abaA, a new pleiotropic regulatory locus for antibiotic production in Streptomyces coelicolor.

Production of the blue-pigmented antibiotic actinorhodin is greatly enhanced in Streptomyces lividans and Streptomyces coelicolor by transformation with a 2.7-kb DNA fragment from the S. coelicolor chromosome cloned on a multicopy plasmid. Southern analysis, restriction map comparisons, and map locations of the cloned genes revealed that these genes were different from other known S. coelicolor genes concerned with actinorhodin biosynthesis or its pleiotropic regulation. Computer analysis of the DNA sequence showed five putative open reading frames (ORFs), which were named ORFA, ORFB, and ORFC (transcribed in one direction) and ORFD and ORFE (transcribed in the opposite direction). Subcloning experiments revealed that ORFB together with 137 bp downstream of it is responsible for antibiotic overproduction in S. lividans. Insertion of a phi C31 prophage into ORFB by homologous recombination gave rise to a mutant phenotype in which the production of actinorhodin, undecylprodigiosin, and the calcium-dependent antibiotic (but not methylenomycin) was reduced or abolished. The nonproducing mutants were not affected in the timing or vigor or sporulation. A possible involvement of ORFA in antibiotic production in S. coelicolor is not excluded. abaA constitutes a new locus which, like the afs and abs genes previously described, pleiotropically regulates antibiotic production. DNA sequences that hybridize with the cloned DNA are present in several different Streptomyces species.     

Identification of a gene negatively affecting antibiotic production and morphological differentiation in Streptomyces coelicolor A3(2)

SC7A1 is a cosmid with an insert of chromosomal DNA from Streptomyces coelicolor A3(2). Its insertion into the chromosome of S. coelicolor strains caused a duplication of a segment of ca. 40 kb and delayed actinorhodin antibiotic production and sporulation, implying that SC7A1 carried a gene negatively affecting these processes. The subcloning of SC7A1 insert DNA resulted in the identification of the open reading frame SCO5582 as nsdA, a gene negatively affecting Streptomyces differentiation. The disruption of chromosomal nsdA caused the overproduction of spores and of three of four known S. coelicolor antibiotics of quite different chemical types. In at least one case (that of actinorhodin), this was correlated with premature expression of a pathway-specific regulatory gene (actII-orf4), implying that nsdA in the wild-type strain indirectly repressed the expression of the actinorhodin biosynthesis cluster. nsdA expression was up-regulated upon aerial mycelium initiation and was strongest in the aerial mycelium. NsdA has DUF921, a Streptomyces protein domain of unknown function and a conserved SXR site. A site-directed mutation (S458A) in this site in NsdA abolished its function. Blast searching showed that NsdA homologues are present in some Streptomyces genomes. Outside of streptomycetes, NsdA-like proteins have been found in several actinomycetes. The disruption of the nsdA-like gene SCO4114 had no obvious phenotypic effects on S. coelicolor. The nsdA orthologue SAV2652 in S. avermitilis could complement the S. coelicolor nsdA-null mutant phenotype.     

产生变活霉素的变株的分离与初步鉴别

对天然无抗菌活性的链霉菌1254菌株进行诱变．获导了二株有抗菌活性的变株。变株113产生的抗生素为一组新蒽环类化合物．有抗病毒活性,定名为变活霉素。变株2—6产生碱性永溶性物质。初步实验结果表明,原株1254及变株2-6均是胞壁类型1,为链霉菌属。变株113为胞壁类型Ⅳ,不含有枝菌酸。原株1254与变株113的阻断变株共合成的产物与变活霉素相局,以放线紫红素聚酮合成酶基因act1为探针与原株1254的总DNA进行Southern杂交为阳性。根据这二个实验的结果推断,在1254菌株中可能存在一条变活霉素的合成途径,但有的基因处于未表达状态,诱发突变使其被活化。     

Genetic analysis of absB, a Streptomyces coelicolor locus involved in global antibiotic regulation.

The filamentous soil bacterium Streptomyces coelicolor is known to produce four antibiotics which are genetically and structurally distinct. An extensive search for antibiotic regulatory mutants led to the discovery of absB mutants, which are antibiotic deficient but sporulation proficient. Genetic analysis of the absB mutants has resulted in definition of the absB locus at 5 o'clock on the genetic map. Multiple cloned copies of the actII-ORF4 gene, an activator of synthesis of the antibiotic actinorhodin, restore actinorhodin biosynthetic capability to the absB mutants. These results are interpreted to mean that the failure of absB mutants to produce antibiotics results from decreased expression of the antibiotic genes. The absB gene is proposed to be involved in global regulation of antibiotic synthesis.     

