Bacterial Plasmids


Plasmids are nonessential genetic elements that can maintain accessory genetic information and facilitate infectious spread of the genes they carry. They regulate their own replication and transmission and were largely responsible for the proliferation of antibiotic resistance during the second half of the twentieth century.

Keywords: extrachromosomal DNA; gene cloning; antibiotic resistance; gene transfer; DNA replication

Figure 1.

Alternative plasmid structures: (a) supercoiled circular plasmid; (b) linear ‘racket frame’ structure and (c) an ‘endless linear’ plasmid (5′‐ and 3′‐DNA termini are joined by a single‐strand loop).

Figure 2.

Segregation of incompatible plasmids.

Figure 3.

Control of replication by antisense RNA in plasmid ColE1. ColE1 illustrates the way that antisense RNA can provide a simple negative feedback loop that controls replication in step with cell growth. RNA I is produced in large quantities but is unstable so its concentration is proportional to plasmid copy number. RNA II is made at low level but is more stable and folds in a way that allows association with the replication origin before RNAaseH processing. RNA II transiently forms a structure that associates with hairpin loops of RNA I and is inactivated, so when copy number is high and RNA I concentration is high, very little RNA I is allowed to proceed to form primer for replication. Rom promotes the association of RNA I and II and so further reduces copy number.

Figure 4.

Replication control of the P1 prophage by plasmid handcuffing (a) the R‐replicon of phage P1; iterons flanking repA are indicated by arrows (b) following replication, plasmid pairing is mediated by iteron‐bound RepA and eventually disrupted by partition and cell division.



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Further Reading

Bushman F (2001) Lateral DNA Transfer. New York: Cold Spring Harbor Laboratory Press.

Funnell BE and Phillips GJ (eds) (2004) Plasmid Biology. Washington, DC: ASM Press.

Grinstead J and Bennett PM (1988) Plasmid technology. In: Methods in Microbiology. London: Academic Press.

Hardy KG (1987) Plasmids: A Practical Approach. Oxford: IRL Press.

Hardy KG (1993) Plasmids: A Practical Approach. Oxford: IRL Press.

Snyder L and Champness W (2002) Molecular Genetics of Bacteria. Washington, DC: ASM Press.

Summers DK (1996) The Biology of Plasmids. Oxford: Blackwell Science.

Syvanen M and Kado CI (2002) Horizontal Gene Transfer. Amsterdam: Elsevier.

Thomas CM (ed.) (2000) The Horizontal Gene Pool: Bacterial Plasmids and Gene Spread. Amsterdam: Harwood Academic Press.

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Thomas, Christopher M, and Summers, David(Sep 2008) Bacterial Plasmids. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0000468.pub2]