The use of a chromosome integration vector to map erythromycin resistance and production genes in Saccharopolyspora erythraea (Streptomyces erythraeus)

In Silico/In Vitro Strategies Leading to the Discovery of New Nonribosomal Peptide and Polyketide Antibiotics Active against Human Pathogens

Antibiotics are majorly important molecules for human health. Following the golden age of antibiotic discovery, a period of decline ensued, characterised by the rediscovery of the same molecules. At the same time, new culture techniques and high-throughput sequencing enabled the discovery of new microorganisms that represent a potential source of interesting new antimicrobial substances to explore. The aim of this review is to present recently discovered nonribosomal peptide (NRP) and polyketide (PK) molecules with antimicrobial activity against human pathogens. We highlight the different in silico/in vitro strategies and approaches that led to their discovery. As a result of technological progress and a better understanding of the NRP and PK synthesis mechanisms, these new antibiotic compounds provide an additional option in human medical treatment and a potential way out of the impasse of antibiotic resistance.

Random transposon mutagenesis of the Saccharopolyspora erythraea genome reveals additional genes influencing erythromycin biosynthesis

A single cycle of strain improvement was performed in Saccharopolyspora erythraea mutB and 15 genotypes influencing erythromycin production were found. Genotypes generated by transposon mutagenesis appeared in the screen at a frequency of ~3%. Mutations affecting central metabolism and regulatory genes were found, as well as hydrolases, peptidases, glycosyl transferases and unknown genes. Only one mutant retained high erythromycin production when scaled-up from micro-agar plug fermentations to shake flasks. This mutant had a knockout of the cwh1 gene (SACE_1598), encoding a cell-wall-associated hydrolase. The cwh1 knockout produced visible growth and morphological defects on solid medium. This study demonstrated that random transposon mutagenesis uncovers strain improvement-related genes potentially useful for strain engineering.

Deoxysugar pathway interchange for erythromycin analogues heterologously produced through Escherichia coli

The overall erythromycin biosynthetic pathway can be sub-divided into macrocyclic polyketide formation and polyketide tailoring to produce the final bioactive molecule. In this study, the native deoxysugar tailoring reactions were exchanged for the purpose of demonstrating the production of alternative final erythromycin compounds. Both the d-desosamine and l-mycarose deoxysugar pathways were replaced with the alternative d-mycaminose and d-olivose pathways to produce new erythromycin analogues through the Escherichia coli heterologous system. Both analogues exhibited bioactivity against multiple antibiotic-resistant Bacillus subtilis strains. Besides demonstrating an intrinsic flexibility for the biosynthetic system to accommodate alternative tailoring pathways, the results offer an initial attempt to leverage the E. coli platform for erythromycin analogue production.

Substrate specificity of the acyl transferase domains of EpoC from the epothilone polyketide synthase

The production of epothilone mixtures is a direct consequence of the substrate tolerance of the module 3 acyltransferase (AT) domain of the epothilone polyketide synthase (PKS) which utilises both malonyl- and methylmalonyl-CoA extender units. Particular amino acid motifs in the active site of AT domains influence substrate selection for methylmalonyl-CoA (YASH) or malonyl-CoA (HAFH). This motif appears in hybrid form (HASH) in epoAT3 and may represent the molecular basis for the relaxed specificity of the domain. To investigate this possibility the AT domains from modules 2 and 3 of the epothilone PKS were examined in the heterologous DEBS1-TE model PKS. Substitution of AT1 of DEBS1-TE by epoAT2 and epoAT3 both resulted in functional PKSs, although lower yields of total products were observed when compared to DEBS1-TE (2% and 11.5% respectively). As expected, epoAT3 was significantly more promiscuous in keeping with its nature during epothilone biosynthesis. When the mixed motif (HASH) of epoAT3 within the hybrid PKS was mutated to HAFH (indicative of malonyl-CoA selection) it resulted in a non-productive PKS. When this mixed motif was converted to YASH (indicative of methylmalonyl-CoA selection) the selectivity of the hybrid PKS for methylmalonyl-CoA showed no statistically significant increase, and was associated with a loss of productivity.

