Here at Kerafast, our portfolio of available plasmids continues to grow; in recent months, for example, we’ve added Wnt pathway plasmids from the University of Rochester, hepatitis B virus expression plasmids from Penn State College of Medicine and human Bcl-xL plasmids from the University of Louisville. We thought we’d take this opportunity to review what exactly plasmids are and how they’re used in molecular biology research to modify and study organisms.

Plasmids are small circular double-stranded DNA molecules that can self-replicate independently from a cell’s chromosomal DNA. They occur naturally in bacteria, archaea and eukaryotes, but it is in bacteria that plasmids play the most significant role. Bacterial plasmids carry genes that provide useful genetic advantages that help hosts survive environmental stress. For example, bacterial plasmids may encode genes for enzymes that digest antibiotics such as penicillin or ampicillin. When bacteria die, their plasmids are released and can be taken up by other bacteria.

Plasmids are easily isolated and manipulated, and scientists have long leveraged these naturally occurring elements as important research tools. Plasmids are the basis for recombinant DNA technology, and they are used to clone, transfer, express and otherwise manipulate genes. The term “vector” is used to refer to a natural plasmid that has been engineered to make a more useful research tool. A vector has typically been manipulated to allow for the easy cloning of foreign DNA and expression of foreign proteins.

The pMeca Plasmid Vector Map. Note the selectable marker (ampicillin resistance gene) and restriction sites within the multiple cloning site.

When used in molecular biology, a plasmid must contain three basic elements:

• Origin of replication: the DNA sequence that initiates plasmid replication
• Selectable marker: a unique trait associated with the plasmid, but not the host, that allows researchers to select for only the plasmid-containing bacterial cells. A commonly used selectable marker is an antibiotic resistance gene.
• Multiple cloning site: a region that contains several restriction enzyme sites. Restriction enzymes cleave the DNA, making the plasmids linear and allowing for easy insertion of new genes or DNA into the plasmid.

Scientists introduce plasmids into bacteria via a process called transformation. Once successfully introduced and selected for, plasmids are used for a variety of purposes, including:

• DNA cloning. An example cloning plasmid from our catalog is the pMeca plasmid from the University of Georgia. This computer-designed synthetic vector contains 44 unique restriction sites, including 9 rare-cutters, providing the ability to transform an organism with up to eighteen different gene cassettes.
• Gene expression or knock-down. For example, our catalog includes the pRSET-mSA2 protein expression vector from the University of Buffalo, used to express monomeric streptavidin 2 (mSA2) protein.
• Studies of a particular genetic function via a reporter such as GFP or luciferase. An example of a reporter plasmid is the Notch1CR2-GFP reporter plasmid from Rutgers University, which uses GFP to study expression of Notch1CR2, a noncoding sequence active throughout the duration of neurogenesis.

If the plasmid can be used in two different species of hosts, it is known as a shuttle vector. For example, our pSVJ21 psychrophilic bacterium shuttle vector from Penn State University can be used with both E. coli and high G+C Gram-positive psychrophilic (cold-loving) bacteria. In addition, many yeast shuttle vectors also grow in E. coli. An advantage of shuttle vectors is that researchers can manipulate them in E. coli before using them in a slower or more advanced system.

To conclude, plasmids and vectors are an important tool for molecular biology researchers, enabling gene cloning, transfer, expression and more. Review our full list of available plasmids here and contact us at community@kerafast.com if your lab has any materials to add.