Molecular cloning and expression of the phenoxazinone synthase gene from Streptomyces antibioticus

Poly(A) polymerase activity and RNA polyadenylation in Streptomyces coelicolor A3(2)

The Streptomyces coelicolor genome sequence was searched for open reading frames (ORFs) similar to Escherichia coli poly(A) polymerase I, revealing an ORF with 36% amino acid sequence identity to that protein. Mycelial extracts prepared from S. coelicolor cultures incorporated radioactive ATP into an acid-insoluble form, and some of the products of this incorporation had the properties expected of poly(A). [3H]-uridine and [3H]-adenosine were used to label the RNA in S. coelicolor cultures of different ages, and total RNA was fractionated by oligo dT cellulose chromatography. Approximately 3% of the total uridine-labelled RNA and 11% of the adenosine-labelled RNA were retained by the oligo dT cellulose columns. Enzymatic digestion of the retained RNA supported the conclusion that a significant fraction of the adenosine label was present in 3'-poly(A) chains. Measurement of poly(A) tail lengths by end labelling of total RNA and RNase digestion revealed a maximum length of approximately 18 residues. Radioactive cDNA prepared from the RNA fraction retained by oligo dT cellulose hybridized to the 16S and 23S genes from a streptomycete ribosomal RNA operon but not to the 5S gene. Reverse transcription-polymerase chain reaction (RT-PCR) revealed the presence of mRNAs in the RNA fraction retained by oligo dT cellulose.

Biochemical engineering of natural product biosynthesis pathways

Metabolic engineering of natural products is a science that has been built on the goals of traditional strain improvement with the availability of modern molecular biological technologies. In the past 15 years, the state of the art in metabolic engineering of natural products has advanced from the first proof-of-principle experiment based on minimal known genetics to a commonplace event using highly specific and sophisticated gene manipulation methods. With the availability of genes, host organisms, vector systems, and standard molecular biological tools, it is expected that metabolic engineering will be translated into industrial reality.

Actinomycin production persists in a strain of Streptomyces antibioticus lacking phenoxazinone synthase

Truncated fragments of the phenoxazinone synthase gene, phsA, were prepared by the PCR. The resulting fragments were cloned into conjugative plasmid pKC1132 and transferred to Streptomyces antibioticus by conjugation from Escherichia coli. Two of the resulting constructs were integrated into the S. antibioticus chromosome by homologous recombination, and each of the resulting strains, designated 3720/pJSE173 and 3720/pJSE174, contained a disrupted phsA gene. Strain 3720/pJSE173 grew poorly, and Southern blotting suggested that genetic changes other than the disruption of the phsA gene might have occurred during the construction of that strain. Strain 3720/pJSE174 sporulated well and grew normally on the medium used to prepare inocula for antibiotic production. Strain 3720/pJSE174 also grew as well as the wild-type strain on antibiotic production medium containing either 1 or 5.7 mM phosphate. Strain 3720/pJSE174 was shown to be devoid of phenoxazinone synthase (PHS) activity, and PHS protein was undetectable in this strain by Western blotting. Despite the absence of detectable PHS activity, strain 3720/pJSE174 produced slightly more actinomycin than did the wild-type parent strain in medium containing 1 or 5.7 mM phosphate. The observation that strain 3720/pJSE174, lacking detectable PHS protein or enzyme activity, retained the ability to produce actinomycin supports the conclusion that PHS is not required for actinomycin biosynthesis in S. antibioticus.

