A Streptomyces griseus gene (sgaA) suppresses the growth disturbance caused by high osmolality and a high concentration of A-factor during early growth

Reviewing a plethora of oxidative-type reactions catalyzed by whole cells of Streptomyces species

Selective oxidation reactions represent a challenging task for conventional organic chemistry. Whole-cell biocatalysis provides a very convenient, easy to apply method to carry out different selective oxidation reactions including chemo-, regio-, and enantio-selective reactions. Streptomyces species are important biocatalysts as they can catalyze these selective reactions very efficiently owing to the wide diversity of enzymes and enzymatic cascades in their cell niche. In this review, we present and analyze most of the examples reported to date of oxidative reactions catalyzed by Streptomyces species as whole-cell biocatalysts. We discuss 33 different Streptomyces species and strains and the role they play in different oxidative reactions over the past five decades. The oxidative reactions have been classified into seven categories that include: hydroxylation of steroids/non-steroids, asymmetric sulfoxidations, oxidation of aldehydes, multi-step oxidations, oxidative cleavage, and N-oxidations. The role played by Streptomyces species as recombinant hosts catalyzing bio-oxidations has also been highlighted.

Structural Elucidation of Trace Components Combining GC/MS, GC/IR, DFT-Calculation and Synthesis-Salinilactones, Unprecedented Bicyclic Lactones from Salinispora Bacteria

The analysis of volatiles released by marine Salinispora bacteria uncovered a new class of natural compounds displaying an unusual bicyclic [3.1.0]-lactone skeleton. Although only sub-μg quantities of the compounds were available, the combination of analytical methods, computational spectroscopy, and synthesis allowed unambiguous structural identification of the compounds, called salinilactones, without the need for isolation. Orthogonal hyphenated methods, GC/MS and solid-phase GC/IR allowed to propose a small set of structures consistent with the data. A candidate structure was selected by comparison of DFT-calculated IR spectra and the experimental IR-spectrum. Synthesis confirmed the structure and absolute configuration of three bicyclic lactones, salinilactones A-C. The salinilactones are structurally closely related to the A-factor class of compounds, autoregulators from streptomycete bacteria. They exhibited inhibitory activity against Salinispora and Streptomyces strains.

SCO3129, a TetR family regulator, is responsible for osmotic stress in Streptomyces coelicolor

Streptomyces are the soil-dwelling bacteria with a complex lifecycle and a considerable ability to produce a variety of secondary metabolites. Osmoregulation is important for their lifecycle in nature. In the genome of Streptomyces coelicolor M145, SCO3128 (encodes a putative fatty acid desaturase), SCO3129 (encodes a putative TetR family regulator) and SCO3130 (encodes a putative l-carnitine dehydratase) constitute a transcriptional unit, and its transcript was found to be in response to osmotic stress. Disruption of SCO3130 led to a bald phenotype on MMG medium and the mycelia lysis on the edge of the colony when KCl/NaCl was added to the medium. These results indicated that SCO3130 is important for the osmotic stress resistance in S. coelicolor. Transcriptional analysis and electrophoretic mobility shift assays (EMSA) demonstrated that SCO3129 repressed the transcription of SCO3128-3130 operon through directly binding to the promoter region of SCO3128, indicating that SCO3129 regulates the transcription of SCO3128-3130 in response to osmotic stress.

Life Stage-specific Proteomes of Legionella pneumophila Reveal a Highly Differential Abundance of Virulence-associated Dot/Icm effectors

