The integrated and free states of Streptomyces griseus plasmid pSG1

Genome sequencing and analysis of a type A Clostridium perfringens isolate from a case of bovine clostridial abomasitis

Clostridium perfringens is a common inhabitant of the avian and mammalian gastrointestinal tracts and can behave commensally or pathogenically. Some enteric diseases caused by type A C. perfringens, including bovine clostridial abomasitis, remain poorly understood. To investigate the potential basis of virulence in strains causing this disease, we sequenced the genome of a type A C. perfringens isolate (strain F262) from a case of bovine clostridial abomasitis. The ∼3.34 Mbp chromosome of C. perfringens F262 is predicted to contain 3163 protein-coding genes, 76 tRNA genes, and an integrated plasmid sequence, Cfrag (∼18 kb). In addition, sequences of two complete circular plasmids, pF262C (4.8 kb) and pF262D (9.1 kb), and two incomplete plasmid fragments, pF262A (48.5 kb) and pF262B (50.0 kb), were identified. Comparison of the chromosome sequence of C. perfringens F262 to complete C. perfringens chromosomes, plasmids and phages revealed 261 unique genes. No novel toxin genes related to previously described clostridial toxins were identified: 60% of the 261 unique genes were hypothetical proteins. There was a two base pair deletion in virS, a gene reported to encode the main sensor kinase involved in virulence gene activation. Despite this frameshift mutation, C. perfringens F262 expressed perfringolysin O, alpha-toxin and the beta2-toxin, suggesting that another regulation system might contribute to the pathogenicity of this strain. Two complete plasmids, pF262C (4.8 kb) and pF262D (9.1 kb), unique to this strain of C. perfringens were identified.

Actinomycete integrative and conjugative elements

This paper reviews current knowledge on actinomycete integrative and conjugative elements (AICEs). The best characterised AICEs, pSAM2 of Streptomyces ambofaciens (10.9 kb), SLP1 (17.3 kb) of Streptomyces coelicolor and pMEA300 of Amycolatopsis methanolica (13.3 kb), are present as integrative elements in specific tRNA genes, and are capable of conjugative transfer. These AICEs have a highly conserved structural organisation, with functional modules for excision/integration, replication, conjugative transfer, and regulation. Recently, it has been shown that pMEA300 and the related elements pMEA100 of Amycolatopsis mediterranei and pSE211 of Saccharopolyspora erythraea form a novel group of AICEs, the pMEA-elements, based on the unique characteristics of their replication initiator protein RepAM. Evaluation of a large collection of Amycolatopsis isolates has allowed identification of multiple pMEA-like elements. Our data show that, as AICEs, they mainly coevolved with their natural host in an integrated form, rather than being dispersed via horizontal gene transfer. The pMEA-like elements could be separated into two distinct populations from different geographical origins. One group was most closely related to pMEA300 and was found in isolates from Australia and Asia and pMEA100-related sequences were present in European isolates. Genome sequence data have enormously contributed to the recent insight that AICEs are present in many actinomycete genera. The sequence data also provide more insight into their evolutionary relationships, revealing their modular composition and their likely combined descent from bacterial plasmids and bacteriophages. Evidence is accumulating that AICEs act as modulators of host genome diversity and are also involved in the acquisition of secondary metabolite clusters and foreign DNA via horizontal gene transfer. Although still speculative, these AICEs may play a role in the spread of antibiotic resistance factors into pathogenic bacteria. The novel insights on AICE characteristics presented in this review may be used for the effective construction of new vectors that allows us to engineer and optimise strains for the production of commercially and medically interesting secondary metabolites, and bioactive proteins.