Cryptic carbapenem antibiotic production genes are widespread in Erwinia carotovora: facile trans activation by the carR transcriptional regulator

Few strains of Erwinia carotovora subsp. carotovora (Ecc) make carbapenem antibiotics. Strain GS101 makes the basic carbapenem molecule, 1-carbapen-2-em-3-carboxylic acid (Car). The production of this antibiotic has been shown to be cell density dependent, requiring the accumulation of the small diffusible molecule N-(3-oxohexanoyl)-L-homoserine lactone (OHHL) in the growth medium. When the concentration of this inducer rises above a threshold level, OHHL is proposed to interact with the transcriptional activator of the carbapenem cluster (CarR) and induce carbapenem biosynthesis. The introduction of the GS101 carR gene into an Ecc strain (SCRI 193) which is naturally carbapenem-negative resulted in the production of Car. This suggested that strain SCRI 193 contained functional cryptic carbapenem biosynthetic genes, but lacked a functional carR homologue. The distribution of trans-activatable antibiotic genes was assayed in Erwinia strains from a culture collection and was found to be common in a large proportion of Ecc strains. Significantly, amongst the Ecc strains identified, a larger proportion contained trans-activatable cryptic genes than produced antibiotics constitutively. Southern hybridization of the chromosomal DNA of cryptic Ecc strains confirmed the presence of both the car biosynthetic cluster and the regulatory genes. Identification of homologues of the transcriptional activator carR suggests that the cause of the silencing of the carbapenem biosynthetic cluster in these strains is not the deletion of carR. In an attempt to identify the cause of the silencing in the Ecc strain SCRI 193 the carR homologue from this strain was cloned and sequenced. The SCRI 193 CarR homologue was 94% identical to the GS101 CarR and contained 14 amino acid substitutions. Both homologues could be expressed from their native promoters and ribosome-binding sites using an in vitro prokaryotic transcription and translation assay, and when the SCRI 193 carR homologue was cloned in multicopy plasmids and reintroduced into SCRI 193, antibiotic production was observed. This suggested that the mutation causing the silencing of the biosynthetic cluster in SCRI 193 was leaky and the cryptic Car phenotype could be suppressed by multiple copies of the apparently mutant transcriptional activator.

Guanosine pentaphosphate synthetase from Streptomyces antibioticus is also a polynucleotide phosphorylase

The gene for the enzyme guanosine pentaphosphate synthetase I (GPSI) from Streptomyces antibioticus has been cloned and sequenced. The cloned gene functioned as a template in the streptomycete coupled transcription-translation system and directed the synthesis of a protein with the properties expected for GPSI. Sequencing of the cloned gene identified an open reading frame of 740 amino acids whose amino terminal sequence corresponded to the N terminus of purified GPSI. The GPSI protein sequence was found to possess significant homology to polynucleotide phosphorylase from Escherichia coli. Indeed, like E. coli polynucleotide phosphorylase, purified GPSI was shown to catalyze the polymerization of ADP and the phosphorolysis of poly(A). However, the E. coli enzyme was unable to catalyze the synthesis of guanosine pentaphosphate under conditions in which GPSI was highly active in that reaction. Overexpression of the cloned gpsI gene in E. coli led to an increase in both polynucleotide phosphorylase and guanosine pentaphosphate synthetase activities in the cloning host. The polynucleotide phosphorylase activities of GPSI and of the E. coli enzyme were strongly inhibited by dCDP, but the pppGpp synthetase activity of GPSI was not inhibited and indeed was slightly stimulated by dCDP. These results strongly support the identity of GPSI as a bifunctional enzyme capable of both pppGpp synthesis and polynucleotide phosphorylase activities.

Nucleotide sequence, transcriptional analysis, and glucose regulation of the phenoxazinone synthase gene (phsA) from Streptomyces antibioticus

