Genetic and biochemical features of spiramycin biosynthesis in Streptomyces ambofaciens--curing, protoplast regeneration and plasmid transfer

The regulatory genes involved in spiramycin and bitespiramycin biosynthesis

Bitespiramycin (biotechnological spiramycin, Bsm) is a new 16-membered macrolide antibiotic produced by Streptomyces spiramyceticus WSJ-1 integrated exogenous genes. The gene cluster for Bsm biosynthesis consists of two parts: spiramycin biosynthetic gene cluster (92 kb) and two exogenous genes including 4"-O-isovaleryltransferase gene (ist) and a positive regulatory gene (acyB2) from S. thermotolerans. Four putative regulatory genes, bsm2, bsm23, bsm27 and bsm42, were identified by sequence analysis in the spiramycin gene cluster. The inactivation of bsm23 or bsm42 in S. spiramyceticus eliminated spiramycin production, while the deletion of bsm2 and bsm27 did not abolish spiramycin biosynthesis. The acyB2 gene, homologous with bsm42 gene, cannot recover the spiramycin production in Δbsm42 mutant. The high expression of bsm42 significantly increased the spiramycin production, but overexpression of bsm23 inhibited its production in Δbsm23 and wild-type strain. Bsm23 was shown to be involved in the regulation of the expression of bsm42 and acyB2 by electrophoretic mobility shift assays. The bsm42 gene was also positive regulator for ist expression inferred from the improved yield of 4"-isovalerylspiramycins in the S. lividans TK24 biotransformation test, but adding bsm23 decreased the production of 4''-isovalerylspiramycins. These results demonstrated Bsm42 was a pathway-specific activator for spiramycin or Bsm biosynthesis, but overexpression of Bsm23 alone was adverse to produce these antibiotics although Bsm23 was essential for positive regulation of spiramycin production.

Identification and functional analysis of genes controlling biosynthesis of 2-methylisoborneol

To identify the genes for biosynthesis of the off-flavor terpenoid alcohol, 2-methylisoborneol (2-MIB), the key genes encoding monoterpene cyclase were located in bacterial genome databases by using a combination of hidden Markov models, protein-family search, and the sequence alignment of their gene products. Predicted terpene cyclases were classified into three groups: sesquiterpene, diterpene, and other terpene cyclases. Genes of the terpene cyclase group that form an operon with a gene encoding S-adenosyl-l-methionine (SAM)-dependent methyltransferase were found in genome data of seven microorganisms belonging to actinomycetes, Streptomyces ambofaciens ISP5053, Streptomyces coelicolor A3(2), Streptomyces griseus IFO13350, Streptomyces lasaliensis NRRL3382R, Streptomyces scabies 87.22, Saccharopolyspora erythraea NRRL2338, and Micromonospora olivasterospora KY11048. Among six microorganisms tested, S. ambofaciens, S. coelicolor A3(2), S. griseus, and S. lasaliensis produced 2-MIB but M. olivasterospora produced 2-methylenebornane (2-MB) instead. The regions containing monoterpene cyclase and methyltransferase genes were amplified by PCR from S. ambofaciens, S. lasaliensis, and Saccharopolyspora erythraea, respectively, and their genes were heterologously expressed in Streptomyces avermitilis, which was naturally deficient of 2-MIB biosynthesis by insertion and deletion. All exoconjugants of S. avermitilis produced 2-MIB. Full-length recombinant proteins, monoterpene cyclase and methyltransferase of S. lasaliensis were expressed at high level in Escherichia coli. The recombinant methyltransferase catalyzed methylation at the C2 position of geranyl diphosphate (GPP) in the presence of SAM. 2-MIB was generated by incubation with GPP, SAM, recombinant methyltransferase, and terpene cyclase. We concluded that the biosynthetic pathway involves the methylation of GPP by GPP methyltransferase and its subsequent cyclization by monoterpene cyclase to 2-MIB.

Effect of intercalating dyes on the production of antibiotics by Micromonospora rosaria and Micromonospora purpurea

The effect of treatment with various intercalating dyes on the ability to produce antibiotics in Micromonospora rosaria and Micromonospora purpurea was studied. Treatment with acriflavine resulted in a high frequency loss of antibiotic productivity in both species. In M. rosaria, the loss of antibiotic-producing ability appeared to be strain-dependent. In M. purpurea, up to 90% of colonies were found to have lost gentamicin-producing ability when protoplasts were used in the test. These antibiotic-nonproducing strains were further studied. The following observations were made: (1) Unlike the producing ability, the resistance to the antibiotics is a very stable character in both species. (2) Protoplast fusion analysis indicates that rosamicin-nonproducing characteristics of MR 217-AF2 and MR 217-AF3 strains induced by the acriflavine treatment is due to chromosomal mutation or rearrangement but not to loss of a plasmid. (3) Gentamicin-nonproducing strains of M. purpurea responded differently to the supplementation of streptamine or DOS in the culture medium. When supplemented with streptamine or DOS, some of these strains regained the ability to produce antibiotic, showing that the biosynthesis of intermediate was affected in these strains.

