of tailored mutants in the regulatory pathways. A region (named bul) contains several genes involved in the synthesis and control of the gamma-butyrolactone autoregulator molecules. This region contains ten genes (bulA, bulZ, bulY, bulR2, bulS2, bulR1, bulW, bluB, bulS1, bulC) including two γ-butyrolactone receptor homologues (bulR1, bulR2), two putative gamma-butyrolactone synthetase homologues (bulS1, bulS2) and two SARP regulatory genes (bulY, bulZ). Analysis of the autoregulatory element (ARE)-like sequences by electrophoretic mobility shift assays and footprinting using the purified BulR1 and BulR2 recombinant proteins revealed six ARE regulatory sequences distributed along the bul cluster. These sequences showed specific binding of both BulR1 (the gamma-butyrolactone receptor) and BulR2, a possible pseudo γ-butyrolactone receptor. The protected region in all cases covered a 28-nt sequence with a palindromic structure. Optimal docking area analysis of BulR1 proved that this protein can be presented as either monomer or dimer but not oligomers and that it binds to the conserved ARE sequence in both strands. The effect on tacrolimus production was analysed by deletion of the bulR1 gene, which resulted in a strong decrease of tacrolimus production. Meanwhile, the ΔbulR2 mutation did not affect the biosynthesis of this immunosuppressant.

Controlling translation elongation efficiency: tRNA regulation of ribosome flux on the mRNA

Gene expression can be regulated by a wide variety of mechanisms. One example concerns the growing body of evidence that the protein-production rate can be regulated at the level of translation elongation by controlling ribosome flux across the mRNA. Variations in the abundance of tRNA molecules cause different rates of translation of their counterpart codons. This, in turn, produces a variable landscape of translational rate across each and every mRNA, with the dynamic formation and deformation of ribosomal queues being regulated by both tRNA availability and the rates of translation initiation and termination. In the present article, a range of examples of tRNA control of gene expression are reviewed, and the use of mathematical modelling to develop a predictive understanding of the consequences of that regulation is discussed and explained. These findings encourage a view that predicting the protein-synthesis rate of each mRNA requires a holistic understanding of how each stage of translation, including elongation, contributes to the overall protein-production rate.

Regulation of the clpP1clpP2 operon by the pleiotropic regulator AdpA in Streptomyces lividans

Insertion of an apramycin resistance cassette in the clpP1clpP2 operon (encoding the ClpP1 and ClpP2 peptidase subunits) affects morphological and physiological differentiation of Streptomyces lividans. Another key factor controlling Streptomyces differentiation is the pleiotropic transcriptional regulator AdpA. We have identified a spontaneous missense mutation (-1 frameshift) in the adpA (bldH) open reading frame in a clpP1clpP2 mutant that led to the synthesis of a non-functional AdpA protein. Electrophoretic mobility shift assays showed that AdpA bound directly to clpP1clpP2 promoter region. Quantitative real-time PCR analysis showed that AdpA regulated the clpP1clpP2 operon expression at specific growth times. In vitro, AdpA and ClgR, a transcriptional activator of clpP1clpP2 operon and other genes, were able to bind simultaneously to clpP1 promoter, which suggests that AdpA binding to clpP1 promoter did not affect that of ClgR. This study allowed to uncover an interplay between the ClpP peptidases and AdpA in S. lividans.

Pleiotropic regulatory genes bldA, adpA and absB are implicated in production of phosphoglycolipid antibiotic moenomycin

Unlike the majority of actinomycete secondary metabolic pathways, the biosynthesis of peptidoglycan glycosyltransferase inhibitor moenomycin in Streptomyces ghanaensis does not involve any cluster-situated regulators (CSRs). This raises questions about the regulatory signals that initiate and sustain moenomycin production. We now show that three pleiotropic regulatory genes for Streptomyces morphogenesis and antibiotic production—bldA, adpA and absB—exert multi-layered control over moenomycin biosynthesis in native and heterologous producers. The bldA gene for tRNA^Leu_UAA is required for the translation of rare UUA codons within two key moenomycin biosynthetic genes (moe), moeO5 and moeE5. It also indirectly influences moenomycin production by controlling the translation of the UUA-containing adpA and, probably, other as-yet-unknown repressor gene(s). AdpA binds key moe promoters and activates them. Furthermore, AdpA interacts with the bldA promoter, thus impacting translation of bldA-dependent mRNAs—that of adpA and several moe genes. Both adpA expression and moenomycin production are increased in an absB-deficient background, most probably because AbsB normally limits adpA mRNA abundance through ribonucleolytic cleavage. Our work highlights an underappreciated strategy for secondary metabolism regulation, in which the interaction between structural genes and pleiotropic regulators is not mediated by CSRs. This strategy might be relevant for a growing number of CSR-free gene clusters unearthed during actinomycete genome mining.