The nucleotide sequence of a 2.3-kb SphI fragment containing the structural gene (phsA) for phenoxazinone synthase (PHS) of Streptomyces antibioticus was determined. The sequence was found to contain an open reading frame (ORF) with a G+C content of 71.5% oriented in the direction of transcription that was confirmed by primer extension. The ORF encodes a protein with an M(r) of 70,223 consisting of 642 amino acids and is preceded by a potential ribosome-binding site. The codon usage pattern is in agreement with the general pattern for streptomycete genes, with a 92.5 mol% G+C content in the third position. The N-terminal sequence of the mature PHS subunit corresponds exactly to that predicted from the nucleotide sequence. Neither ATG nor GTG initiator codons were identified for the protein. However, a TTG codon was located near the amino terminus of the mature protein and is a good candidate for the initiator codon. The transcriptional start point of phsA was located 36 bp upstream of the start codon by primer extension. The -10 region of the putative promoter showed some similarity to the consensus sequence for the major class of prokaryotic promoters, but the -35 region was less similar. Comparison of the primary amino acid sequence of PHS of S. antibioticus with other amino acid sequences indicated that PHS is a blue copper protein with copper binding domains in the N-terminal and C-terminal regions of the polypeptide chain. A BsrBI fragment containing the promoter region of phsA and a portion of the ORF was shown to promote xylE expression when cloned in the streptomycete promoter probe vector pIJ2843. This phsA promoter-dependent xylE expression could be repressed by glucose in S. antibioticus when the organism was grown on glucose or galactose plus glucose. Thus, the cloned promoter region appears to contain the sequences responsible for catabolite repression of PHS production.

Stoichiometry and spectroscopic identity of copper centers in phenoxazinone synthase: a new addition to the blue copper oxidase family

Phenoxazinone synthase catalyzes the oxidative condensation of two molecules of substituted o-aminophenols to the phenoxazinone chromophore of actinomycin. Cyclization occurs with the concomitant reduction of molecular oxygen to water. We have shown that the enzyme requires 4-5 copper atoms/monomer for full activity and the additional copper inhibits the enzyme. The optical absorption spectrum of phenoxazinone synthase is also dependent on the Cu per monomer ratio, and the absorption peak at 598 nm has a maximum extinction coefficient of 4000 +/- 150 M-1 cm-1 at a ratio of 4-5 Cu atoms per monomer. The electron paramagnetic resonance (EPR) spectrum of enzyme as isolated with low copper content (0.8 Cu/monomer) only shows the presence of type 1 (blue) copper centers (g parallel = 2.24, A = 0.0067 cm-1, and g perpendicular = 2.07). Enzyme incubated with 4-5 Cu per monomer demonstrates the presence of both type 1 and type 2 copper centers with a stoichiometry of one type 1 center per monomer and the remainder bound as type 2 Cu2+. Anaerobic incubation of substrate with enzyme containing five Cu atoms per subunit results in bleaching of the blue center. The EPR spectrum of the enzyme reduced under these conditions suggests that one of the type 2 Cu2+ centers with a g parallel = 2.34, A = 0.016 cm-1, and g perpendicular = 2.07 remains oxidized and is not involved in catalysis.(ABSTRACT TRUNCATED AT 250 WORDS)

Review of the Streptomyces lividans/vector pIJ702 system for gene cloning

Interest in the biology of the Streptomyces and application of these soil bacteria to production of commercial antibiotics and enzymes has stimulated the development of efficient cloning techniques and a variety of streptomycete plasmid and phage vectors. Streptomyces lividans is routinely employed as a host for gene cloning, largely because this species recognizes a large number of promoters and appears to lack a restriction system. Vector pIJ702 was constructed from a variant of a larger autonomous plasmid and is often used as a cloning vehicle in conjunction with S. lividans. The host range of vector pIJ702 extends beyond Streptomyces spp., and its high copy number has been exploited for the overproduction of cloned gene products. This combination of host and vector has been used successfully to investigate antibiotic biosynthesis, gene structure and expression, and to map various Streptomyces mutants.