Initiation of actinorhodin export in Streptomyces coelicolor

Many microorganisms produce molecules having antibiotic activity and expel them into the environment, presumably enhancing their ability to compete with their neighbours. Given that these molecules are often toxic to the producer, mechanisms must exist to ensure that the assembly of the export apparatus accompanies or precedes biosynthesis. Streptomyces coelicolor produces the polyketide antibiotic actinorhodin in a multistep pathway involving enzymes encoded by genes that are clustered together. Embedded within the cluster are genes for actinorhodin export, two of which, actR and actA resemble the classic tetR and tetA repressor/efflux pump-encoding gene pairs that confer resistance to tetracycline. Like TetR, which represses tetA, ActR is a repressor of actA. We have identified several molecules that can relieve repression by ActR. Importantly (S)-DNPA (an intermediate in the actinorhodin biosynthetic pathway) and kalafungin (a molecule related to the intermediate dihydrokalafungin), are especially potent ActR ligands. This suggests that along with the mature antibiotic(s), intermediates in the biosynthetic pathway might activate expression of the export genes thereby coupling export to biosynthesis. We suggest that this could be a common feature in the production of many bioactive natural products.     

Novel approach for improving the productivity of antibiotic-producing strains by inducing combined resistant mutations

We developed a novel approach for improving the production of antibiotic from Streptomyces coelicolor A3(2) by inducing combined drug-resistant mutations. Mutants with enhanced (1.6- to 3-fold-higher) actinorhodin production were detected at a high frequency (5 to 10%) among isolates resistant to streptomycin (Str(r)), gentamicin (Gen(r)), or rifampin (Rif(r)), which developed spontaneously on agar plates which contained one of the three drugs. Construction of double mutants (str gen and str rif) by introducing gentamicin or rifampin resistance into an str mutant resulted in further increased (1.7- to 2.5-fold-higher) actinorhodin productivity. Likewise, triple mutants (str gen rif) thus constructed were found to have an even greater ability for producing the antibiotic, eventually generating a mutant able to produce 48 times more actinorhodin than the wild-type strain. Analysis of str mutants revealed that a point mutation occurred within the rpsL gene, which encodes the ribosomal protein S12. rif mutants were found to have a point mutation in the rpoB gene, which encodes the beta-subunit of RNA polymerase. Mutation points in gen mutants still remain unknown. These single, double, and triple mutants displayed in hierarchical order a remarkable increase in the production of ActII-ORF4, a pathway-specific regulatory protein, as determined by Western blotting analysis. This reflects the same hierarchical order observed for the increase in actinorhodin production. The superior ability of the triple mutants was demonstrated by physiological analyses under various cultural conditions. We conclude that by inducing combined drug-resistant mutations we can continuously increase the production of antibiotic in a stepwise manner. This new breeding approach could be especially effective for initially improving the production of antibiotics from wild-type strains.     

S-adenosyl-L-methionine activates actinorhodin biosynthesis by increasing autophosphorylation of the Ser/Thr protein kinase AfsK in Streptomyces coelicolor A3(2)

S-Adenosyl-L-methionine (SAM) is one of the major methyl donors in all living organisms. The exogenous treatment with SAM leads to increased actinorhodin production in Streptomyces coelicolor A3(2). In this study, mutants from different stages of the AfsK-AfsR signal transduction cascade were used to test the possible target of SAM. SAM had no significant effect on actinorhodin production in afsK, afsR, afsS, or actII-open reading frame 4 (ORF4) mutant. This confirms that afsK plays a critical role in delivering the signal generated by exogenous SAM. The afsK-pHJL-KN mutant did not respond to SAM, suggesting the involvement of the C-terminal of AfsK in binding with SAM. SAM increased the in vitro autophosphorylation of kinase AfsK in a dose-dependent manner, and also abolished the effect of decreased actinorhodin production by a Ser/Thr kinase inhibitor, K252a. In sum, our results suggest that SAM activates actinorhodin biosynthesis in S. coelicolor M130 by increasing the phosphorylation of protein kinase AfsK.     

