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Bidirection replication from a unique origin in a mini-F plasmid
Abstract:
Replicating molecules of the mini F-kanamycin resistance plasmid, pML31, derived from F'lac, have been isolated from Escherichia coli as covalently-closed circular DNA molecules. These molecules were examined in the electron microscope after digestion with either EcoRI or BamHI restriction endonuclease. The structure of the majority of the molecules replicating was consistent with a bidirectional mode of replication starting at a unique origin on the F-fragment. This origin is located approximately 2.3 kilobases from one of the EcoRI sites. Orientation of the F-fragment relative to the physical map of F showed the position of the origin to be at 42.6 kilobases. A small proportion of molecules appeared to be replicating unidirectionally in either direction from this origin. Termination of replication of pML31 apparently occurs in the fragment containing the locus for kanamycin resistance in a unique region opposite the origin in the circular DNA molecule.
Insights
Replication of the mini F-kanamycin resistance plasmid (pML31) in Escherichia coli primarily occurs bidirectionally from a specific origin. Termination sites are located opposite this origin, near the kanamycin resistance gene.
Area of Science:
- Molecular Biology
- Genetics
- Microbiology
Background:
- The mini F-kanamycin resistance plasmid, pML31, is a derivative of the F'lac plasmid.
- Understanding plasmid replication mechanisms is crucial for genetic engineering and molecular biology.
Purpose of the Study:
- To investigate the replication mode and origin of the pML31 plasmid in Escherichia coli.
- To determine the replication termination sites of pML31.
Main Methods:
- Isolation of covalently-closed circular DNA molecules of pML31 from Escherichia coli.
- Electron microscopy analysis of DNA molecules after digestion with EcoRI and BamHI restriction endonucleases.
Main Results:
- The majority of pML31 molecules exhibited bidirectional replication, initiating from a unique origin on the F-fragment.
- This origin was mapped to 42.6 kilobases on the F-fragment, approximately 2.3 kilobases from an EcoRI site.
- A small fraction of molecules showed unidirectional replication from the origin.
- Replication termination was observed in a unique region opposite the origin, within the fragment containing the kanamycin resistance locus.
Conclusions:
- Plasmid pML31 replicates via a predominantly bidirectional mechanism in Escherichia coli.
- The identified origin and termination sites provide insights into the control of plasmid DNA replication.