Analysis of an 8.1-kb DNA fragment contiguous with the erythromycin gene cluster of Saccharopolyspora erythraea in the eryCI-flanking region

An 8.1-kb region of the Saccharopolyspora erythraea genome, significant for its contiguity to the known genes of the erythromycin biosynthetic gene cluster, was mutationally analyzed and its DNA sequence was determined. The region lies immediately adjacent to eryCI. The newly characterized region is notable for a large, 3.0-kb segment, predicted not to be translated, followed by four probable genes: an acetyltransferase gene, a protease inhibitor gene, a methyltransferase gene, and a transposase gene. Because the probable functions of the genes in this region are not required for erythromycin biosynthesis or resistance and because a deletion of a 6.0-kb portion of this region had no effect on erythromycin biosynthesis, this region marks the outside boundary of the erythromycin gene cluster. Therefore, eryCI represents the end of the cluster. These results complete the analysis of the erythromycin gene cluster and eliminate the possibility that additional sought-after pathway-specific structural or regulatory genes might be found within or adjacent to the cluster.

Construction of new vectors for high-level expression in actinomycetes

A new integrative vector (pCJR24) was constructed for use in the erythromycin producer Saccharopolyspora erythraea and in other actinomycetes. It includes the pathway-specific activator gene actII-ORF4 from the actinorhodin biosynthetic gene cluster of Streptomyces coelicolor. The actI promoter and the associated ribosome binding site are located upstream of an NdeI site (5'-CATATG-3') which encompasses the actI start codon allowing protein(s) to be produced at high levels in response to nutritional signals if these signals are faithfully mediated by the ActII-ORF4 activator. Several polyketide synthase genes were cloned in pCJR24 and overexpressed in S. erythraea after integration of the vector into the chromosome by homologous recombination, indicating the possibility that the S. coelicolor promoter/activator functions appropriately in S. erythraea. pCJR24-mediated recombination was also used to place the entire gene set for the erythromycin-producing polyketide synthase under the control of the actI promoter. The resulting strain produced copious quantities of erythromycins and precursor macrolides when compared with wild-type S. erythraea. The use of this system provides the means for rational strain improvement of antibiotic-producing actinomycetes.

The loading domain of the erythromycin polyketide synthase is not essential for erythromycin biosynthesis in Saccharopolyspora erythraea

6-Deoxyerythronolide B synthase (DEBS) is a large multifunctional enzyme that catalyses the biosynthesis of the erythromycin polyketide aglycone. DEBS is organized into six modules, each containing the enzymic domains required for a single condensation of carboxylic acid residues which make up the growing polyketide chain. Module 1 is preceded by loading acyltransferase (AT-L) and acyl carrier protein (ACP-L) domains, hypothesized to initiate polyketide chain growth with a propionate-derived moiety. Using recombinant DNA technology several mutant strains of Saccharopolyspora erythraea were constructed that lack the initial AT-L domain or that lack both the AT-L and ACP-L domains. These strains were still able to produce erythromycin, although at much lower levels than that produced by the wild-type strain. In addition, the AT-L domain expressed as a monofunctional enzyme was able to complement the deletion of this domain from the PKS, resulting in increased levels of erythromycin production. These findings indicate that neither the initial AT-L nor the ACP-L domains are required to initiate erythromycin biosynthesis; however, without these domains the efficiency of erythromycin biosynthesis is decreased significantly. It is proposed that in these mutants the first step in erythromycin biosynthesis is the charging of KS1 with propionate directly from propionyl-CoA.