Prevalence and distribution of nucleotide sequences typical for pMEA-like accessory genetic elements in the genus Amycolatopsis

The prevalence and distribution of pMEA-like elements in the genus Amycolatopsis was studied. For this purpose, a set of 95 recently isolated Amycolatopsis strains and 16 Amycolatopsis type strains were examined for the presence of two unique pMEA-sequences (repAM and traJ), encoding proteins essential for replication and conjugative transfer. Homologues of repAM and traJ were found in 10 and 26 of 111 investigated strains, respectively, a result which shows that pMEA-like sequences, though not very abundant, can be found in several Amycolatopsis strains. Phylogenetic analysis of the deduced RepAM and TraJ protein sequences revealed clustering with the protein sequences of either pMEA300 or pMEA100. Furthermore, two geographically different populations of pMEA-like elements were distinguished, one originating in Europe and the other in Australia and Asia. Linkage between the distribution of repAM and traJ and the chromosomal identifier, the 16S rRNA gene, indicated that these elements coevolved with their hosts, suggesting that they evolved in an integrated form rather than by horizontal gene transfer of the free replicating form.

RepAM of the Amycolatopsis methanolica integrative element pMEA300 belongs to a novel class of replication initiator proteins

Accessory genetic elements, such as plasmids and integrative elements, are widespread amongst actinomycetes, but little is known about their functions and mode of replication. The conjugative element pMEA300 from Amycolatopsis methanolica is present mostly in an integrated state at a single specific site in the chromosome, but it can also replicate autonomously. Complete nucleotide sequencing, in combination with deletion studies, has revealed that orfB of pMEA300 is essential for autonomous replication in its host. In this study, it was shown that purified OrfB protein binds specifically to the 3' end of its own coding sequence. Within this short sequence, a putative hairpin structure is located, which contains several direct and inverted repeats, and a nucleotide stretch that resembles the nicking site of the pC194 family of rolling circle replicating plasmids. Additional binding studies revealed that OrfB binds to an 8 bp inverted repeat that occurs three times within the hairpin structure. The data presented show that OrfB is the replication initiator (Rep) protein of pMEA300, and is therefore termed RepAM. Surprisingly, RepAM lacks significant sequence similarity with known prokaryotic Rep proteins, but it is highly similar to a number of yet uncharacterized ORFs that are located on integrative and conjugative elements of other actinomycetes. It is concluded that RepAM and its homologues are members of a novel class of Rep proteins.

Characterization of the Micromonospora rosaria pMR2 plasmid and development of a high G+C codon optimized integrase for site-specific integration

pMR2, an 11.1 kb plasmid was isolated from Micromonospora rosaria SCC2095, NRRL3718, and its complete nucleotide sequence determined. Analysis revealed 13 ORFs including homologs of a KorSA regulatory protein and TraB plasmid transfer protein found on other actinomycete plasmids. pMR2 contains att/int functions consisting of an integrase, an excisionase, and a putative plasmid attachment site (attP). The integrase gene contained a high frequency of codons rarely used in high G+C actinomycete coding regions. The gene was codon optimized for actinomycete codon usage to create the synthetic gene int-OPT. pSPRX740, containing an rpsL promoter and the att/int-OPT region, was introduced into Micromonospora halophytica var. nigra ATCC33088. Analysis of DNA flanking the pSPRX740 integration site confirmed site-specific integration into a tRNA(Phe) gene in the M. halopytica var. nigra chromosome. The pMR2 attP element and chromosomal attachment (attB) site contain a 63 bp region of sequence identity overlapping the 3' end of the tRNA(Phe) gene. Plasmids comprising the site-specific att/int-OPT functions of pMR2 can be used to integrate genes into the chromosome of actinomycetes with an appropriate tRNA gene. The development of an integrative system for Micromonospora will expand our ability to study antibiotic biosynthesis in this important actinomycete genus.