13C nuclear magnetic resonance and gas chromatography-mass spectrometry studies of carbon metabolism in the actinomycin D producer Streptomyces parvulus by use of 13C-labeled precursors

Fructose and glutamate metabolism was monitored in cell suspensions of streptomyces parvulus by 13C nuclear magnetic resonance. The experiments were performed for cells grown with various 13C sources in a growth medium containing D-[U-13C]fructose, L-[13C]glutamate, or L-[U-13C]aspartate and with nonlabeled precursors to compare intracellular pools in S. parvulus cells at different periods of the cell life cycle. The transport of fructose into the cells was biphasic in nature; during rapid transport, mannitol, fructose, and glucose 6-phosphate were accumulated intracellularly, whereas during the passive diffusion of fructose, the intracellular carbohydrate pool comprised mainly trehalose (1,1'-alpha-alpha-D-glucose). The regulation of fructokinase activity by the intracellular intermediates may play an important role in fructose catabolism in S. parvulus. Transaldolase activity in S. parvulus was determined from the 13C nuclear magnetic resonance labeling pattern of trehalose carbons obtained from cells grown in medium containing either L-[U-13C]aspartate or L-[U-13C]glutamate. Only carbons 4, 5, and 6 of the disaccharide were labeled. Isotopomer analysis of the trehalose carbons led us to conclude that the flux through the reverse glycolytic pathway, condensation of glyceraldehyde 3-phosphate with dihydroxyacetone phosphate, makes at best a minor contribution to the 13C-labeled glucose units observed in trehalose. The pentose pathway and transaldolase activity can explain the labeling pattern of 4,5,6-13C3 of trehalose. Moreover, the transfer of the 13C label of L-[U-13C]aspartate into the different isotopomers of trehalose C4, C5, and C6 by the transaldolase activity allowed us to calculate the relative fluxes from oxaloacetate via gluconeogenesis and through the tricarboxylic acid cycle. The ratio of the two fluxes is approximately 1. However, the main carbon source for trehalose synthesis in S. parvulus is fructose and not glutamate or aspartate. The 13C enrichment and isotopomer population, measured by nuclear magnetic resonance and gas chromatography-mass spectrometry, of the actinomycin D peptide ring enabled us to specify the origins of the five amino acids of actinomycin D. Threonine and proline exhibited isotopomer populations similar to that of the extracellular L-[13C]glutamate, indicating that protein catabolism is the origin of their 13C label, whereas the isotopomer populations of sarcosine and N-methylvaline were similar to those of the new intracellular pool of S. parvulus that originated from D-[U-13C]fructose during the production of actinomycin D.

Pleiotropic effects of a relC mutation in Streptomyces antibioticus

Ochi (Agric. Biol. Chem. 51:829-835, 1987) has isolated a relaxed mutant of Streptomyces antibioticus, designated relC49, relC49 accumulates significantly lower levels of ppGpp than the parent stain, IMRU3720. At its maximum, the ppGpp level in relC49 was only one-fourth that observed in strain IMRU3720. Interestingly, a burst of ppGpp synthesis between 18 and 22 h of growth in IMRU3720 coincided with the onset of actinomycin production in that strain. As shown previously, the activity in protein synthesis of ribosomes from strain IMRU3720 decreases with the age of the culture. The decrease in activity was less pronounced in cultures of relC49. relC49 mycelium contains reduced levels of phenoxazinone synthase, a key enzyme involved in actinomycin biosynthesis. The rel mutation prevents the normal increase in the activity of one of the other enzymes required for production of the antibiotic, 3-hydroxyanthanilate-4-methyltransferase, and a third enzyme, actinomycin synthetase I, appears to be completely absent from relC49 mycelium. Levels of phenoxazinone synthease mRNA were examined by RNA dot blotting with the cloned phenoxazinone synthase gene as a probe. mRNA levels for phenoxazinone synthase were dramatically reduced in relC49 compared with strain IMRU3720. These results are discussed in terms of the possible regulation of the onset of actinomycin production by ppGpp.

