Abstract:
The plasmids pDR401 and pDR412 are disclosed which contain a selectable chloramphenicol resistance gene and which are able to replicate in both T. ferrooxidans and E. coli. A process for constructing the plasmids is also described.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to the construction of recombinant DNA cloning plasmid vectors for Thiobacillus ferrooxidans which contain a selectable chloramphenicol resistance gene and are able to replicate in both T. ferrooxidans and Escherichia coli. 
     Although selectable cloning vectors have been constructed for other bacteria, there are, to the applicant&#39;s knowledge, no previous reports of the construction of selectable cloning vectors for T. ferrooxidans which can also replicate in E. coli. 
     SUMMARY OF THE INVENTION 
     According to the present invention a plasmid vector is constructed by extracting a cryptic DNA plasmid from a T. ferrooxidans strain, cleaving the T. ferrooxidans plasmid and a second plasmid which contains a chloramphenicol resistance gene with the same restriction enzyme, and ligating the plasmids to form a recombinant plasmid. 
     The second plasmid may be the plasmid pBR325, although other suitable plasmids could possibly by used in place of pBR 325. 
     The resulting recombinant plasmid, pDR401, replicated in E. coli. 
     According to a further aspect of the invention the pDR401 plasmid may be reisolated and modified by cleavage with at least one restriction enzyme to remove the E. coli origin of replication. 
     The deleted recombinant plasmid, pDR412, contains the chloramphenicol resistance gene and replicated in E. coli using the T. ferrooxidans origin of replication. 
     The invention also extends to the recombinant plasmids, pDR401 and pDR412, as herein defined, which are novel selectable shuttle vectors and which can be used for genetic manipulation experiments involving T. ferrooxidans and E. coli. 
     Thus the invention provides a plasmid pDR401 which has a size of about 17.8 kb and which is divided into three fragments having the sizes 3.5 kb, 6.2 kb and 8.1 kb, respectively, by the restriction enzyme Pvul. 
     The invention also provides a plasmid pDR412 which has a size of about 14.8 kb and which is divided into three fragments having the sizes 3.5 kb, 5.1 kb and 6.2 kb, respectively, by the restriction enzyme Pvul. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     An example of the invention is described with reference to the accompanying drawing which illustrates restriction maps of the cryptic plasmid, and the recombinant plasmids of the invention. 
    
    
     A 12.4 kilobase (kb) cryptic plasmid, pTF-FC2, was extracted from a T. ferrooxidans FC strain isolated from acid leach liquor from Fairview Mine, General Mining Union Corporation Limited, South Africa. Restriction mapping of pTF-FC2 shows that it has unique Pst1, Xho1, Kpn1, EcoR1, Apa1, restriction sites and two Pvu1 restriction sites, (refer to the accompanying drawing). 
     A recombinant plasmid, pDR401, was constructed by insertion of the E. coli plasmid, pBR325, into the Pst1 site of pTF-FC2. pBR325 contains the genes for ampicillin (Ap), chloramphenicol (Cm) and tetracycline (Tc) resistance. Cloning at the Pst1 site insertionally inactivated the Ap R  gene and E. coli transformants which were Cm R , Tc R  and Ap S  were isolated. The recombinant plasmid pDR401 (about 17.8 kb) was extracted from the E. coli transformants and characterized by restriction analysis. This novel recombinant plasmid contains the genes for Cm R  and Tc R  and is able to replicate in T. ferrooxidans and E. coli. 
     A deletion plasmid, pDR412 (about 14.8 kb), was constructed following Sal1/Xho1 digestion of pDR401 and removal of the Sal1/Xho1 fragment (about 3.0 kb) which contained the pBR325 origin of replication and approximately half of the Tc gene, and a 0.3 kb piece of the T. ferrooxidans pTF-FC2 DNA. Restriction analysis confirmed that the Sal1/Xho1 3,1 kb fragment had been deleted in the plasmid pDR412. This novel recombinant plasmid contains the gene for Cm R  and is able to replicate in T. ferrooxidans and E. coli. 
     The two plasmids pDR401 and pDR412 were characterized by endonucleolytic cleavage with a restriction enzyme. 
     Thus the restriction enzyme Pvu1 cleaves the plasmid pDR401 into three fragments having the sizes 3.5 kb, 6.2 kb and 8.1 kb, respectively. 
     The same restriction enzyme Pvu1 cleaves the plasmid pDR412 into three fragments having the sizes 3.5 kb, 5.1 kb and 6.2 kb, respectively. 
     The T. Ferrooxidans strain has been deposited with the American Type Culture Collection, 12301 Parklawn Drive, Rockville, MD 20852 and has been assigned ATCC number 39838. It has a taxonometric description as follows: It is an autotrophic aerobic, Gram-negative, rod shaped bacterium. It is able to oxidize ferrous iron to ferric iron and reduced and partially reduced sulfur compounds to sulfuric acid. Its optimum pH is between 1.6 and 2.2 and its optimum temperature is between 25° C. and 30° C. It has a G-C ratio of 59-60 mol percent.