In accordance with the invention, there is provided a method for preparing
plasmid from host cells which contain the
plasmid, comprising the steps: (a) preparing a cleared lysate of the host cells, wherein the cleared lysate comprises unligatable open circular
plasmid, wherein the open circular plasmid is not 3'-hydroxyl, 5-
phosphate nicked plasmid; (b) incubating the unligatable open circular plasmid with one or more enzymes in the presence of their appropriate
nucleotide cofactors, whereby the unligatable open circular plasmid is converted to 3'-hydroxyl, 5'-
phosphate nicked plasmid; (c) incubating the 3'-hydroxyl, 5'-
phosphate nicked plasmid with
DNA ligase in the presence of
DNA ligase
nucleotide cofactor, whereby 3'-hydroxyl, 5'-phosphate nicked plasmid is converted to relaxed covalently closed circular plasmid; and (d) incubating the relaxed covalently closed circular plasmid with
DNA gyrase in the presence of DNA gyrase
nucleotide cofactor, whereby relaxed covalently closed circular plasmid is converted to negatively supercoiled plasmid. Preferably, the enzymatic steps (b), (c), and (d) are performed in a
single step using an
enzyme mixture comprising
DNA polymerase,
DNA ligase, and DNA gyrase. Preferably, the mixture further comprises a 3' terminus deblocking
enzyme, such as
exonuclease III or 3'-
phosphatase. Preferably, the mixture further comprises one or more regenerating enzymes and a
high energy phosphate donor, whereby the nucleotide by-products of the nucleotide cofactors generated by
DNA ligase and DNA gyrase are converted to back to nucleotide
cofactor. Preferably, the
enzyme mixture further comprises one or more exonucleases, such as ATP dependent
exonuclease, whereby linear
chromosomal DNA is selectively degraded.