Major differences in the transcriptional program underlying the phenotypic switch between exponential and post-exponential growth of Legionella pneumophila were formerly described characterizing important alterations in infection capacity. Additionally, a third state is known where the bacteria transform in a viable but nonculturable state under stress, such as starvation. We here describe phase-related proteomic changes in exponential phase (E), postexponential phase (PE) bacteria, and unculturable microcosms (UNC) containing viable but nonculturable state cells, and identify phase-specific proteins. We present data on different bacterial subproteomes of E and PE, such as soluble whole cell proteins, outer membrane-associated proteins, and extracellular proteins. In total, 1368 different proteins were identified, 922 were quantified and 397 showed differential abundance in E/PE. The quantified subproteomes of soluble whole cell proteins, outer membrane-associated proteins, and extracellular proteins; 841, 55, and 77 proteins, respectively, were visualized in Voronoi treemaps. 95 proteins were quantified exclusively in E, such as cell division proteins MreC, FtsN, FtsA, and ZipA; 33 exclusively in PE, such as motility-related proteins of flagellum biogenesis FlgE, FlgK, and FliA; and 9 exclusively in unculturable microcosms soluble whole cell proteins, such as hypothetical, as well as transport/binding-, and metabolism-related proteins. A high frequency of differentially abundant or phase-exclusive proteins was observed among the 91 quantified effectors of the major virulence-associated protein secretion system Dot/Icm (> 60%). 24 were E-exclusive, such as LepA/B, YlfA, MavG, Lpg2271, and 13 were PE-exclusive, such as RalF, VipD, Lem10. The growth phase-related specific abundance of a subset of Dot/Icm virulence effectors was confirmed by means of Western blotting. We therefore conclude that many effectors are predominantly abundant at either E or PE which suggests their phase specific function. The distinct temporal or spatial presence of such proteins might have important implications for functional assignments in the future or for use as life-stage specific markers for pathogen analysis.

Natural products in soil microbe interactions and evolution

Gram positive bacteria from the soil have historically been a deep source of useful natural products. This article considers how natural products may mediate microbial interactions in the soil environment.

Butyrolactone I Quantification from Lovastatin Producing Aspergillus terreus Using Tandem Mass Spectrometry-Evidence of Signalling Functions

Aspergillus terreus is an industrially important filamentous fungus producing a wide spectrum of secondary metabolites, including lovastatin and itaconic acid. It also produces butyrolactone I which has shown potential as an antitumour agent. Additionally, butyrolactone I has been implicated to have a regulating role in the secondary metabolism and morphology of A. terreus. In this study, a quantitative time-course liquid chromatography—electrospray ionisation—tandem mass spectrometry (LC-ESI-MS-MS) analysis of butyrolactone I is reported for the first time in nine-day long submerged cultures of A. terreus. Butyrolactone I was fragmented in the mass analysis producing a reproducible fragmentation pattern of four main daughter ions (m/z 307, 331, 363 and 393) in all the samples tested. Supplementing the cultures with 100 nM butyrolactone I caused a statistically significant increase (up to two-fold) in its production, regardless of the growth stage but was constitutive when butyrolactone I was added at high cell density during the stationary phase. Furthermore, the extracellular butyrolactone I concentration peaked at 48 h post inoculation, showing a similar profile as has been reported for bacterial quorum sensing molecules. Taken together, the results support the idea of butyrolactone I as a quorum sensing molecule in A. terreus.

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.

DNA microarray analysis of global gene regulation by A-factor in Streptomyces griseus

A-factor (2-isocapryloyl-3R-hydroxymethyl-gamma-butyrolactone) is a microbial hormone that triggers morphological differentiation and secondary metabolism in Streptomyces griseus. The effects of A-factor on global gene expression were determined by DNA microarray analysis of transcriptomes obtained with the A-factor-deficient mutant DeltaafsA. A-factor was added at a concentration of 25 ng ml(-1) to mutant DeltaafsA at the middle of the exponential growth phase, and RNA samples were prepared from the cells grown after A-factor addition for a further 5, 15 and 30 min, and 1, 2, 4, 8 and 12 h. The effects of A-factor on transcription of all protein-coding genes of S. griseus were evaluated by comparison of the transcriptomes with those obtained from cells grown in the absence of A-factor. Analysis of variance among the transcriptomes revealed that 477 genes, which were dispersed throughout the chromosome, were differentially expressed during the 12 h after addition of A-factor, when evaluated by specific criteria. Quality threshold clustering analysis with regard to putative polycistronic transcriptional units and levels of upregulation predicted that 152 genes belonging to 74 transcriptional units were probable A-factor-inducible genes. Competitive electrophoretic mobility shift assays using DNA fragments including putative promoter regions of these 74 transcriptional units suggested that AdpA bound 37 regions to activate 72 genes in total. Many of these A-factor-inducible genes encoded proteins of unknown function, suggesting that the A-factor regulatory cascade of S. griseus affects gene expression at a specific time point more profoundly than expected.