Acyltransferase domain substitutions in erythromycin polyketide synthase yield novel erythromycin derivatives

The methylmalonyl coenzyme A (methylmalonyl-CoA)-specific acyltransferase (AT) domains of modules 1 and 2 of the 6-deoxyerythronolide B synthase (DEBS1) of Saccharopolyspora erythraea ER720 were replaced with three heterologous AT domains that are believed, based on sequence comparisons, to be specific for malonyl-CoA. The three substituted AT domains were "Hyg" AT2 from module 2 of a type I polyketide synthase (PKS)-like gene cluster isolated from the rapamycin producer Streptomyces hygroscopicus ATCC 29253, "Ven" AT isolated from a PKS-like gene cluster of the pikromycin producer Streptomyces venezuelae ATCC 15439, and RAPS AT14 from module 14 of the rapamycin PKS gene cluster of S. hygroscopicus ATCC 29253. These changes led to the production of novel erythromycin derivatives by the engineered strains of S. erythraea ER720. Specifically, 12-desmethyl-12-deoxyerythromycin A, which lacks the methyl group at C-12 of the macrolactone ring, was produced by the strains in which the resident AT1 domain was replaced, and 10-desmethylerythromycin A and 10-desmethyl-12-deoxyerythromycin A, both of which lack the methyl group at C-10 of the macrolactone ring, were produced by the recombinant strains in which the resident AT2 domain was replaced. All of the novel erythromycin derivatives exhibited antibiotic activity against Staphylococcus aureus. The production of the erythromycin derivatives through AT replacements confirms the computer predicted substrate specificities of "Hyg" AT2 and "Ven" AT and the substrate specificity of RAPS AT14 deduced from the structure of rapamycin. Moreover, these experiments demonstrate that at least some AT domains of the complete 6-deoxyerythronolide B synthase of S. erythraea can be replaced by functionally related domains from different organisms to make novel, bioactive compounds.

Sequencing and mutagenesis of genes from the erythromycin biosynthetic gene cluster of Saccharopolyspora erythraea that are involved in L-mycarose and D-desosamine production

The nucleotide sequence on both sides of the eryA polyketide synthase genes of the erythromycin-producing bacterium Saccharopolyspora erythraea reveals the presence of ten genes that are involved in L-mycarose (eryB) and D-desosamine (eryC) biosynthesis or attachment. Mutant strains carrying targeted lesions in eight of these genes indicate that three (eryBIV, eryBV and eryBVI) act in L-mycarose biosynthesis or attachment, while the other five (eryCII, eryCIII, eryCIV, eryCV and eryCVI) are devoted to D-desosamine biosynthesis or attachment. The remaining two genes (eryBII and eryBVII) appear to function in L-mycarose biosynthesis based on computer analysis and earlier genetic data. Three of these genes, eryBII, eryCIII and eryCII, lie between the eryAIII and eryG genes on one side of the polyketide synthase genes, while the remaining seven, eryBIV, eryBV, eryCVI, eryBVI, eryCIV, eryCV and eryBVII lie upstream of the eryAI gene on the other side of the gene cluster. The deduced products of these genes show similarities to: aldohexose 4-ketoreductases (eryBIV), aldoketo reductases (eryBII), aldohexose 5-epimerases (eryBVII), the dnmT gene of the daunomycin biosynthetic pathway of Streptomyces peucetius (eryBVI), glycosyltransferases (eryBV and eryCIII), the AscC 3,4-dehydratase from the ascarylose biosynthetic pathway of Yersinia pseudotuberculosis (eryCIV), and mammalian N-methyltransferases (eryCVI). The eryCII gene resembles a cytochrome P450, but lacks the conserved cysteine residue responsible for coordination of the haem iron, while the eryCV gene displays no meaningful similarity to other known sequences. From the predicted function of these and other known eryB and eryC genes, pathways for the biosynthesis of L-mycarose and D-desosamine have been deduced.