A cryptic polyene biosynthetic gene cluster in Streptomyces calvus is expressed upon complementation with a functional bldA gene

Streptomyces calvus is best known as the producer of the fluorinated natural product nucleocidin. This strain of Streptomycetes is also unusual for displaying a "bald" phenotype that is deficient in the formation of aerial mycelium and spores. Genome sequencing of this organism revealed a point mutation in the bldA gene that is predicted to encode a misfolded Leu-tRNA(UUA) molecule. Complementation of S. calvus with a correct copy of bldA restored sporulation and additionally promoted production of a polyeneoic acid amide, 4-Z-annimycin, and a minor amount of the isomer, 4-E-annimycin. Bioassays reveal that these compounds inhibit morphological differentiation in other Actinobacteria. The annimycin gene cluster encoding a type 1 polyketide synthase was identified and verified through disruption studies. This study underscores the importance of the bldA gene in regulating the expression of cryptic biosynthetic genes.

AdpA, key regulator for morphological differentiation regulates bacterial chromosome replication

AdpA, one of the most pleiotropic transcription regulators in bacteria, controls expression of several dozen genes during Streptomyces differentiation. Here, we report a novel function for the AdpA protein: inhibitor of chromosome replication at the initiation stage. AdpA specifically recognizes the 5′ region of the Streptomyces coelicolor replication origin (oriC). Our in vitro results show that binding of AdpA protein decreased access of initiator protein (DnaA) to the oriC region. We also found that mutation of AdpA-binding sequences increased the accessibility of oriC to DnaA, which led to more frequent replication and acceleration of Streptomyces differentiation (at the stage of aerial hyphae formation). Moreover, we also provide evidence that AdpA and DnaA proteins compete for oriC binding in an ATP-dependent manner, with low ATP levels causing preferential binding of AdpA, and high ATP levels causing dissociation of AdpA and association of DnaA. This would be consistent with a role for ATP levels in determining when aerial hyphae emerge.

Repression of antibiotic downregulator WblA by AdpA in Streptomyces coelicolor

The upstream region of antibiotic downregulatory wblA in Streptomyces coelicolor was found to contain AdpA binding motifs. A key morphological regulator, AdpA was shown to specifically bind these motifs by electrophoretic mobility shift assay. An adpA disruption mutant exhibited increased wblA transcription, suggesting that AdpA negatively regulates wblA transcription in S. coelicolor.

Molecular regulation of antibiotic biosynthesis in streptomyces

Streptomycetes are the most abundant source of antibiotics. Typically, each species produces several antibiotics, with the profile being species specific. Streptomyces coelicolor, the model species, produces at least five different antibiotics. We review the regulation of antibiotic biosynthesis in S. coelicolor and other, nonmodel streptomycetes in the light of recent studies. The biosynthesis of each antibiotic is specified by a large gene cluster, usually including regulatory genes (cluster-situated regulators [CSRs]). These are the main point of connection with a plethora of generally conserved regulatory systems that monitor the organism's physiology, developmental state, population density, and environment to determine the onset and level of production of each antibiotic. Some CSRs may also be sensitive to the levels of different kinds of ligands, including products of the pathway itself, products of other antibiotic pathways in the same organism, and specialized regulatory small molecules such as gamma-butyrolactones. These interactions can result in self-reinforcing feed-forward circuitry and complex cross talk between pathways. The physiological signals and regulatory mechanisms may be of practical importance for the activation of the many cryptic secondary metabolic gene cluster pathways revealed by recent sequencing of numerous Streptomyces genomes.

A yeast tRNA mutant that causes pseudohyphal growth exhibits reduced rates of CAG codon translation

In Saccharomyces cerevisiae, the SUP70 gene encodes the CAG-decoding tRNA(Gln)(CUG). A mutant allele, sup70-65, induces pseudohyphal growth on rich medium, an inappropriate nitrogen starvation response. This mutant tRNA is also a UAG nonsense suppressor via first base wobble. To investigate the basis of the pseudohyphal phenotype, 10 novel sup70 UAG suppressor alleles were identified, defining positions in the tRNA(Gln)(CUG) anticodon stem that restrict first base wobble. However, none conferred pseudohyphal growth, showing altered CUG anticodon presentation cannot itself induce pseudohyphal growth. Northern blot analysis revealed the sup70-65 tRNA(Gln)(CUG) is unstable, inefficiently charged, and 80% reduced in its effective concentration. A stochastic model simulation of translation predicted compromised expression of CAG-rich ORFs in the tRNA(Gln)(CUG)-depleted sup70-65 mutant. This prediction was validated by demonstrating that luciferase expression in the mutant was 60% reduced by introducing multiple tandem CAG (but not CAA) codons into this ORF. In addition, the sup70-65 pseudohyphal phenotype was partly complemented by overexpressing CAA-decoding tRNA(Gln)(UUG), an inefficient wobble-decoder of CAG. We thus show that introducing codons decoded by a rare tRNA near the 5' end of an ORF can reduce eukaryote translational expression, and that the mutant tRNA(CUG)(Gln) constitutive pseudohyphal differentiation phenotype correlates strongly with reduced CAG decoding efficiency.