Molecular analysis of three Aeromonas hydrophila AH-3 (serotype O34) lipopolysaccharide core biosynthesis gene clusters

By the isolation of three different Aeromonas hydrophila strain AH-3 (serotype O34) mutants with an altered lipopolysaccharide (LPS) migration in gels, three genomic regions encompassing LPS core biosynthesis genes were identified and characterized. When possible, mutants were constructed using each gene from the three regions, containing seven, four, and two genes (regions 1 to 3, respectively). The mutant LPS core structures were elucidated by using mass spectrometry, methylation analysis, and comparison with the full core structure of an O-antigen-lacking AH-3 mutant previously established by us. Combining the gene sequence and complementation test data with the structural data and phenotypic characterization of the mutant LPSs enabled a presumptive assignment of all LPS core biosynthesis gene functions in A. hydrophila AH-3. The three regions and the genes contained are in complete agreement with the recently sequenced genome of A. hydrophila ATCC 7966. The functions of the A. hydrophila genes waaC in region 3 and waaF in region 2 were completely established, allowing the genome annotations of the two heptosyl transferase products not previously assigned. Having the functions of all genes involved with the LPS core biosynthesis and most corresponding single-gene mutants now allows experimental work on the role of the LPS core in the virulence of A. hydrophila.     

Novel Approach for Improving the Productivity of Antibiotic-Producing Strains by Inducing Combined Resistant Mutations

We developed a novel approach for improving the production of antibiotic from Streptomyces coelicolor A3(2) by inducing combined drug-resistant mutations. Mutants with enhanced (1.6- to 3-fold-higher) actinorhodin production were detected at a high frequency (5 to 10%) among isolates resistant to streptomycin (Str^r), gentamicin (Gen^r), or rifampin (Rif^r), which developed spontaneously on agar plates which contained one of the three drugs. Construction of double mutants (str gen and str rif) by introducing gentamicin or rifampin resistance into an str mutant resulted in further increased (1.7- to 2.5-fold-higher) actinorhodin productivity. Likewise, triple mutants (str gen rif) thus constructed were found to have an even greater ability for producing the antibiotic, eventually generating a mutant able to produce 48 times more actinorhodin than the wild-type strain. Analysis of str mutants revealed that a point mutation occurred within the rpsL gene, which encodes the ribosomal protein S12. rif mutants were found to have a point mutation in the rpoB gene, which encodes the β-subunit of RNA polymerase. Mutation points in gen mutants still remain unknown. These single, double, and triple mutants displayed in hierarchical order a remarkable increase in the production of ActII-ORF4, a pathway-specific regulatory protein, as determined by Western blotting analysis. This reflects the same hierarchical order observed for the increase in actinorhodin production. The superior ability of the triple mutants was demonstrated by physiological analyses under various cultural conditions. We conclude that by inducing combined drug-resistant mutations we can continuously increase the production of antibiotic in a stepwise manner. This new breeding approach could be especially effective for initially improving the production of antibiotics from wild-type strains.     

Targeted gene replacements in a Streptomyces polyketide synthase gene cluster: role for the acyl carrier protein

A methodology was developed to construct any desired chromosomal mutation in the gene cluster that encodes the actinorhodin polyketide synthase (PKS) of Streptomyces coelicolor A3(2). A positive selection marker (resistance gene) is first introduced by double crossing-over into the chromosomal site of interest by use of an unstable delivery plasmid. This marker is subsequently replaced by the desired mutant allele via a second high-frequency double recombination event. The technology has been used to: (i) explore the significance of translational coupling between two adjacent PKS genes; (ii) prove that the acyl carrier protein (ACP) encoded by a gene in the cluster is necessary for the function of the actinorhodin PKS; (iii) provide genetic evidence supporting the hypothesis that serine 42 is the site of phosphopantetheinylation in the ACP of the actinorhodin PKS; and (iv) demonstrate that this ACP can be replaced by a Saccharopolyspora fatty acid synthase ACP to generate an active hybrid PKS.     

Role of phosphopantetheinyl transferase genes in antibiotic production by Streptomyces coelicolor

The phosphopantetheinyl transferase genes SCO5883 (redU) and SCO6673 were disrupted in Streptomyces coelicolor. The redU mutants did not synthesize undecylprodigiosin, while SCO6673 mutants failed to produce calcium-dependent antibiotic. Neither gene was essential for actinorhodin production or morphological development in S. coelicolor, although their mutation could influence these processes.