Inhibition of erythromycin synthesis by disruption of malonyl-coenzyme A decarboxylase gene eryM in Saccharopolyspora erythraea

Malonyl-coenzyme A (malonyl-CoA) decarboxylase is widely distributed in prokaryotes and eukaryotes. However, the biological function of this enzyme has not been established in any organism. To elucidate the structure and function of this enzyme, the malonyl-CoA decarboxylase gene from Saccharopolyspora erythraea (formerly Streptomyces erythreaus) was cloned and sequenced. This gene would encode a polypeptide of 417 amino acids. The deduced amino acid sequence matched the experimentally determined amino acid sequences of 25 N-terminal residues each of the enzyme and of an internal peptide obtained by proteolysis of the purified enzyme. This decarboxylase showed homology with aminoglycoside N6'-acetyltransferases of Pseudomonas aeruginosa, Serratia marcescens, and Klebsiella pneumoniae. Northern (RNA) blot analysis revealed a single transcript. The transcription initiation site was 220 bp upstream of the start codon. When expressed in Escherichia coli, the S. erythraea malonyl-CoA decarboxylase gene yielded a protein that cross-reacted with antiserum prepared against S. erythraea malonyl-CoA decarboxylase and catalyzed decarboxylation of [3-14C]malonyl-CoA to acetyl-CoA and 14CO2. The S. erythraea malonyl-CoA decarboxylase gene was disrupted by homologous recombination using an integrating vector pWHM3. The gene-disrupted transformant did not produce immunologically cross-reacting 45-kDa decarboxylase, lacked malonyl-CoA decarboxylase activity, and could not produce erythromycin. Exogenous propionate restored the ability to produce erythromycin. These results strongly suggest that the decarboxylase provides propionyl-CoA for erythromycin synthesis probably via decarboxylation of methylmalonyl-CoA derived from succinyl-CoA, and therefore the malonyl-CoA decarboxylase gene is designated eryM. The gene disrupted mutants also did not produce pigments.

Use of new Escherichia coli/Streptomyces conjugative vectors to probe the functions of the two groEL-like genes of Streptomyces albus G by gene disruption

Streptomyces albus G contains two groEL-like genes encoding three related proteins [Guglielmi et al., J. Bacteriol. 173 (1991) 7374-7381; Mazodier et al., J. Bacteriol. 173 (1991) 7382-7386]. Two proteins, HSP58 and HSP18, are synthesized from a single start codon site in groEL1. HSP18 may be a processed form of HSP58 or the result of early termination after frameshifting. The third protein, HSP56 is encoded by groEL2. In order to determine the physiological roles of these different proteins, both groEL genes were mutagenized by using a new approach for obtaining insertions in the streptomycete chromosome. Escherichia coli plasmids containing fragments homologous to groEL1 or groEL2 are unable to replicate in Streptomyces. They were introduced into S. albus by conjugation with E. coli. We then screened for mutants in which groEL1 or groEL2 had been disrupted due to recombination events (single or double crossover) at specific sites. Using this approach, the functionally indispensable domain of HSP58 was localized to within 249 amino acids of the N-terminus. HSP58 was not detected in the mutant generated by the most upstream insertion into the groEL1 coding sequence. However, HSP18 was synthesized in this mutant after heat shock. This groEL1 mutant was not impaired in growth in the 30-41 degrees C temperature range and SDS-PAGE analysis showed its overall pattern of gene expression to be indistinguishable from the parental strain. The inability to generate strains containing groEL2 disruptions strongly suggests that HSP56 is indispensable for growth.(ABSTRACT TRUNCATED AT 250 WORDS)

An erythromycin analog produced by reprogramming of polyketide synthesis

The polyketide-derived macrolactone of the antibiotic erythromycin is made through successive condensation and processing of seven three-carbon units. The fourth cycle involves complete processing of the newly formed beta-keto group (beta-keto reduction, dehydration, and enoyl reduction) to yield the methylene that will appear at C-7 of the lactone ring. Synthesis of this molecule in Saccharopolyspora erythraea is determined by the three large eryA genes, organized in six modules, each governing one condensation cycle. Two amino acid substitutions were introduced in the putative NAD(P)H binding motif in the proposed enoyl reductase domain encoded by eryAII. The metabolite produced by the resulting strain was identified as delta 6,7-anhydroerythromycin C resulting from failure of enoyl reduction during the fourth cycle of synthesis of the macrolactone. This result demonstrates the involvement of at least the enoyl reductase from the fourth module in the fourth cycle and indicates that a virtually complete macrolide can be produced through reprogramming of polyketide synthesis.