Conditionally positive effect of the TetR-family transcriptional regulator AtrA on streptomycin production by Streptomyces griseus

AtrA, a transcriptional activator for actII-ORF4, encoding the pathway-specific transcriptional activator of the actinorhodin biosynthetic gene cluster in Streptomyces coelicolor A3(2), has been shown to bind the region upstream from the promoter of strR, encoding the pathway-specific transcriptional activator of the streptomycin biosynthetic gene cluster in Streptomyces griseus [Uguru et al. (2005) Mol Microbiol 58, 131-150]. The atrA orthologue (atrA-g) in S. griseus was constitutively transcribed throughout growth from a promoter located about 250 nt upstream of the translational start codon, as determined by S1 nuclease mapping. DNase I footprinting showed that histidine-tagged AtrA-g bound an inverted repeat located upstream of strR at positions -117 to -142 relative to the transcriptional start point of strR as +1. This AtrA-g-binding site was between two AdpA-binding sites at approximately nucleotide positions -270 and -50. AdpA is a central transcriptional activator in the A-factor regulatory cascade and essential for the transcription of strR. AtrA-g and AdpA simultaneously bound the respective binding sites. In contrast to AdpA, AtrA-g was non-essential for strR transcription; an atrA-g-disrupted strain produced streptomycin on routine agar media to the same extent as the wild-type strain. However, the atrA-g-disrupted strain tended to produce a smaller amount of streptomycin than the wild-type strain under some conditions, for example, on Bennett agar containing 1 % maltose and on a minimal medium. Therefore, AtrA-g had a conditionally positive effect on streptomycin production, as a tuner, probably by enhancing the AdpA-dependent transcriptional activation of strR in a still unknown manner.

Control of the Streptomyces Subtilisin inhibitor gene by AdpA in the A-factor regulatory cascade in Streptomyces griseus

AdpA in the A-factor regulatory cascade in Streptomyces griseus activates a number of genes required for secondary metabolism and morphological differentiation, forming an AdpA regulon. The Streptomyces subtilisin inhibitor (SSI) gene, sgiA, in S. griseus was transcribed in response to AdpA, showing that sgiA is a member of the AdpA regulon. AdpA bound a single site upstream of the sgiA promoter at approximately position -70 with respect to its transcriptional start point. Mutational analysis of the AdpA-binding site showed that the AdpA-binding site was essential for transcriptional activation. Mutants in which sgiA was disrupted had higher trypsin, chymotrypsin, metalloendopeptidase, and total protease activities than the wild-type strain, which showed that SgiA modulated the activities of these extracellularly produced proteases. Because a number of genes encoding chymotrypsins, trypsins, and metalloendopeptidases, most of which are SSI-sensitive proteases, are also under the control of AdpA, the A-factor regulatory cascade was thought to play a crucial role in modulating the extracellular protease activities by triggering simultaneous production of the proteases and their inhibitor at a specific timing during growth. Mutants in which sgiA was disrupted grew normally and formed aerial hyphae and spores with the same time course as the wild-type strain. However, exogenous addition of purified SgiA to substrate mycelium grown on agar medium resulted in a delay in aerial mycelium formation, indicating that SgiA is involved in aerial hypha formation in conjunction with proteases.

Three chymotrypsin genes are members of the AdpA regulon in the A-factor regulatory cascade in Streptomyces griseus

AdpA is a key transcriptional activator in the A-factor regulatory cascade in Streptomyces griseus, activating a number of genes required for secondary metabolism and morphological differentiation. Of the five chymotrypsin-type serine protease genes, sprA, sprB, and sprD were transcribed in response to AdpA, showing that these protease genes are members of the AdpA regulon. These proteases were predicted to play the same physiological role, since these protease genes were transcribed in a similar time course during growth and the matured enzymes showed high end-to-end similarity to one another. AdpA bound two sites upstream of the sprA promoter approximately at positions -375 and -50 with respect to the transcriptional start point of sprA. Mutational analysis of the AdpA-binding sites showed that both AdpA-binding sites were essential for transcriptional activation. AdpA bound a single site at position -50 in front of the sprB promoter and greatly enhanced the transcription of sprB. The AdpA-binding site at position -40 was essential for transcription of sprD, although there was an additional AdpA-binding site at position -180. Most chymotrypsin activity excreted by S. griseus was attributed to SprA and SprB, because mutant deltasprAB, having a deletion in both sprA and sprB, lost almost all chymotrypsin activity, as did mutant deltaadpA. Even the double mutant deltasprAB and triple mutant deltasprABD grew normally and developed aerial hyphae and spores over the same time course as the wild-type strain.