Nonribosomal biosynthesis of peptide antibiotics

Peptide antibiotics are known to contain non-protein amino acids, D-amino acids, hydroxy acids, and other unusual constituents. In addition they may be modified by N-methylation and cyclization reactions. Their biosynthetic origin has been connected in many cases to an enzymatic system referred to as the 'thiotemplate multienzymic mechanism'. This mechanism includes the activation of the constituent residues as adenylates on the enzymic template, the acylation of specific template thiol groups, epimerization or N-methylation at this thioester stage, and polymerization in the sequence directed by the multienzymic structure with the aid of 4'-phosphopantetheine as a cofactor, including possible cyclization or terminal modification reactions. The reaction sequences leading to gramicidin S, tyrocidine, cyclosporine, bacitracin, polymyxin, actinomycin, enniatin, beauvericin, delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine and linear gramicidin are discussed. The structures of the multienzymes, their genetic organization, the biological functions of these peptides and results on related systems are discussed.

The Improving Prospects for Yield Increase by Genetic Engineering in Antibiotic-Producing Streptomycetes

Molecular genetics has spawned a dramatic expansion of the biotechnology industry in the direction of the products of single genes. On the other hand, antibiotics--some of the classical products of biotechnology--result from the concerted action of many genes, and it is therefore less straightforward to apply the new techniques to antibiotic production. Studies of cloned genes for antibiotic biosynthesis are now providing information that should allow the application of a combination of traditional and recombinant DNA methodology to the improvement of yield in antibiotic-producing Streptomyces species.

Molecular biology of antibiotic production in Bacillus

Several species of the genus Bacillus produce peptide antibiotics which are synthesized either through a ribosomal or non-ribosomal mechanism. The antibiotics gramicidin, tyrocidine, and bacitracin are synthesized nonribosomally by the multienzyme thiotemplate mechanism. Surfactin and mycobacillin are also synthesized nonribosomally but by a mechanism that, apparently, is distinct from that of the multienzyme thiotemplate. Other antibiotics such as subtilin are gene encoded and are ribosomally synthesized. Molecular genetic and DNA sequence analysis have shown that biosynthesis genes for some antibiotics are clustered into polycistronic transcription units and are under the control of global regulatory systems that govern the expression of genes that are induced when Bacillus cells enter stationary phase of growth. Future experiments involving the molecular dissection of peptide antibiotic biosynthesis genes in Bacillus will be attempted in hopes of further examining the mechanism and regulation of antibiotic production.

Molecular cloning and in vitro expression of a silent phenoxazinone synthase gene from Streptomyces lividans

Phenoxazinone synthase (PHS) catalyzes a step in actinomycin D biosynthesis in Streptomyces antibioticus. Two sequences from Streptomyces lividans that hybridize to the phs gene of S. antibioticus have been cloned in Escherichia coli K-12 using the plasmid pBR322. Although there was some similarity in the restriction maps of the two cloned fragments, neither insert appeared to be a direct subset of the other nor of the S. antibioticus phs gene. In vitro expression studies, in a streptomycete coupled transcription-translation system, showed that a 3.98-kb SphI fragment encoded a PHS-related protein. These observations provide additional support for the existence of silent genes for antibiotic production in streptomycetes.

Physical and functional analyses of the syrA and syrB genes involved in syringomycin production by Pseudomonas syringae pv. syringae

The syrA and syrB genes involved in syringomycin production in Pseudomonas syringae pv. syringae B301D were identified from an EcoRI-pLAFR3 cosmid library and then physically and functionally analyzed in relation to plant pathogenicity. Homologous recombination of the genes required for syringomycin production from cosmids pGX183 (syrA) and pGX56 (syrB), respectively, introduced into nontoxigenic (Tox-) Tn5 mutants W4S2545 and W4S770 resulted in the concomitant restoration of toxin production and full virulence. The disease indices of the Tox+ strains obtained by recombination of the cloned, homologous DNA into the corresponding Tn5 mutant were essentially equivalent to that of strain B301D-R and significantly higher than those of W4S2545 and W4S770. A 12-kilobase (kb) EcoRI fragment from pGX183 was subcloned (i.e., pGX15) and found to contain the sequences necessary for syringomycin production. A map of pGX15 prepared by a combination of restriction endonuclease digestions and Tn5 mutagenesis showed that the syrA sequence was 2.3 to 2.8 kb. Marker exchange of syrA::Tn5 from pGX15 into B301D-R yielded nonpathogenic phenotypes, indicating that syrA is a regulatory gene since it is necessary for both syringomycin production and pathogenicity. The 4.9-kb EcoRI fragment from pGX56 was subcloned (i.e., pGX4) and shown to carry the syrB sequence which was 2.4 to 3.3 kb. Sodium dodecyl sulfate-polyacrylamide gel electrophoretic analysis of protein extracts from B301D-R associated five proteins, ranging from approximately 130,000 to approximately 470,000 in molecular weight, with syringomycin production. The syrA and syrB genes were required for the formation of proteins SR4 (approximately 350,000) and SR5 (approximately 130,000), which are believed to be components of the syringomycin synthetase complex.