A cloned replicon of Saccharopolyspora phages JHJ-1 and JHJ-3 is stably maintained as a plasmid in various actinomycetes

A replicon of phage JHJ-1 (and JHJ-3) was cloned. The autonomously replicating phage element was maintained as a medium-copy-number shuttle plasmid in many actinomycetes, and was efficiently transmitted to spores without antibiotic selection. One gene was shown to be expressed in a vector containing the JHJ-3 replicon.

Identification of a Saccharopolyspora erythraea gene required for the final hydroxylation step in erythromycin biosynthesis

In analyzing the region of the Saccharopolyspora erythraea chromosome responsible for the biosynthesis of the macrolide antibiotic erythromycin, we identified a gene, designated eryK, located about 50 kb downstream of the erythromycin resistance gene, ermE. eryK encodes a 44-kDa protein which, on the basis of comparative analysis, belongs to the P450 monooxygenase family. An S. erythraea strain disrupted in eryK no longer produced erythromycin A but accumulated the B and D forms of the antibiotic, indicating that eryK is responsible for the C-12 hydroxylation of the macrolactone ring, one of the last steps in erythromycin biosynthesis.

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.

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.

The isolation and characterization of antibiotic biosynthesis genes

Antibiotic biosynthesis pathways are found in a broad range of Gram positive prokaryotes, a smaller range of Gram negative prokaryotes and a limited range of eukaryotes. A variety of techniques can be used to identify the genes involved in the biosynthesis of these compounds ranging from genetic complementation and interspecific gene transfer to polymerase chain reaction amplification and transposon mutagenesis. The dissection of these cloned pathways and the understanding of their structure and regulation has led to insights into the structure and function of antibiotic biosynthesis genes. With new knowledge of the structural similarities and relationships between related antibiotic biosynthesis pathways, the possibility of directed manipulation of specific genes to allow synthesis of novel antibiotics is now possible.

Cloning and sequence analysis of genes involved in erythromycin biosynthesis in Saccharopolyspora erythraea: sequence similarities between EryG and a family of S-adenosylmethionine-dependent methyltransferases

The gene cluster (ery) responsible for production of the macrolide antibiotic erythromycin by Saccharopolyspora erythraea is also known to contain ermE, the gene conferring resistance to the antibiotic. The nucleotide sequence has been determined of a 4.5 kb portion of the biosynthetic gene cluster, from a region lying between 3.7 kb and 8.2 kb 3' of ermE. This has revealed the presence of four complete open reading frames, including the previously known ery gene eryG, which catalyses the last step in the biosynthetic pathway. Comparison of the amino acid sequence of EryG with the sequence of other S-adenosylmethionine (SAM)-dependent methyltransferases has revealed that one of the sequence motifs previously suggested to be part of the SAM-binding site is present not only in EryG but also in many other recently sequenced SAM-dependent methyltransferases. Previous genetic studies have shown that this region also contains gene(s) involved in hydroxylation of the intermediate 6-deoxyerythronolide B. One of the three other open reading frames (eryF) in fact shows very high sequence similarity to known cytochrome P450 hydroxylases. An adjacent gene (ORF5) shows a strikingly high degree of similarity to prokaryotic and eukaryotic acyltransferases and thioesterases.

Cloning vectors, mutagenesis, and gene disruption (ermR) for the erythromycin-producing bacterium Aeromicrobium erythreum

Genetic systems for study of Aeromicrobium erythreum, a gram-positive, G + C-rich (72%) bacterium with the capacity for erythromycin biosynthesis, are described. High-copy-number plasmids suitable as gene cloning vectors include derivatives of the Streptomyces plasmids pIJ101, pVE1, and pJV1. pIJ101 derivatives with missense substitutions at the rep gene BamHI site do not replicate in A. erythreum. Ethyl methanesulfonate treatment generated several amino acid auxotrophs and non-erythromycin-producing (Ery-) strains. Using the Ery- strain AR1807 as a recipient for plasmid-directed integrative recombination, the chromosomal ermR gene (encoding 23S rRNA methyltransferase) was disrupted. Phenotypic characterizations demonstrated that ermR is the sole determinant of macrolide antibiotic resistance in A. erythreum.

