Plasmids are nonessential extrachromosomal genetic elements that replicate autonomously and control their own replication. Most bacterial plasmids are circular supercoiled DNA molecules that range in size from one to several hundred kilobase pairs and exist in a characteristic number of copies per cell, ranging from one to several hundred.

Keywords: plasmids; DNA replication; antibiotic resistance; extrachromosomal genes

Figure 1.

Bacterial chromosomal and plasmid DNAs. (a) Electron micrograph of the chromosomal DNA of a single Escherichia coli cell (2.6 Mb) prepared by ammonium acetate disruption of the bacterial nucleoid followed by surface spreading in a cytochrome c monolayer. Molecules were shadowed with platinum palladium and photographed at a magnification of about ×1400. Reproduced with permission from Kavenoff R and Bowen B (1976) Chromosome (Berl.)59: 89–101. Copyright © Springer‐Verlag. (b) Electron micrograph of plasmid DNA (30 kb) purified from Staphylococcus aureus by equilibrium density gradient centrifugation. Purified plasmid DNA was spread in a cytochrome c monolayer, shadowed with platinum palladium and photographed at a magnification of about 32 000; it is reproduced here at an original magnification of ×14 400. The plasmid is about 1/100 the size of the E. coli chromosome. Note that both relaxed and supercoiled molecules are present (unpublished data).

Figure 2.

Rolling circle replication: possible termination mechanism. Circular supercoiled plasmid molecule (A) is bound by dimeric initiator protein (RepC), which causes melting in the region of the double‐strand origin (DSO) nicking site, resulting in the formation of a double stern‐loop (cruciform) structure (B). This structure is nicked at a specific nucleotide site by one RepC subunit, which remains attached to the 5′ side of the nick by a phosphotyrosine bond. DNA polymerase holoenzyme plus helicase and possibly single‐strand binding protein combine to catalyse polymerization, using the 3′ end of the nicked leading strand as the primer (C). At the end of the replication cycle, the nascent leading strand is extended a short distance past the DSO nick site, which becomes unpaired from the template (D). This allows RepC subunit A to perform a strand transfer, switching the two leading strand segments and thus circularizing the new leading strand and transferring the short leading strand extension to the 3′ end of the old leading strand (E). The attached RepC, subunit B, then catalyses a second strand transfer, circularizing the displaced leading strand and leaving the short leading strand extension attached to the protein (RepC*), which is then released (F). Replication of the displaced leading strand is then initiated by the host RNA polymerase, using a unique palindromic single‐strand initiation site (SSO) (G).


Further Reading

Chang ACY and Cohen SN (1974) Genome construction between bacterial species in vitro: replication and expression of Staphylococcus plasmid genes in Escherichia coli. Proceedings of the National Academy of Sciences of the USA 71: 1030–1034.

Gerdes K, Rasmussen PB and Molin S (1986) Unique type of plasmid maintenance function: postsegregational killing of plasmid‐free cells. Proceedings of the National Academy of Sciences of the USA 83: 3116–3120.

Kinashi H, Shimaji M and Sakai A (1987) Giant linear plasmids in Streptomyces which code for antibiotic biosynthesis genes. Nature 328: 454–456.

Novick RP (1980) Plasmids. Scientific American 243(6): 102–4, 106, 110 passim.

Novick RP (1987) Plasmid incompatibility. Microbiological Reviews 51: 381–395.

Novick RP, Iordanescu S, Projan SJ, Kornblum J and Edelman I (1989) pT181 plasmid replication is regulated by a countertranscript‐driven transcriptional attenuator. Cell 59: 395–404.

Portnoy DA and Martinez RJ (1985) Role of a plasmid in the pathogenicity of Yersinia species. Current Topics in Microbiology and Immunology 118: 29–51.

Rasooly A and Novick RP (1993) Replication‐specific inactivation of the pT181 plasmid initiator protein. Science 262: 1048–1050.

Nordstrom K, Ingram LC and Lundback A (1972) Mutations in R factors of \fIEscherichia\fR \fIcoli\fR causing an increased number of R‐factor copies per chromosome. Journal of Bacteriology 110: 562–569.

Tomizawa J and Itoh T (1981) Plasmid ColE1 incompatibility determined by interaction of RNA I with primer transcript. Proceedings of the National Academy of Sciences of the USA 78(10): 6096–6100.

Timmis K, Cabello F and Cohen SN (1975) Cloning, isolation, and characterization of replication regions of complex plasmid genomes. Proceedings of the National Academy of Sciences of the USA 72(6): 2242–2246.

Inuzuka M and Helinski DR (1978) Replication of antibiotic resistance plasmid R6K DNA in vitro. Biochemistry 17(13): 2567–2573.

Bazaral M and Helinski DR (1968) Circular DNA forms of colicinogenic factors E1, E2 and E3 from Escherichia coli. Journal of Molecular Biology 36(2): 185–194.

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Novick, Richard P(Jan 2002) Plasmids. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1038/npg.els.0001490]