Genetics of actinomycin C production in Streptomyces chrysomallus

Three distinct classes of mutations affecting the biosynthesis of actinomycin have been established in Streptomyces chyrsomallus by crossing various actinomycin-nonproducing mutants with each other by protoplast fusion. In crosses between members of different classes of mutations, actinomycin-producing recombinant progeny arose, whereas in crosses between members of the same class, no actinomycin-producing recombinants were seen. Biochemical examination of a number of mutants revealed that the expression of all actinomycin synthetases was reduced by about 1 order of magnitude in mutants belonging to class II. In mutants of class I, the specific activities of the actinomycin synthetases were comparable with those measured in their actinomycin-producing parents. Feeding experiments with 4-methyl-3-hydroxyanthranilic acid (4-MHA), the biosynthetic precursor of the chromophore moiety of actinomycin, with representative mutants of the three genetic classes revealed formation of actinomycin in minute amounts by mutants of class I. It is suggested that mutants belonging to class I are mutated at a genetic locus involved in the biosynthesis of 4-MHA. Mutants belonging to class II appear to carry mutations at a locus involved in the regulation of the expression of the actinomycin synthetases. The role of the locus in class III mutations could not be assigned. Mapping studies in S. chrysomallus based on conjugal matings revealed the chromosomal linkage of all three loci. Mutations belonging to classes I and III were closely linked. Their genetic loci could be localized in a map interval of the chromosomal linkage group which is significantly distant from the gene locus represented by mutations belonging to class II.

Actinomycin synthesis in Streptomyces antibioticus: enzymatic conversion of 3-hydroxyanthranilic acid to 4-methyl-3-hydroxyanthranilic acid

A methyltransferase which utilizes 3-hydroxyanthranilic acid (HAA) as a substrate was identified in detergent-treated extracts of the bacterium Streptomyces antibioticus. The enzyme catalyzes the transfer of methyl groups from [14C]S-adenosylmethionine to HAA, but does not catalyze the methylation of 3-hydroxy-DL-kynurenine. Enzyme, substrate, time, and pH dependencies for the methyl transfer reaction were examined. Reaction products obtained from scaled-up reaction mixtures were fractionated by chromatography on Dowex 1, and the Dowex 1 fractions were examined by paper and thin-layer chromatography. One Dowex fraction was shown to contain a radioactive product with the chromatographic properties of 4-methyl-3-hydroxyanthranilic acid (MHA), a known intermediate in the biosynthesis of actinomycin. Available evidence indicates that the conversion of HAA to MHA is an early step in the biosynthesis of actinomycin by S. antibioticus and other actinomycin-producing streptomycetes.

The impact of genetic engineering on the commercial production of antibiotics by Streptomyces and related bacteria

Developments in Streptomyces genetics that have laid a foundation for this field over the past ten years are reviewed and discussed to suggest how this knowledge might useful for improving the commercial production of antibiotics. This brief analysis predicts a bright future for the application of Streptomyces genetics in antibiotic production.