Manipulation of Corynebacterium glutamicum by gene disruption and replacement

We have developed a system for the genetic manipulation of the amino acid-producing Corynebacterium glutamicum. Gene disruption and replacement were achieved by introducing, via conjugation, Escherichia coli vector plasmids carrying manipulated C. glutamicum DNA fragments. We obtained stable mutants in which the chromosomal lysA gene, encoding meso-diaminopimelate decarboxylase, was interrupted by a chloramphenicol resistance cartridge, or in which an essential internal part of the lysA gene was deleted. The deletion mutants retain neither antibiotic resistance markers nor vector sequences. This strategy is generally applicable to the construction of industrial strains to be used in fermentation processes.

Cloning and characterization of the Saccharopolyspora erythraea fdxA gene encoding ferredoxin

The Saccharopolyspora erythraea gene (fdxA) corresponding to a previously purified ferredoxin [Shafiee and Hutchinson, J. Bacteriol., 170 (1988) 1548-1553] was cloned using an oligodeoxyribonucleotide probe based on the N-terminal sequence of the ferredoxin. The nucleotide sequence of a 1.3-kb segment encompassing fdxA indicates that the corresponding protein, SeFdI, is 105 amino acids long, and very similar to other 7Fe ferredoxins. A partial open reading frame closely linked to fdxA was also detected. Disruption of fdxA was attempted by replacing the wild-type allele with an in vitro mutated copy. The failure to construct an fdxA mutant strain suggests that fdxA lies in an essential region of the S. erythraea chromosome.

An unusually large multifunctional polypeptide in the erythromycin-producing polyketide synthase of Saccharopolyspora erythraea

Erythromycin A, a clinically important polyketide antibiotic, is produced by the Gram-positive bacterium Saccharopolyspora erythraea. In an arrangement that seems to be generally true of antibiotic biosynthetic genes in Streptomyces and related bacteria like S. erythraea, the ery genes encoding the biosynthetic pathway to erythromycin are clustered around the gene (ermE) that confers self-resistance on S. erythraea. The aglycone core of erythromycin A is derived from one propionyl-CoA and six methylmalonyl-CoA units, which are incorporated head-to-tail into the growing polyketide chain, in a process similar to that of fatty-acid biosynthesis, to generate a macrolide intermediate, 6-deoxyerythronolide B. 6-Deoxyerythronolide B is converted into erythromycin A through the action of specific hydroxylases, glycosyltransferases and a methyltransferase. We report here the analysis of about 10 kilobases of DNA from S. erythraea, cloned by chromosome 'walking' outwards from the erythromycin-resistance determinant ermE, and previously shown to be essential for erythromycin biosynthesis. Partial sequencing of this region indicates that it encodes the synthase. Our results confirm this, and reveal a novel organization of the erythromycin-producing polyketide synthase, which provides further insight into the mechanism of chain assembly.

Antibiotics--cloning of biosynthetic pathways

Biosynthetic pathways leading to antibiotics have often been found to be clustered, and new organizational forms of multifunctional enzymes have been discovered. Such polyenzymes accomplish the synthesis of complex metabolites such as peptides or polyketides by a sequence of enzymatic reactions. So, reactions leading to the tripeptide precursor of beta-lactam antibiotics, ACV, or to the cycloundecapeptide cyclosporine have been fused into single polypeptide chain synthetases, respectively. In certain isofunctional sites restricted similarities have been detected.