Autorepression of AdpA of the AraC/XylS family, a key transcriptional activator in the A-factor regulatory cascade in Streptomyces griseus

AdpA belonging to the AraC/XylS family is a key transcriptional activator in the A-factor regulatory cascade in Streptomyces griseus, activating a number of genes required for physiological and morphological differentiation. On the other hand, AdpA repressed its own transcription by cooperative binding to the promoter region containing multiple operator sites. AdpA contained three operator sites, site 1 approximately at nucleotide position -100, site 2 at the promoter elements, and site 3 at position +80. AdpA bound to a strong binding site 1 increased the affinity for AdpA of a weak site 2, probably by forming a DNA loop via the two molecules of AdpA dimer, thus preventing RNA polymerase from access to the promoter. AdpA bound to site 3 with rather weak affinity repressed the AdpA promoter activity independently of sites 1 and 2, perhaps preventing RNA polymerase from chain elongation. Consistent with this model, the in vivo transcription of AdpA containing mutated site 1 or site 3 was greatly increased. In addition, streptomycin production, one of the phenotypes controlled positively by AdpA, was greatly increased in the mutants containing AdpA with a mutation at site 1 and site 3. The in vitro transcription of AdpA containing mutated site 1 was also increased. Thus, the transcription of AdpA, encoding an important transcriptional factor for ordered physiological and morphological development, is self-controlled.

Regulation of sigmaB by an anti- and an anti-anti-sigma factor in Streptomyces coelicolor in response to osmotic stress

sigmaB, a homolog of stress-responsive sigmaB of Bacillus subtilis, controls both osmoprotection and differentiation in Streptomyces coelicolor A3 (2). Its gene is preceded by rsbA and rsbB genes encoding homologs of an anti-sigma factor, RsbW, and its antagonist, RsbV, of B. subtilis, respectively. Purified RsbA bound to sigmaB and prevented sigmaB-directed transcription from the sigBp1 promoter in vitro. An rsbA-null mutant exhibited contrasting behavior to the sigB mutant, with elevated sigBp1 transcription, no actinorhodin production, and precocious aerial mycelial formation, reflecting enhanced activity of sigmaB in vivo. Despite sequence similarity to RsbV, RsbB lacks the conserved phosphorylatable serine residue and its gene disruption produced no distinct phenotype. RsbV (SCO7325) from a putative six-gene operon (rsbV-rsbR-rsbS-rsbT-rsbU1-rsbU) was strongly induced by osmotic stress in a sigmaB-dependent manner. It antagonized the inhibitory action of RsbA on sigmaB-directed transcription and was phosphorylated by RsbA in vitro. These results support the hypothesis that the rapid induction of sigmaB target genes by osmotic stress results from modulation of sigmaB activity by the kinase-anti-sigma factor RsbA and its phosphorylatable antagonist RsbV, which function by a partner-switching mechanism. Amplified induction could result from a rapid increase in the synthesis of both sigmaB and its inhibitor antagonist.

The Mycobacterium tuberculosis complex-restricted gene cfp32 encodes an expressed protein that is detectable in tuberculosis patients and is positively correlated with pulmonary interleukin-10

Human tuberculosis (TB) is caused by the bacillus Mycobacterium tuberculosis, a subspecies of the M. tuberculosis complex (MTC) of mycobacteria. Postgenomic dissection of the M. tuberculosis proteome is ongoing and critical to furthering our understanding of factors mediating M. tuberculosis pathobiology. Towards this end, a 32-kDa putative glyoxalase in the culture filtrate (CF) of growing M. tuberculosis (originally annotated as Rv0577 and hereafter designated CFP32) was identified, cloned, and characterized. The cfp32 gene is MTC restricted, and the gene product is expressed ex vivo as determined by the respective Southern and Western blot testing of an assortment of mycobacteria. Moreover, the cfp32 gene sequence is conserved within the MTC, as no polymorphisms were found in the tested cfp32 PCR products upon sequence analysis. Western blotting of M. tuberculosis subcellular fractions localized CFP32 predominantly to the CF and cytosolic compartments. Data to support the in vivo expression of CFP32 were provided by the serum recognition of recombinant CFP32 in 32% of TB patients by enzyme-linked immunosorbent assay (ELISA) as well as the direct detection of CFP32 by ELISA in the induced sputum samples from 56% of pulmonary TB patients. Of greatest interest was the observation that, per sample, sputum CFP32 levels (a potential indicator of increasing bacterial burden) correlated with levels of expression in sputum of interleukin-10 (an immunosuppressive cytokine and a putative contributing factor to disease progression) but not levels of gamma interferon (a key cytokine in the protective immune response in TB), as measured by ELISA. Combined, these data suggest that CFP32 serves a necessary biological function(s) in tubercle bacilli and may contribute to the M. tuberculosis pathogenic mechanism. Overall, CFP32 is an attractive target for drug and vaccine design as well as new diagnostic strategies.