Development of the Micromonospora carbonacea var. africana ATCC 39149 bacteriophage pMLP1 integrase for site-specific integration in Micromonospora spp

Micromonospora carbonacea var. africana ATCC 39149 contains a temperate bacteriophage, pMLP1, that is present both as a replicative element and integrated into the chromosome. Sequence analysis of a 4.4 kb KpnI fragment revealed pMLP1 att/int functions consisting of an integrase, an excisionase and the phage attachment site (attP). Plasmids pSPRH840 and pSPRH910, containing the pMLP1 att/int region, were introduced into Micromonospora spp. by conjugation from Escherichia coli. Sequence analysis of DNA flanking the integration site confirmed site-specific integration into a tRNAHis gene in the chromosome. The pMLP1 attP element and chromosomal bacterial attachment (attB) site contain a 24 bp region of sequence identity located at the 3' end of the tRNA. Integration of pMLP1-based plasmids in M. carbonacea var. africana caused a loss of the pMLP1 phage. Placement of an additional attB site into the chromosome allowed integration of pSPRH840 into the alternate attB site. Plasmids containing the site-specific att/int functions of pMLP1 can be used to integrate genes into the chromosome.

Characterization of the attP site of the integrative element pSAM2 from Streptomyces ambofaciens

pSAM2 is integrated into the Streptomyces ambofaciens chromosome through site-specific recombination between the element (attP) and the chromosomal (attB) site. The 43 kDa integrase protein encoded by pSAM2 catalyses this recombination event. Tools have been developed to study site-specific recombination in Escherichia coli. In vivo studies showed that a 360 bp fragment of attP is required for efficient site-specific recombination and that int can be provided in trans. pSAM2 integrase was purified and overexpressed in E. coli and Int binding at the attP site was studied. DNaseI footprinting revealed two sites that bind integrase strongly and appear to be symmetrical with regard to the core site. These two P1/P2 arm-type sites both contain a 17 bp motif that is identical except at one position, GTCACGCAG(A/T)TAGACAC. P1 and P2 are essential for site-specific recombination.

Structure of the chromosomal insertion site for pSAM2: functional analysis in Escherichia coli

The element pSAM2 from Streptomyces ambofaciens integrates into the chromosome through site-specific recombination between the element (attP) and the chromosomal (attB) sites. These regions share an identity segment of 58bp extending from the anti-codon loop through the 3' end of a tRNA(Pro) gene. To facilitate the study of the attB site, the int and xis genes, expressed from an inducible promoter, and attP from pSAM2 were cloned on plasmids in Escherichia coil. Compatible plasmids carrying the different attB regions to be tested were introduced in these E. coli strains. Under these conditions, Int alone could promote site-specific integration; Int and Xis were both required for site-specific excision. This experimental system was used to study the sequences required in attB for efficient site-specific recombination. A 26 bp sequence, centred on the anti-codon loop region and not completely included in the identity segment, retained all the functionality of attB; shorter sequences allowed integration with lower efficiencies. By comparing the 26-bp-long attB with attP, according to the Lambda model, we propose that B and B', C and C' core-type Int binding sites consist of 9 bp imperfect inverted repeats separated by a 5 bp overlap region.

Identification and functional analysis of the transfer region of plasmid pMEA300 of the methylotrophic actinomycete Amycolatopsis methanolica

Amycolatopsis methanolica contains a 13.3-kb plasmid (pMEA300) that is present either as an integrated element or as an autonomously replicating plasmid. Conjugational transfer of pMEA300 results in pock formation, zones of growth inhibition that become apparent when plasmid-carrying donor cells develop in a confluent lawn of plasmid-lacking recipient cells. A 6.2-kb pMEA300 DNA region specifying the functions of conjugation and pock formation was sequenced, revealing 10 open reading frames. This is the first sequence of the transfer region of a plasmid from a nonstreptomycete actinomycete. No clear similarities were found between the deduced sequences of the 10 putative Tra proteins of pMEA300 and those of Streptomyces plasmids. All Tra proteins of pMEA300 thus may represent unfamiliar types. A detailed mutational analysis showed that at least four individual proteins, TraG (9,488 Da), TraH (12,586 Da), TraI (40,468 Da), and TraJ (81,109 Da), are required for efficient transfer of pMEA300. Their disruption resulted in a clear reduction in the conjugational transfer frequencies, ranging from (5.2 x 10(1))-fold (TraG) to (2.3 x 10(6))-fold (TraJ), and in reduced pock sizes. At least two putative proteins, TraA (10,698 Da) and TraB (31,442 Da), were shown to be responsible for pock formation specifically. Specific binding of the pMEA300-encoded KorA protein to the traA-korA intragenic region was observed.