Multiple antibiotics produced by Pseudomonas fluorescens HV37a and their differential regulation by glucose

Pseudomonas fluorescens HV37a inhibited growth of the fungus Pythium ultimum on potato dextrose agar (PDA). An antibiotic activity produced under these conditions was fractionated and partially characterized. Extracts prepared from the PDA on which HV37a was grown revealed a single peak of antibiotic activity on thin-layer chromatograms. Similar extracts were prepared from mutants of HV37a. Their analysis indicated that the antibiotic observed in thin-layer chromatograms was responsible for fungal inhibition observed on PDA. The production of the PDA antibiotic required the presence of glucose, whereas two other antibiotic activities were produced only on potato agar without added glucose. Two mutants (denoted AfuIa and AfuIb) previously characterized as deficient in fungal inhibition on PDA showed altered regulation of the production of all three antibiotics in response to glucose. These mutants were also deficient in glucose dehydrogenase. Mutants isolated as deficient in glucose dehydrogenase were also deficient in fungal inhibition and were grouped into two classes on the basis of complementation analysis with an AfuI cosmid. Glucose regulation of antibiotic biosynthesis therefore involves at least two components and requires glucose dehydrogenase.

Structural stability of heterologous genes cloned in Streptomyces plasmid pIJ702

A recombinant plasmid pWCL1 containing Streptomyces plasmid pIJ702, E. coli plasmid pUC12, and hepatitis B viral surface antigen (HBsAg) gene was stably maintained in E. coli, but exhibited structural instability in S. lividans 1326. The deletions were found ranging from 2.75 to 5.65 kilobases (kb) and most of them occurred within the melanin (mel) gene of pIJ702, resulting in the loss of part of the mel gene sequence plus the insert. The removal of the pUC12 sequence from pWCL1 eliminated the instability. However, pUC12 alone inserted in either orientation on pIJ702 also caused the deletion in S. lividans 1326. The results indicated that the structural instability of hybrid plasmid of pIJ702 depended on the interaction between the mel sequence and the inserted sequence.

Construction and characterization of new cloning vectors derived from Streptomyces griseobrunneus plasmid pBT1 and containing amikacin and sulfomycin resistance genes

Three cryptic plasmids, designated pBT1 (5.6 kb), pBT2 (9.7 kb), and pBT3 (16.6 kb), were isolated from Streptomyces griseobrunneus ISP5066 and physically characterized. pBT1 and pBT2, which differ by a 4.1-kb segment, are high copy-number plasmids (40-100 copies per chromosome) that coexist with each other. pBT3 is a low copy-number plasmid. Vectors containing amikacin (or kanamycin) and sulfomycin (or thiostrepton) resistance genes from Streptomyces litmocidini ISP5164 and Streptomyces viridochromogenes subsp. sulfomycini ATCC 29776, respectively, were constructed from pBT1. One such vector, pBT37, has unique restriction sites for cloning, including BglII, XhoI, PvuII, ClaI, and SacI, with the PvuII and ClaI sites allowing clone recognition by insertional inactivation of sulfomycin resistance. Since many Streptomyces species were very sensitive to amikacin and sulfomycin, these resistance genes serve as useful selective markers. pBT37 could transform several Streptomyces strains that produce antibiotics such as tetracyclines, macrolides, beta-lactams, and aminoglycosides. This plasmid is a potentially useful vector for cloning antibiotic biosynthetic genes.

Cloning of the tyrocidine synthetase 1 gene from Bacillus brevis and its expression in Escherichia coli

The entire structural gene for tyrocidine synthetase 1 from Bacillus brevis ATCC 8185 has been cloned and expressed in Escherichia coli. Transformed E. coli cells were screened for their ability to produce tyrocidine synthetase 1 by in situ immunoassay using antibodies against gramicidin S synthetase 2 which cross-react with tyrocidine synthetase 1. The cloned gene is within a 5.2 kb fragment of B. brevis genomic DNA and requires no external promoter for its expression in E. coli. It was also observed that cloning of the 5.2 kb insert in the opposite orientation still resulted in a high level of tyrocidine synthetase 1 expression in transformed E. coli cells. In addition, protein blotting and partial purification of the gene product by gel filtration revealed a major protein of molecular weight about 100,000 with specific D-phenylalanine dependent ATP-32PPi and 2'deoxy ATP-32PPi exchange activities. These unique activities of tyrocidine synthetase 1 were not detected in protein extracts of E. coli strains carrying the vector.