amfR, an essential gene for aerial mycelium formation, is a member of the AdpA regulon in the A-factor regulatory cascade in Streptomyces griseus

In Streptomyces griseus, A-factor (2-isocapryloyl-3R-hydroxymethyl-gamma-butyrolactone) acts as a chemical signalling molecule that triggers morphological differentiation and secondary metabolism. A transcriptional activator, AdpA, in the A-factor regulatory cascade switches on a number of genes required for both processes, thus forming an AdpA regulon. amfR encoding a regulatory protein similar to response regulators of bacterial two-component regulatory systems and essential for aerial mycelium formation was found to be a member of the AdpA regulon. AdpA bound two sites at nucleotide positions approximately -200 (site 1) and -60 (site 2), with respect to the major transcriptional start point of amfR, and accelerated the transcription of amfR by assisting RNA polymerase in forming an open complex at an appropriate region including the transcriptional start point. Site 2 contributed more to the transcriptional activation of amfR by AdpA than site 1, although AdpA showed a much lower affinity to site 2 than to site 1. The amfR transcription enhanced by AdpA subsequently ceased at day 2 when aerial hyphae began to be formed in the wild-type strain, whereas in an adsA null mutant amfR was continuously transcribed even until day 3. This implied that amfR was repressed growth dependently by a gene product under the control of sigma-AdsA. Transcription of the promoter upstream of amfT depended on amfR, which is consistent with the idea that AmfR serves as an activator for amfTSBA in the amf operon. The observations that the amfR gene contains a TTA codon, a potential target for bldA-mediated regulation, and a conserved Asp-54 residue, which might be phosphorylated by a sensor kinase, suggest that the amf operon is under transcriptional, translational and post-translational control systems.

Transcriptional switch on of ssgA by A-factor, which is essential for spore septum formation in Streptomyces griseus

A-factor (2-isocapryloyl-3R-hydroxymethyl-gamma-butyrolactone) triggers morphological development and secondary metabolism in Streptomyces griseus. A transcriptional activator (AdpA) in the A-factor regulatory cascade switches on a number of genes required for both processes. AdBS11 was identified in a library of the DNA fragments that are bound by AdpA and mapped upstream of ssgA, which is essential for septum formation in aerial hyphae. Gel mobility shift assays and DNase I footprinting revealed three AdpA-binding sites at nucleotide positions about -235 (site 1), -110 (site 2), and +60 (site 3) with respect to the transcriptional start point, p1, of ssgA. ssgA had two transcriptional start points, one starting at 124 nucleotides (p1) and the other starting at 79 nucleotides (p2) upstream of the start codon of ssgA. Of the three binding sites, only sites 1 and 2 were required for transcriptional activation of p1 and p2 by AdpA. The transcriptional switch on of ssgA required the extracytoplasmic function sigma factor, sigma(AdsA), in addition to AdpA. However, it was unlikely that sigma(AdsA) recognized the two ssgA promoters, since their -35 and -10 sequences were not similar to the promoter sequence motifs recognized by sigma(BldN), a sigma(AdsA) homologue of Streptomyces coelicolor A3(2). An ssgA disruptant formed aerial hyphae, but did not form spores, irrespective of the carbon source of the medium, which indicated that ssgA is a member of the whi genes. Transcriptional analysis of ssfR, located just upstream of ssgA and encoding an IclR-type transcriptional regulator, suggested that no read-through from ssfR into ssgA occurred, and ssgA was transcribed in the absence of ssfR. ssgA was thus found to be controlled by AdpA and not by SsfR to a detectable extent. SsfR appeared to regulate spore septum formation independently of SsgA or through interaction with SsgA in some unknown way, because an ssfR disruptant also showed a whi phenotype.