Characterization of the genes and attachment sites for site-specific integration of plasmid pSE101 in Saccharopolyspora erythraea and Streptomyces lividans

The 11.3 kb plasmid pSE101 integrates into the chromosome of Saccharopolyspora erythraea at a specific attB site and into the chromosome of Streptomyces lividans at many sites. Multisite integration in S. lividans was also observed when a 1.9 kb segment of pSE101 containing attP and adjacent plasmid sequence was used to transform a pSE101- S. lividans host. Nucleotide sequencing of this segment revealed the presence of a complete open reading frame (ORF) designated int, encoding a putative polypeptide of 448 amino acids that shows similarities to site-specific recombinases of the integrase family. Sequencing of the 1.3 kb segment upstream of int revealed the presence of three additional ORFs: the one most distal to int encodes a putative 76 amino acid basic polypeptide analogous to the Xis proteins of a number of bacteriophages. Nucleotide sequencing of attP, and the attB, attL and attR sites from Sac. erythraea revealed a 46 bp sequence common to all sites with no duplications of chromosomal sequences in the integrated state. A putative structural gene for a tRNA(Thr) was found to overlap the 46 bp common sequence at attB. Sequencing of four pSE101 integration sites (attB') and corresponding attL' and attR' sites in S. lividans showed that the 46 bp sequence was present at each attR' site, whereas only the first three bases, CTT, were retained at each attL' and attB' site. A feature common to the four attB' sites and to attB is a highly conserved 21 bp segment with inverted repeats flanking the CTT sequence.(ABSTRACT TRUNCATED AT 250 WORDS)

Mode and origin of replication of pSAM2, a conjugative integrating element of Streptomyces ambofaciens

pSAM2 is an 11 kb integrating element from Streptomyces ambofaciens that is capable of replication. It generates single-stranded DNA during replication, and is therefore the first Streptomyces integrating element to be described that may belong to the family of elements, called the ssDNA elements, that replicate by a rolling-circle mechanism. The direction of replication has been identified. The plus origin (ori) of replication and minus origin (M-O) have been located. Streptomyces lividans harbouring replicating pSAM2 also contain numerous small covalently closed circular DNA molecules (scm) derived from pSAM2. These scm contain ori and extend on both sides of the putative nick site. Sequences at the junction points of these scm are heterogeneous but short direct repeats were always found in the vicinity of these junctions.

tDNA(ser) sequences are involved in the excision of Streptomyces griseus plasmid pSG1

Plasmid pSG1 is maintained in some derivatives of Streptomyces griseus NRRL3851 mainly in the chromosomally integrated form (pSG1int). In others, the integrated plasmid is co-maintained with free pSG1 [Cohen et al., Plasmid 13 (1985) 41-50]. The pSG1 plasmid integration site (attP) and the pSG1int-chromosome boundaries (attL and attR) were cloned and sequenced. The results indicate that pSG1int is flanked at attL by a functional tRNA(ser) gene and at attR by a 60-nt sequence of the 3' end of the same tRNA(ser) gene. A single mismatch distinguishes the 60-nt sequence at attR from its direct repeat at attL. The attP site contains a single copy of the 60-nt repeat, identical to the one at attL. This observation indicates that pSG1, like integrating plasmids of other Actinomycetes, is integrated in a tRNA gene and suggests that the exact excision of pSG1int occurs by a recombinational crossing-over event at the first 43 nt of the 60-nt repeat.

