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.

Key Concepts

  • By definition, a plasmid is an autonomously replicating nonessential genetic element.
  • Plasmids are classified by the relatedness of their backbone functions which are responsible for replication, stable inheritance and transfer.
  • All plasmids encode a mechanism that inhibits plasmid replication when its copy number rises and the genes that achieve this can be inserted into a vector that can displace the plasmid by blocking its replication.
  • Natural plasmids have evolved a variety of systems to ensure that they are inherited stably including addiction systems that prevent growth of bacteria that lose the plasmid.
  • Many plasmids can spread from one bacterium to another by a replicative transfer process that donates a copy to the new cell while leaving at least one copy in the donor cell.
  • Plasmids can be pictured as cargo trains with the backbone functions represented as the engines that drive the system and different mobile DNA as the trucks that can join the engine and the phenotypic markers as the cargo carried by the trucks.

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. Based on Ebersbach G, Sherratt DJ and Gerdes K (2005) Partition‐associated incompatibility caused by random assortment of pure plasmid clusters. Molecular Microbiology 56: 1430–1440.
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.
Figure 5. Summary of the most standard conjugative transfer process. A plasmid‐encoded pilus is assembled on the bacterial surface by its cognate T4SS and initiates mating pair formation when it contacts a suitable recipient. The mating bridge is recognised by the relaxasome which nicks one strand and creates a nucleoprotein complex at the 5' end that leads the DNA across the bridge. Ilangovan A, Kay CWM, Roier S, El Mkami H, Salvadori E, Zechner E, Zanetti G, Waksman G Cryo‐EM Structure of a Relaxase Reveals the Molecular Basis of DNA Unwinding during Bacterial Conjugation. Cell 169: 708–721. Public Domain.


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Further Reading

Addgene (2019) Plasmids 101: A Desktop Resource, 3rd edn.‐101‐eBook‐3rd‐Ed‐Final

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

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

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

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

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

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

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

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

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

Tolmasky ME and Alonso JC (eds) (2015) Plasmids: Biology and Impact in Biotechnology and Discovery. ASM Press: Washington, DC.

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Thomas, Christopher M, and Summers, David(Sep 2020) Bacterial Plasmids. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0029193]