Regulation of phenoxazinone synthase expression in Streptomyces antibioticus

The cloned gene for the subunit of phenoxazinone synthase (PHS), an enzyme implicated in the biosynthesis of actinomycin in Streptomyces antibioticus, was used as a probe to study the regulation of the enzyme. The direction of transcription of the PHS gene was determined with end-labeled restriction fragments derived from the gene. Low-resolution S1 mapping revealed that transcription was initiated at a position which may lie within the SphI restriction site, which represents the limit of the cloned sequence. Northern blotting allowed the identification of the putative PHS message. This RNA appeared to be significantly larger than the size required to encode the PHS subunit. RNA dot blotting showed that the increase in PHS specific activity observed in cultures grown on antibiotic production medium, with galactose as a carbon source, was due in part to an increased production of PHS mRNA. PHS was also more stable than most cellular proteins and appeared to be protected against degradation under conditions in which most other proteins are broken down. This protective effect also contributed to the increase in PHS specific activity observed in S. antibioticus cultures grown on production medium. The repression of PHS synthesis by glucose was also reflective of a transcriptional control mechanism. At early time points postinoculation, PHS mRNA levels were lower in cultures grown on glucose as a carbon source than in cultures of the same age grown on galactose. mRNA levels presumably begin to increase only after all the glucose in the medium is utilized. The ability of 5-fluorouracil to stimulate PHS production in young cultures was also due to the synthesis of new mRNA for the enzyme.

Efficient plasmid transformation of Streptomyces ambofaciens and Streptomyces fradiae protoplasts

A procedure for efficient transformation of Streptomyces ambofaciens and Streptomyces fradiae protoplasts with plasmid DNA was developed. Transformation frequencies with S. fradiae protoplasts were strongly influenced by the temperatures for cell growth, protoplast formation, and protoplast regeneration. Transformation frequencies for both species were also influenced by the culture age before protoplast formation, the source and concentration of polyethylene glycol, the transformation-inducing agent, the concentration of protoplasts used in the transformation procedure, and the number of protoplasts added to regeneration plates. Transformation frequencies were substantially higher for both species when calf thymus DNA and protamine sulfate were added to the transformation mix. With S. fradiae, transformation frequencies were much lower with plasmid DNA prepared from other species than with the same plasmids prepared from S. fradiae, suggesting that S. fradiae expresses restriction and modification. With the modified transformation procedures using DNA prepared from homologous hosts, S. ambofaciens and S. fradiae are now transformed routinely at frequencies of 10(6) to 10(7) transformants per micrograms of plasmid DNA.

Production of 'hybrid' antibiotics by genetic engineering

The recent development of molecular cloning systems in Streptomyces has made possible the isolation of biosynthetic genes for some of the many antibiotics produced by members of this important genus of bacteria. Such clones can now be used to test the idea that novel antibiotics could arise through the transfer of biosynthetic genes between streptomycetes producing different antibiotics. The likelihood of a 'hybrid' compound being produced must depend on the substrate specificities of the biosynthetic enzymes, about which little is known. In attempts to demonstrate hybrid antibiotic production, we therefore began with strains producing different members of the same chemical class of compounds in order to maximize the chance of success. Here we report the production of novel compounds by gene transfer between strains producing the isochromanequinone antibiotics actinorhodin, granaticin and medermycin. These experiments were made possible by the recent cloning of the whole set of genes for the biosynthetic pathway of actinorhodin from Streptomyces coelicolor A3(2) (ref. 8). We believe that this represents the first report of the production of hybrid antibiotics by genetic engineering.