Characterization of an 8.7-kilobase thiostrepton resistance-encoding plasmid (pGIF3) of Streptomyces incarnatus

The low-copy-number 8.7-kb plasmid pGIF3 of Streptomyces incarnatus was studied after cloning in the Escherichia coli vector pBR322; a restriction map was constructed. Southern blot analysis showed that pGIF3 in S. incarnatus occurs predominantly as integrated in a larger replicon. The plasmid carries a gene for thiostrepton resistance having no homology with the known thiostrepton resistance gene from Streptomyces azureus.

Characterization of the genetic elements required for site-specific integration of plasmid pSE211 in Saccharopolyspora erythraea

The 18.1-kilobase plasmid pSE211 integrates into the chromosome of Saccharopolyspora erythraea at a specific attB site. Restriction analysis of the integrated plasmid, pSE211int, and adjacent chromosomal sequences allowed identification of attP, the plasmid attachment site. Nucleotide sequencing of attP, attB, attL, and attR revealed a 57-base-pair sequence common to all sites with no duplications of adjacent plasmid or chromosomal sequences in the integrated state, indicating that integration takes place through conservative, reciprocal strand exchange. An analysis of the sequences indicated the presence of a putative gene for Phe-tRNA at attB which is preserved at attL after integration has occurred. A comparison of the attB site for a number of actinomycete plasmids is presented. Integration at attB was also observed when a 2.4-kilobase segment of pSE211 containing attP and the adjacent plasmid sequence was used to transform a pSE211- host. Nucleotide sequencing of this segment revealed the presence of two complete open reading frames (ORFs) and a segment of a third ORF. The ORF adjacent to attP encodes a putative polypeptide 437 amino acids in length that shows similarity, at its C-terminal domain, to sequences of site-specific recombinases of the integrase family. The adjacent ORF encodes a putative 98-amino-acid basic polypeptide that contains a helix-turn-helix motif at its N terminus which corresponds to domains in the Xis proteins of a number of bacteriophages. A proposal for the function of this polypeptide is presented. The deduced amino acid sequence of the third ORF did not reveal similarities to polypeptide sequences in the current data banks.

Excision of pIJ408 from the chromosome of Streptomyces glaucescens and its transfer into Streptomyces lividans

Streptomyces glaucescens GLA000 contains the integrated 15 kb DNA element pIJ408 which, during mating of the parent strain with S. lividans, can be transferred into recipient cells. In S. lividans cells, pIJ408 was found in an autonomously replicating form and in a chromosomally integrated state. In the majority of the S. lividans transconjugants studied, a deletion derivative pIJ408.1 (12.4 kb) occurred. The deletion form was found in some strains only as a free plasmid, in others it was also chromosomally integrated. The integration region of pIJ408 was subcloned and precisely mapped by hybridization, restriction and sequencing analyses. The DNA junction fragments of the integrated plasmid in S. glaucescens, as well as the DNA fragment containing the attachment site of the S. lividans chromosome, were also cloned, submitted to detailed restriction analysis and sequenced. The attachment site of pIJ408 (attP) and the junctions of its integrated form with the chromosomal DNA in S. glaucescens (attL and attR) contain an identical 43 bp sequence. The chromosomal attachment site in S. lividans (attB) differs from the S. glaucescens att sequence by a single base substitution. The similarities between attachment sites of SLP1, pMEA100, pSAM2 and pIJ408 are discussed.