Cloning of genes involved in erythromycin biosynthesis from Saccharopolyspora erythraea using a novel actinomycete-Escherichia coli cosmid

Two plasmids were constructed that replicate in Saccharopolyspora (Sac.) erythraea, Escherichia coli and Streptomyces (S.) lividans, and used for the cloning of a locus involved in the synthesis of the macrolide antibiotic erythromycin (Er). Plasmid pAL7002 contains the thiostrepton-resistance gene (tsr), a replicon-containing fragment from pJVI and pUC9. Plasmid pNJI contains the lambda cos site but is otherwise similar to pAL7002. A library of total DNA from Sac. erythraea was constructed in pNJI and probed in colony hybridizations with a DNA fragment containing ermE, the Sac. erythraea ErR-encoding gene. Plasmids obtained were subsequently introduced into EryA mutants of Sac. erythraea blocked in synthesis of Er (Ery-) and transformants were screened for restoration of Er production (Ery+). Several plasmids were found to convert two mutants to Ery+, but a third EryA strain could not be restored to Ery+ by any of the plasmids employed. A 5-kb segment, designated eryAI, responsible for restoring the Ery+ phenotype in the EryA strains, was identified and mapped in the segment 12 to 17 kb downstream from ermE. Gene disruption experiments indicated that the 5-kb length of eryAI is fully internal to an eryAI-containing transcript. In Southern blots it was shown that one of the EryA strains carried a small deletion in eryAI and that, in at least some of the transformants restored to Ery+, the deletion had been replaced by the wild-type eryAI allele.(ABSTRACT TRUNCATED AT 250 WORDS)

Organization of a cluster of erythromycin genes in Saccharopolyspora erythraea

We used a series of gene disruptions and gene replacements to mutagenically characterize 30 kilobases of DNA in the erythromycin resistance gene (ermE) region of the Saccharopolyspora erythraea chromosome. Five previously undiscovered loci involved in the biosynthesis of erythromycin were found, eryBI, eryBII, eryCI, eryCII, and eryH; and three known loci, eryAI, eryG, and ermE, were further characterized. The new Ery phenotype, EryH, was marked by (i) the accumulation of the intermediate 6-deoxyerythronolide B (DEB), suggesting a defect in the operation of the C-6 hydroxylase system, and (ii) a block in the synthesis or addition reactions for the first sugar group. Analyses of ermE mutants indicated that ermE is the only gene required for resistance to erythromycin, and that it is not required for production of the intermediate erythronolide B (EB) or for conversion of the intermediate 3-alpha-mycarosyl erythronolide B (MEB) to erythromycin. Mutations in the eryB and eryC loci were similar to previously reported chemically induced eryB and eryC mutations blocking synthesis or attachment of the two erythromycin sugar groups. Insertion mutations in eryAI, the macrolactone synthetase, defined the largest (at least 9-kilobase) transcription unit of the cluster. These mutants help to define the physical organization of the erythromycin gene cluster, and the eryH mutants provide a source for the production of the intermediate DEB.

Disruption of a rhodaneselike gene results in cysteine auxotrophy in Saccharopolyspora erythraea

A 3,373-base-pair DNA segment from a clone fortuitously isolated from Saccharopolyspora erythraea by hybridization to an oligodeoxynucleotide probe was sequenced. Computer-assisted analysis of the nucleotide sequence reveals three closely linked Streptomyces open reading frames plus a fourth converging on the others. The deduced product of one of them, ORF2, shows considerable similarity to bovine liver rhodanese. orf2, and the closely linked orf3 located just downstream of it, were disrupted by insertion of an apramycin resistance cassette into the orf2 coding sequence along with inversion of the fragment carrying most of orf2 and orf3 via two successive recombinational events in the wild-type strain. The mutant strain thus created contains wild-type levels of rhodanese activity but cannot grow on minimal medium. It is a cysteine auxotroph, capable of utilizing efficiently only thiosulfate among the inorganic sulfur sources tested. orf2 has been designated cysA. The possible role of the rhodaneselike cysA gene product in thiosulfate formation is discussed.