Autophosphorylation of a bacterial serine/threonine kinase, AfsK, is inhibited by KbpA, an AfsK-binding protein

A protein serine/threonine kinase, AfsK, and its target protein AfsR globally control physiological and morphological differentiation in the bacterial genus Streptomyces. A protein (KbpA) of 252 amino acids encoded by an open reading frame in a region upstream of afsK in Streptomyces coelicolor A3(2) was identified as an AfsK-interacting protein. The interaction site of AfsK was in the N-terminal portion containing the kinase catalytic domain. KbpA bound a nonphosphorylated form of AfsK and inhibited its autophosphorylation at serine and threonine residues. KbpA in the reaction mixture containing AfsK and AfsR also inhibited the phosphorylation of AfsR by AfsK, presumably because KbpA inhibited the conversion from the inactive, nonphosphorylated form of AfsK to the active, phosphorylated form. kbpA was transcribed throughout growth, and the transcription was enhanced when production of actinorhodin had already started. KbpA thus appeared to play an inhibitory role in a negative feedback system in the AfsK-AfsR regulatory pathway. Consistent with these in vitro observations, kbpA served as a repressor for actinorhodin production in S. coelicolor A3(2); disruption of kbpA greatly enhanced actinorhodin production, and overexpression of kbpA reduced the production.

Use of direct-infusion electrospray mass spectrometry to guide empirical development of improved conditions for expression of secondary metabolites from actinomycetes

A major barrier in the discovery of new secondary metabolites from microorganisms is the difficulty of distinguishing the minor fraction of productive cultures from the majority of unproductive cultures and growth conditions. In this study, a rapid, direct-infusion electrospray mass spectrometry (ES-MS) technique was used to identify chemical differences that occurred in the expression of secondary metabolites by 44 actinomycetes cultivated under six different fermentation conditions. Samples from actinomycete fermentations were prepared by solid-phase extraction, analyzed by ES-MS, and ranked according to a chemical productivity index based on the total number and relative intensity of ions present in each sample. The actinomycete cultures were tested for chemical productivity following treatments that included nutritional manipulations, autoregulator additions, and different agitation speeds and incubation temperatures. Evaluation of the ES-MS data from submerged and solid-state fermentations by paired t test analyses showed that solid-state growth significantly altered the chemical profiles of extracts from 75% of the actinomycetes evaluated. Parallel analysis of the same extracts by high-performance liquid chromatography-ES-MS-evaporative light scattering showed that the chemical differences detected by the ES-MS method were associated with growth condition-dependent changes in the yield of secondary metabolites. Our results indicate that the high-throughput ES-MS method is useful for identification of fermentation conditions that enhance expression of secondary metabolites from actinomycetes.

Cloning and characterization of the pknA gene from Streptomyces coelicolor A3(2), coding for the Mn2+ dependent protein Ser/Thr kinase

A gene pknA, coding for an eukaryotic-type protein Ser/Thr kinase, was cloned from the Streptomyces coelicolor A3(2) chromosome. The PknA protein kinase, containing the C-terminal eukaryotic-type kinase domain with an N-terminal extension, was expressed in Escherichia coli and Streptomyces lividans. The affinity purified MBP-PknA fusion protein was assayed for kinase activity that showed its ability to autophosphorylate in vitro in the presence of [gamma-32P]ATP. The activity was Mn2+ dependent. The preautophosphorylated kinase phosphorylated at least two proteins (sizes 30 and 32 kDa) in the S. coelicolor J1501 cell-free extracts of all developmental stages. The larger of them was also phosphorylated in vitro by an endogenous protein kinase in late stages extracts, but not earlier. Although Mn2+ dependent protein phosphorylation has previously been described in Streptomyces, this is the first report of a gene encoding such an enzyme in this genus.

