J Bacteriol, May 1998, p. 2796-2799, Vol. 180, No. 10
Department of Molecular Biotechnology,
Graduate School of Engineering, Hiroshima University,
Higashi-Hiroshima 739-8527, Japan,1 and
Genome Research Unit, Department of Genetics,
Kaiserslautern University, D-67653 Kaiserslautern,
Germany2
Received 25 November 1997/Accepted 10 March 1998
A cosmid library was constructed for the 350-kb giant linear
plasmid SCP1 and aligned on a successive linear map. Only a
0.8-kb gap has remained uncloned in the terminal inverted repeats close to both ends. Partial digestion of the aligned cosmids
with EcoRI and hybridization with the flanking fragments of
the vector enabled physical mapping of all of the
EcoRI fragments. On this map, the methylenomycin
biosynthetic gene cluster, the insertion sequence IS466,
and the sapCDE genes coding for spore-associated proteins were localized.
Linear plasmids have been found in a
variety of eucaryotic species, but have rarely been identified in
procaryotic organisms. However, the filamentous soil bacteria
Streptomyces spp. possess frequently linear plasmids, which
seem therefore to be a significant feature of this genus
(12). The plasmids vary in size between 12 kb
(28) and 1 Mb (24a). They contain terminal
inverted repeats (TIRs) which range between 44 bp in SLP2 of
Streptomyces lividans (3) and 95 kb in pPZG101 of
Streptomyces rimosus (6). The 5' ends of all of
the plasmids investigated were shown to be blocked by a protein.
Lin et al. (20) first demonstrated that S. lividans has an 8-Mb linear chromosome. The linear chromosome
topology has been proved for Streptomyces griseus
(19), Streptomyces ambofaciens (18),
Streptomyces coelicolor A3(2) (26), and S. rimosus (25). The close structural similarities between
Streptomyces chromosomes and linear plasmids suggest a
hypothetical idea that linear plasmids were generated by recombination
of linear chromosomes, or circular chromosomes were linearized by
integration of a linear plasmid.
SCP1 from S. coelicolor A3(2) is genetically the
best-studied linear plasmid. The element long remained unisolated but
was postulated on the basis of classical genetic analysis
(27). It was shown to carry genes for the biosynthesis of
and resistance to the antibiotic methylenomycin (2, 17).
With the introduction of pulsed-field gel electrophoresis (PFGE), SCP1
was revealed to be a 350-kb giant linear plasmid containing 80-kb-long
TIRs (13, 15).
Considerable attention has been paid to SCP1 because of its
interaction with the host chromosome. Freely replicating SCP1 mobilizes chromosomal markers randomly, as seen for all fertility plasmids in Streptomyces. In addition, Hopwood and
colleagues isolated mutants carrying an SCP1-chromosome hybrid
structure which promotes a directional DNA transfer during conjugation
(9, 10). Free SCP1-prime plasmids containing a certain
chromosomal DNA stretch were found as well as mutants
possessing SCP1 integrated in a central region of the
S. coelicolor chromosome. Those SCP1-integrated strains showed either unidirectional or bidirectional DNA
transfer with respect to the SCP1 integration site. Preliminary
structural analysis of the integrated copies of SCP1 has been carried
out for S. coelicolor A3(2) strain 2612 (an NF strain)
(8) and strains A608 (a pabA donor) and A634 (an
NF-like donor strain) (16).
To reveal in more detail the genetic organization of SCP1 and its role
in conjugative DNA transfer, we established an ordered contig of cosmid
clones covering most parts of the element.
Generation of an ordered cosmid library.
A cosmid library was
prepared for total DNA of S. coelicolor 1147, a wild-type
A3(2) strain, which carries both SCP1 and a 31-kb circular plasmid,
SCP2 (22). Total DNA was isolated and partially
digested with Sau3AI to an average size of 40 to 60 kb.
As described previously (26), the DNA was ligated to
the vector Supercos-1 (5) (Stratagene, La
Jolla, Calif.) and packaged into lambda phages which were
subsequently used to transfect Escherichia coli
Sure. Two thousand cosmid clones representing the total genome of
S. coelicolor A3(2) were screened with the SCP1 DNA, which was isolated by contour-clamped homogeneous electric fields (CHEF) (4) and nonradioactively labeled with dig-11-dUTP
(Boehringer Mannheim, Mannheim, Germany). A total of 192 clones which
hybridized strongly with the SCP1 DNA were obtained.
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Cloning and Physical Mapping of the
EcoRI Fragments of the Giant Linear Plasmid
SCP1
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FIG. 1.