Cloning of spiramycin biosynthetic genes and their use in constructing Streptomyces ambofaciens mutants defective in spiramycin biosynthesis

Several cosmid clones from Streptomyces ambofaciens containing the spiramycin resistance gene srmB were introduced into S. fradiae PM73, a mutant defective in tylosin synthesis, resulting in tylosin synthesis. The DNA responsible for this complementation was localized to a 10.5-kilobase EcoRI fragment. A 32-kilobase DNA segment which included the srmB spiramycin resistance gene and DNA which complemented the defect in strain PM73 were mutagenized in vivo with Tn10 carrying the gene for Nmr (which is expressed in Streptomyces spp.) or in vitro by insertional mutagenesis with a drug resistance gene (Nmr) cassette. When these mutagenized DNA segments were crossed into the S. ambofaciens chromosome, three mutant classes blocked in spiramycin synthesis were obtained. One mutant accumulated two precursors of spiramycin, platenolide I and platenolide II. Two mutants, when cofermented with the platenolide-accumulating mutant, produced spiramycin. Tylactone supplementation of these two mutants resulted in the synthesis of a group of compounds exhibiting antibiotic activity. Two other mutants failed to coferment with any of the other mutants or to respond to tylactone supplementation.

Genetic instability and DNA amplification in Streptomyces lividans 66

Streptomyces lividans 66 exhibits genetic instability, involving sequential loss of resistance to chloramphenicol (Cams) and subsequent mutation of argG. Associated with this instability is the amplification of a 5.7-kilobase (kb) amplified DNA sequence (ADS). We have characterized a second, independent pathway of genetic instability, involving sequential loss of resistance to tetracycline (Tets) followed by mutation in nitrogen assimilation (Ntr). We detected DNA amplification in many of these mutant strains, as well as other reiterations coresident with the 5.7-kb ADS in Cams Arg mutants. However, in contrast to the 5.7-kb ADS, none of the novel elements were observed to amplify at high frequency. The mutation of argG is due to a deletion, one endpoint of which is defined by the 5.7-kb ADS. This amplification derives from a structure, the tandemly duplicated amplifiable unit of DNA (AUD), present in the wild-type genome. We found that progenitor strains containing just a single-copy AUD failed to reproducibly generate amplification of this element in Cams argG mutants, and DNA deletion endpoints proximal to the element were found to be unspecific. These results suggest that a duplicated AUD structure is required for high-frequency amplification and that this reiteration can subsequently buffer the extent of deletion formation in the relevant chromosomal region.

Relationship of an unstable argG gene to a 5.7-kilobase amplifiable DNA sequence in Streptomyces lividans 66

The relationship between an unstable argG gene and a 5.7-kilobase (kb) amplifiable DNA sequence in Streptomyces lividans 66 was investigated. Spontaneous, high-frequency Arg mutants deleted for this gene typically contain 200 to 300 copies of the tandemly reiterated sequence. A library of S. lividans 66 (strain 1326) wild-type genomic DNA was prepared in the vector lambda Charon 35. Chromosome walking over 44 kb established that argG is located 25 kb distant from a duplicated amplifiable DNA structure. A sequence was characterized, located farther distal from the amplifiable structure, containing strong homology with an internal sequence of the amplifiable DNA, which may have a role in the deletion of argG. Genetic mapping showed that argG and the 5.7-kb amplifiable sequence are linked to another unstable gene, determining chloramphenicol resistance (Camr) and that together these genes may be located in a silent chromosomal arc.

Plasmids in different strains of Streptomyces ambofaciens: free and integrated form of plasmid pSAM2

Five strains of Streptomyces ambofaciens were examined for their plasmid content. Among these strains, four belong to the same lineage (strains B) and the other was isolated independently (strain A). A large plasmid (ca. 80 kb), called pSAM1 in this paper and already described, was present in all B strains, and absent in strain A. A second plasmid, not described before, was found as covalently closed circular DNA in two of the four B strains. This plasmid with a size of 11.1 kb was called pSAM2. A restriction map for 14 enzymes was established. Hybridization experiments showed that a unique sequence homologous to this plasmid is integrated in a larger replicon, which is not pSAM1 and is probably the chromosome, in all B strains and not in strain A. It seems probable that the integrated sequence is the origin of the free plasmid found in two strains of the B family. It is noteworthy that the integrated form and the free plasmid may be found together. Transformation experiments proved that pSAM2 may be maintained autonomously in S. ambofaciens strain A and in S. lividans. pSAM2 is a self-transmissible plasmid, able to elicit the lethal zygosis reaction. pSAM2 was compared to the plasmids SLP1, pIJ110 and pIJ408, which all come from integrated sequences in three Streptomyces species and are found as autonomous plasmids after transfer to S. lividans. If pSAM2 resembles these plasmids in its origin, it does not appear to be related directly to them. Concerning their plasmid content, the two isolates of S. ambofaciens are very different.(ABSTRACT TRUNCATED AT 250 WORDS)