The A-factor regulatory cascade leading to streptomycin biosynthesis in Streptomyces griseus : identification of a target gene of the A-factor receptor

In Streptomyces griseus, A-factor (2-isocapryloyl-3R-hydroxymethyl-gamma-butyrolactone) at an extremely low concentration triggers streptomycin biosynthesis and cell differentiation by binding a repressor-type receptor protein (ArpA) and dissociating it from DNA. An A-factor-responsive transcriptional activator (AdpA) able to bind the promoter of strR, a pathway-specific regulatory gene responsible for transcription of other streptomycin biosynthetic genes, was purified to homogeneity and adpA was cloned by PCR on the basis of amino acid sequences of purified AdpA. adpA encoding a 405-amino-acid protein containing a helix-turn-helix DNA-binding motif at the central region showed sequence similarity to transcriptional regulators in the AraC/XylS family. The -35 and -10 regions of the adpA promoter were found to be a target of ArpA; ArpA bound the promoter region in the absence of A-factor and exogenous addition of A-factor to the DNA-ArpA complex immediately released ArpA from the DNA. Consistent with this, S1 nuclease mapping showed that adpA was transcribed only in the presence of A-factor and strR was transcribed only in the presence of intact adpA. Furthermore, adpA disruptants produced no streptomycin and overexpression of adpA caused the wild-type S. griseus strain to produce streptomycin at an earlier growth stage in a larger amount. On the basis of these findings, we propose here a model to demonstrate how A-factor triggers streptomycin biosynthesis at a late exponential growth stage.

A new pathway for polyketide synthesis in microorganisms

Chalcone synthases, which biosynthesize chalcones (the starting materials for many flavonoids), have been believed to be specific to plants. However, the rppA gene from the Gram-positive, soil-living filamentous bacterium Streptomyces griseus encodes a 372-amino-acid protein that shows significant similarity to chalcone synthases. Several rppA-like genes are known, but their functions and catalytic properties have not been described. Here we show that a homodimer of RppA catalyses polyketide synthesis: it selects malonyl-coenzyme-A as the starter, carries out four successive extensions and releases the resulting pentaketide to cyclize to 1,3,6,8-tetrahydroxynaphthalene (THN). Site-directed mutagenesis revealed that, as in other chalcone synthases, a cysteine residue is essential for enzyme activity. Disruption of the chromosomal rppA gene in S. griseus abolished melanin production in hyphae, resulting in 'albino' mycelium. THN was readily oxidized to form 2,5,7-trihydroxy-1,4-naphthoquinone (flaviolin), which then randomly polymerized to form various coloured compounds. THN formed by RppA appears to be an intermediate in the biosynthetic pathways for not only melanins but also various secondary metabolites containing a naphthoquinone ring. Therefore, RppA is a chalcone-synthase-related synthase that synthesizes polyketides and is found in the Streptomyces and other bacteria.

Doxorubicin overproduction in Streptomyces peucetius: cloning and characterization of the dnrU ketoreductase and dnrV genes and the doxA cytochrome P-450 hydroxylase gene

Doxorubicin-overproducing strains of Streptomyces peucetius ATCC 29050 can be obtained through manipulation of the genes in the region of the doxorubicin (DXR) gene cluster that contains dpsH, the dpsG polyketide synthase gene, the putative dnrU ketoreductase gene, dnrV, and the doxA cytochrome P-450 gene. These five genes were characterized by sequence analysis, and the effects of replacing dnrU, dnrV, doxA, or dpsH with mutant alleles and of doxA overexpression on the production of the principal anthracycline metabolites of S. peucetius were studied. The exact roles of dpsH and dnrV could not be established, although dnrV is implicated in the enzymatic reactions catalyzed by DoxA, but dnrU appears to encode a ketoreductase specific for the C-13 carbonyl of daunorubicin (DNR) and DXR or their biosynthetic precursors. The highest DXR titers were obtained in a dnrX dnrU (N. Lomovskaya, Y. Doi-Katayama, S. Filippini, C. Nastro, L. Fonstein, M. Gallo, A. L. Colombo, and C. R. Hutchinson, J. Bacteriol. 180:2379-2386, 1998) double mutant and a dnrX dnrU dnrH (C. Scotti and C. R. Hutchinson, J. Bacteriol. 178:7316-7321, 1996) triple mutant. Overexpression of doxA in a doxA::aphII mutant resulted in the accumulation of DXR precursors instead of in a notable increase in DXR production. In contrast, overexpression of dnrV and doxA jointly in the dnrX dnrU double mutant or the dnrX dnrU dnrH triple mutant increased the DXR titer 36 to 86%.