Ordered cosmid and restriction maps of SCP1. In the
upper part, 12 cosmids covering almost the entire region of SCP1 are
aligned on a continuous linear map. The open and solid circles indicate
T3 and T7 promoters, respectively. pSCP201, which carries the 4.1-kb
terminal SpeI fragment of SCP1 (14), is included
at both ends. A gap has remained uncloned between pSCP201 and cosmid
31. The restriction map of SCP1 is drawn in the center. Sites for
EcoRI and rare cutting enzymes are shown above and below the
SCP1 line. Previous sequence analysis of pSCP201 revealed that five
EcoRI sites are present in the 342 nucleotides at the end of
SCP1. Among them, four EcoRI sites, the total size of whose
fragments is 0.2 kb, were omitted here to avoid complexity, but they
are included in Fig. 3A. The sizes of fragments are indicated in
kilobases. The positions of the mmr, IS466, and
sapCDE genes and two TIRs, left (TIR-L) and right (TIR-R),
are shown at the bottom. The transcription direction of the
mmr gene (24) is indicated by an arrow. Af,
AflII; As, AseI; V, EcoRV; Sp,
SpeI; Ss, SspI; X, XbaI.
Construction of the EcoRI fragment map. To establish a precise restriction map of SCP1, we chose EcoRI as an appropriate enzyme. EcoRI generated a suitable number (about 40) of observable SCP1 fragments and could excise the insert from the vector. Since SCP1 and Supercos-1 contain only two AseI sites, the 4.3- and 0.9-kb AseI-EcoRI fragments of the latter could be used as probes to determine the order of EcoRI fragments. Each cosmid clone was first completely digested with AseI and then partially digested with EcoRI. The partial digest was separated by CHEF gel electrophoresis and hybridized with the 4.3- and 0.9-kb AseI-EcoRI probes separately. This method also confirmed the direction of the insert, because the 4.3- and 0.9-kb fragments carry the T7 and T3 promoters, respectively.
In Fig. 2, the analysis of cosmid 53 is shown as an example. The complete restriction digest of cosmid 53 with AseI and EcoRI gave seven fragments (Fig. 2A): four from the SCP1 DNA (14.0, 13.0, 8.6, and 6.5 kb) and three from the vector DNA (4.3, 1.7, and 0.9 kb). Hybridization of the 4.3-kb AseI-EcoRI probe to the partial EcoRI digest revealed bands of 4.3, 13, 27, 33, 46, and 47 kb (Fig. 2B). This result determined the order of EcoRI fragments from the T7 promoter (right-handed direction) as (4.3)-8.6-14.0-6.5-13.0-(0.9 kb). The same analysis was carried out with the 0.9-kb AseI-EcoRI probe, which showed hybridizing bands of 0.9, 14, 20, 34, 43, and 47 kb (Fig. 2C). The order in the opposite direction from the T3 promoter agreed with the result described above.
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Location of the mmy, IS466, and sapCDE genes. Several genes were reported to be present on SCP1. Chater and Bruton (2) constructed the restriction maps for the methylenomycin biosynthetic gene (mmy) clusters on plasmids SCP1 and pSV1; the latter was suggested to be a 170-kb circular plasmid (1), in contrast to the linear structure of the former. Hybridization experiments with pIJ518 as a probe, which carries the 7.5-kb mmy fragment of SCP1 (2) (Fig. 3B), revealed that the resistance gene (mmr) is located on the 10-kb EcoRI fragment of cosmid 73. The restriction map of the mmy region on cosmid 73 agreed with that of Chater and Bruton (2).
We previously reported that the insertion sequence IS466 (11) is present just at the inside end of the right TIR of SCP1 (14). Analysis of cosmid 63 determined its location on the EcoRI map as shown in Fig. 3C. Guijarro et al. (7) isolated spore-associated proteins SapA and SapB from S. coelicolor A3(2). The sapA gene coding for the former protein has been cloned (7) and was located on AseI fragment F of the chromosome (26). McCormick et al. (23) cloned additional sap genes, sapCDE, which, however, seemed not to be essential for spore formation, because they were mapped to SCP1, and SCP1
strains show no effect on sporulation. Two sets of the
sapCDE genes were located in each of both TIR regions, and
their restriction maps were constructed (22a). Analysis of
cosmid 53 could locate the sapCDE genes on the
EcoRI map (Fig. 3C).
Ordered clone libraries have proved to be an excellent tool for
analysis of large genomic DNAs. For the 8-Mb S. coelicolor A3(2) chromosome, an ordered cosmid library has been established and
used extensively for gene mapping (27), and it is now being used for the genome project in collaboration with the Sanger Centre and
the John Innes Centre. Here we reported the generation of an ordered
cosmid contig of the giant linear plasmid SCP1, leaving only two 0.8-kb
gaps uncloned. A detailed EcoRI restriction map was
constructed, and the genetic markers IS466,
mmy, and sapCDE were assigned to specific SCP1
fragments. These results will be of great value for study of the
structure and function of SCP1 and analysis of the interaction of
SCP1 with the chromosome of S. coelicolor A3(2).
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ACKNOWLEDGMENTS |
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We thank Keith Chater for plasmid pIJ518; John Cullum for pMT644, which carries IS466; and Joe McCormick for pRC2 and pRD2, which carry a part of sapC and sapD, respectively.
M. Redenbach has been supported by a scholarship from the Japan Society for the Promotion of Sciences.
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FOOTNOTES |
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* Corresponding author. Mailing address: Department of Molecular Biotechnology, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan. Phone: 81-824-7869. Fax: 81-824-24-7869. E-mail: kinashi{at}ipc.hiroshima-u.ac.jp.
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