Site-specific integration in Streptomyces ambofaciens: localization of integration functions in S. ambofaciens plasmid pSAM2

In Streptomyces ambofaciens ATCC 15154, an 11.1-kilobase element, pSAM2, exists as a single integrated copy in the chromosome. In S. ambofaciens 3212 (a derivative of ATCC 15154), pSAM2 exists as a free, circular plasmid as well as an integrated element. BclI fragments from the free form of pSAM2 were cloned into an Escherichia coli plasmid vector. By using gene transplacement methods, the chromosomally integrated form of pSAM2 was marked with a gene coding for apramycin resistance. This enabled us to isolate both a segregant that had lost the integrated pSAM2 element and a cosmid clone containing integrated pSAM2 along with the flanking chromosomal sequences. One of the BclI fragments derived from free pSAM2 was shown to contain all the plasmid-specified information required to direct site-specific recombination in a derivative of S. ambofaciens lacking the resident pSAM2 element as well as in a number of other Streptomyces strains. The attachment sites used by the plasmid and the chromosome in site-specific recombination and the junctions created after integration were cloned and sequenced. Certain structural features in common with other integrating elements in actinomycetes were noted.

Analysis of recombination occurring at SLP1 att sites

SLP1int is a conjugative Streptomyces coelicolor genetic element that can transfer to Streptomyces lividans and integrate site specifically into the genome of the new bacterial host. Recombination of SLP1 previously has been shown to occur within nearly identical 112-base-pair att sequences on the plasmid and host chromosome. We report here that both integrative recombination and intermolecular transfer of SLP1int require no more than a 48-base-pair segment of the att sequence and that SLP1 transfer occurs by a conservative rather than a replicative mechanism. The functions responsible for the excision of the element as a discrete DNA segment are induced during the conjugal transfer of SLP1.

Site-specific insertion of biologically functional adventitious genes into the Streptomyces lividans chromosome

We report that transformation of Streptomyces lividans with cloned DNA of the SLP1 genetic element results in integration of the element at the same chromosomal locus (attB) normally occupied by SLP1 in its original host, Streptomyces coelicolor, and in S. lividans that has received SLP1 by mating. We constructed SLP1 derivatives that can integrate foreign DNA at the attB site and used these to introduce adventitious DNA sequences into the S. lividans chromosome. We also identified three regions of SLP1 essential for its integration and demonstrated that integration of the SLP1 element does not require expression of functions necessary for stable maintenance or transfer of extrachromosomal forms of SLP1.

Site-specific integration in Saccharopolyspora erythraea and multisite integration in Streptomyces lividans of actinomycete plasmid pSE101

An 11.3-kilobase-pair plasmid, designated pSE101, exists in Saccharopolyspora erythraea NRRL 2338 as an integrated sequence (pSE101int) at a unique chromosomal location and in the free form in less than an average of 1 copy per 10 chromosomes. The plasmid sequence is missing from S. erythraea NRRL 2359. Restriction maps of the free and integrated forms of pSE101 showed point-to-point correspondence. Plasmid pECT2 was constructed by ligation of pSE101, pBR322, and the gene for thiostrepton resistance (tsr). When introduced by polyethylene glycol-mediated transformation into protoplasts of S. erythraea NRRL 2359, all thiostrepton-resistant regenerants examined were found to carry a single copy of pECT2 in the integrated state at a single chromosomal site. The chromosomal site of pECT2 integration in strain NRRL 2359 (attB) corresponded to the chromosomal location of pSE101int in strain NRRL 2338. The plasmid crossover site (attP) was mapped to the plasmid site that corresponded to the site of interruption of the plasmid sequence in the host carrying pSE101int, indicating that site-specific integrative recombination had occurred. An additional 2.8-kilobase-pair chromosomal sequence homologous to a segment of pSE101 was also observed in strains NRRL 2338 and NRRL 2359. After introduction of pECT2 into Streptomyces lividans, approximately half of the transformants examined were found to carry the plasmid as a stable, autonomously replicating element. The other half carried a single copy of pECT2 as an integrated sequence, but the location of pECT2int in Streptomyces lividans varied from one transformant to another. In each case, integrative crossover used the attP site. A model is proposed to account for the determination of the particular state of pSE101 in Streptomyces lividans.