Interspecies modulation of bacterial development through iron competition and siderophore piracy

While soil-dwelling actinomycetes are renowned for secreting natural products, little is known about the roles of these molecules in mediating actinomycete interactions. In a previous co-culture screen, we found that one actinomycete, Amycolatopsis sp. AA4, inhibited aerial hyphae formation in adjacent colonies of Streptomyces coelicolor. A siderophore, amychelin, mediated this developmental arrest. Here we present genetic evidence that confirms the role of the amc locus in the production of amychelin and in the inhibition of S. coelicolor development. We further characterize the Amycolatopsis sp. AA4 - S. coelicolor interaction by examining expression of developmental and iron acquisition genes over time in co-culture. Manipulation of iron availability and/or growth near Amycolatopsis sp. AA4 led to alterations in expression of the critical developmental gene bldN, and other key downstream genes in the S. coelicolor transcriptional cascade. In Amycolatopsis sp. AA4, siderophore genes were downregulated when grown near S. coelicolor, leading us to find that deferrioxamine E, produced by S. coelicolor, could be readily utilized by Amycolatopsis sp. AA4. Collectively these results suggest that competition for iron via siderophore piracy and species-specific siderophores can alter patterns of gene expression and morphological differentiation during actinomycete interactions.

Expression of the chaplin and rodlin hydrophobic sheath proteins in Streptomyces venezuelae is controlled by σ(BldN) and a cognate anti-sigma factor, RsbN

The chaplin and rodlin proteins together constitute the major components of the hydrophobic sheath that coats the aerial hyphae and spores in Streptomyces, and mutants lacking the chaplins are unable to erect aerial hyphae and differentiate on minimal media. We have gained insight into the developmental regulation of the chaplin (chp) and rodlin (rdl) genes by exploiting a new model species, Streptomyces venezuelae, which sporulates in liquid culture. Using microarrays, the chaplin and rodlin genes were found to be highly induced during submerged sporulation in a bldN-dependent manner. Using σ(BldN) ChIP-chip, we show that this dependence arises because the chaplin and rodlin genes are direct biochemical targets of σ(BldN) . sven3186 (here named rsbN for regulator of sigma BldN), the gene lying immediately downstream of bldN, was also identified as a target of σ(BldN) . Disruption of rsbN causes precocious sporulation and biochemical experiments demonstrate that RsbN functions as a σ(BldN) -specific anti-sigma factor.

Evaluation of Streptomyces coelicolor A3(2) as a heterologous expression host for the cyanobacterial protein kinase C activator lyngbyatoxin A

Filamentous marine cyanobacteria are extremely rich sources of bioactive natural products and often employ highly unusual biosynthetic enzymes in their assembly. However, the current lack of techniques for stable DNA transfer into these filamentous organisms, combined with the absence of heterologous expression strategies for nonribosomal cyanobacterial gene clusters, prohibit the creation of mutant strains or the heterologous production of these cyanobacterial compounds in other bacteria. In this study, we evaluated the capability of a derivative of the model actinomycete Streptomyces coelicolor A3(2) to express enzymes involved in the biosynthesis of the protein kinase C activator lyngbyatoxin A from a Hawaiian strain of Moorea producta (previously classified as Lyngbya majuscula). Despite large differences in GC content between these two bacteria and the presence of rare TTA/UUA leucine codons in lyngbyatoxin ORFs we were able to achieve expression of the cytochrome P450 monooxygenase LtxB and reverse prenyltransferase LtxC in S. coelicolor M512 and confirmed the in vitro functionality of S. coelicolor overexpressed LtxC. Attempts to express the entire lyngbyatoxin A gene cluster in S. coelicolor M512 were not successful because of transcript termination observed for the ltxA gene, which encodes a large nonribosomal peptide synthetase. However, these attempts did show a detectable level of cyanobacterial promoter recognition in Streptomyces. Successful expression of lyngbyatoxin A proteins in Streptomyces provides a new platform for biochemical investigation of natural product enzymes from Moorea strains.

Signals and regulators that govern Streptomyces development

Streptomyces coelicolor is the genetically best characterized species of a populous genus belonging to the gram-positive Actinobacteria. Streptomycetes are filamentous soil organisms, well known for the production of a plethora of biologically active secondary metabolic compounds. The Streptomyces developmental life cycle is uniquely complex and involves coordinated multicellular development with both physiological and morphological differentiation of several cell types, culminating in the production of secondary metabolites and dispersal of mature spores. This review presents a current appreciation of the signaling mechanisms used to orchestrate the decision to undergo morphological differentiation, and the regulators and regulatory networks that direct the intriguing development of multigenomic hyphae first to form specialized aerial hyphae and then to convert them into chains of dormant spores. This current view of S. coelicolor development is destined for rapid evolution as data from '-omics' studies shed light on gene regulatory networks, new genetic screens identify hitherto unknown players, and the resolution of our insights into the underlying cell biological processes steadily improve.