Cloning of genes governing the deoxysugar portion of the erythromycin biosynthesis pathway in Saccharopolyspora erythraea (Streptomyces erythreus)

Genes that govern the formation of deoxysugars or their attachment to erythronolide B and 3 alpha-mycarosyl erythronolide B, intermediates of the biosynthesis of the 14-membered macrolide antibiotic erythromycin, were cloned from Saccharopolyspora erythraea (formerly Streptomyces erythreus). Segments of DNA that complement the eryB25, eryB26, eryB46, eryC1-60, and eryD24 mutations blocking the formation of erythronolide B or 3 alpha-mycarosyl erythronolide B, when cloned in Escherichia coli-Streptomyces shuttle cosmids or plasmid vectors that can transform S. erythraea, were located in a ca. 18-kilobase-pair region upstream of the erythromycin resistance (ermE) gene. The eryC1 gene lies just to the 5' side of ermE, and one (or possibly two) eryB gene is approximately 12 kilobase pairs farther upstream. Another eryB gene may be in the same region, while an additional eryB mutation appears to be located elsewhere. The eryD gene lies between the eryB and eryC1 genes and may regulate their function on the basis of the phenotype of an EryD- mutant.

Molecular characterization of a gene from Saccharopolyspora erythraea (Streptomyces erythraeus) which is involved in erythromycin biosynthesis

A 7.3 kbp DNA fragment, encompassing the erythromycin (Em) resistance gene (ermE) and a portion of the gene cluster encoding the biosynthetic genes for erythromycin biosynthesis in Saccharopolyspora erythraea (formerly Streptomyces erythraeus) has been cloned in Streptomyces lividans using the plasmid vector pIJ702, and its nucleotide sequence has been determined using a modified dideoxy chain-termination procedure. In particular, we have examined the region immediately 5' of the resistance determinant, where the tandem promoters for ermE overlap the promoters for a divergently transcribed coding sequence (ORF). Disruption of this ORF using an integrational pIJ702-based plasmid vector gave mutants which were specifically blocked in erythromycin biosynthesis, and which accumulated 3-O-alpha-L-mycarosylerythronolide B: this behaviour is identical to that of previously described eryC1 mutants. The eryC1-gene product, a protein of subunit Mr 39,200, is therefore involved either as a structural or as a regulatory gene in the formation of the deoxyamino-sugar desosamine or in its attachment to the macrolide ring.

Cloning, characterization, and heterologous expression of the Saccharopolyspora erythraea (Streptomyces erythraeus) gene encoding an EF-hand calcium-binding protein

The regulatory effects of Ca2+ in eucaryotic cells are mostly mediated by a superfamily of Ca2+-binding proteins (CABs) that contain one or more characteristic Ca2+-binding structural motifs, referred to as EF hands. We have cloned and sequenced the structural gene for an authentic EF-hand CAB from the spore-forming gram-positive bacterium Saccharopolyspora erythraea (formerly Streptomyces erythraeus). When the gene was introduced into Streptomyces lividans on the high-copy plasmid vector pIJ702, CAB was found to be expressed at higher levels than in S. erythraea, with no apparent effects on either growth or sporulation. A more convenient expression system for CAB was obtained by introducing an NdeI site at the initiation codon by using oligonucleotide-directed mutagenesis and placing the gene in the expression vector pT7-7 in Escherichia coli. In this system, CAB was efficiently expressed at levels up to 20 to 30% of total cell protein. When purified to homogeneity from either E. coli or Streptomyces lividans, CAB was found to be identical to the protein previously obtained from S. erythraea.

Identification of a gene required for the terminal step in erythromycin A biosynthesis in Saccharopolyspora erythraea (Streptomyces erythreus)

We have identified a transcription unit in the ermE region of the chromosome of the erythromycin (Er)-producing bacterium Saccharopolyspora erythraea (Streptomyces erythreus) that is briefly switched on at about the time that macrolide production commences. Disruption of the transcription unit, herein designated eryG, by insertion of an integrational plasmid vector, caused a block at the terminal step in the biosynthesis of erythromycin, the conversion of erythromycin C to A by O-methylation.