Site-specific integration and excision of pMEA100 in Nocardia mediterranei

Nocardia mediterranei strain LBG A3136 contains the 23.7 kb element pMEA100 in a chromosomally integrated form as well as in the free state (Moretti et al. 1985). The integrated form of this element can be excised precisely from the Nocardia chromosome without any accompanying rearrangements in flanking chromosomal DNA. After transfer into plasmid-free mutant strains, pMEA100 reintegrates site specifically into its original chromosomal locus. The exact mapping of the pMEA100 integration site was accomplished by restriction analysis and DNA sequencing. The attachment site of pMEA100, the junctions of its integrated form and plasmid-free chromosomal DNA of N. mediterranei contain an identical 47 bp long sequence which is probably required for site-specific recombination connected with integration and excision of pMEA100. Only one such sequence was found in the chromosome of pMEA100-free N. mediterranei derivatives as suggested by the single integration locus.

Extrachromosomal systems and gene transmission in anaerobic bacteria

Obligately anaerobic bacteria are important in terms of their role as medical pathogens as well as their degradative capacities in a variety of natural ecosystems. Two major anaerobic genera, Bacteroides and Clostridium, are examined in this review. Plasmid elements in both genera are reviewed within the context of conjugal transfer and drug resistance. Genetic systems that facilitate the study of these anaerobic bacteria have emerged during the past several years. In large part, these developments have been linked to work centered on extrachromosomal genetic systems in these organisms. Conjugal transfer of antibiotic resistance has been a central focus in this regard. Transposable genetic elements in the Bacteroides are discussed and the evolution and spread of resistance to lincosamide antibiotics are considered at the molecular level. Recombinant DNA systems that employ shuttle vectors which are mobilized by conjugative plasmids have been developed for use in Bacteroides and Clostridium. The application of transmission and recombinant DNA genetic systems to study these anaerobes is under way and is likely to lead to an increased understanding of this important group of procaryotes.

Excision and integration of a self-transmissible replicon of Streptomyces ambofaciens

When Streptomyces ambofaciens OSF was crossed with the plasmid-free Streptomyces lividans TK24, almost all S. lividans exconjugants contained the free 11.1-kb plasmid pOS1. Southern hybridizations showed that pOS1 was derived from the integrated copy of previously recognized plasmid pSAM2 present in strain OSF. A shorter derivative of pOS1 was constructed carrying the tsr gene in a non-essential region, and this pOS7 plasmid was used in transformation experiments with protoplasts of S. ambofaciens ATCC23877 (containing pSAM2 only as an integrated sequence) and S. ambofaciens DSM40697 (devoid of pSAM2-related forms). In both cases, some clones carrying pOS7 in an integrated state were found. Integration into strain ATCC23877 was into the pre-existing integrated copy of pSAM2. In contrast, plasmid pOS7 integrated through specific plasmidic and chromosomal sites into strain DSM40697. Thus it is probable that pSAM2 integrates by interaction between preferred regions of the plasmid and host genomes.

Characterization of the Streptomyces plasmid pVE1

pVE1 (11.0 kb) was isolated from Streptomyces venezuelae ATCC 14585 and characterized. pVE1 has a broad host range and an apparent copy number of about 100 during exponential growth, rising to 1500 during stationary phase. It is a pock-forming, self-transmissible fertility factor which promotes chromosome recombination in S. lividans at frequencies of about 0.1% per total parents. A detailed restriction map for 15 enzymes was determined. Genes for ThioR (thiostrepton resistance), NeoR (neomycin resistance), and pBR322 derivatives were inserted into pVE1 and the resulting plasmids were analyzed for self-transmissibility and stability. The plasmid has an essential region of approximately 2.5 kb and a region of 1.0-3.6 kb required for conjugation. NeoR and ThioR vectors were constructed with unique HindIII, PvuI, BamHI, EcoRI, BglII, and EcoRV sites available for insertion of foreign DNA.