phagemids

Phagemids are plasmids that combine a bacterial replication origin with a filamentous phage origin and packaging signals, enabling their DNA to be packaged into phage particles during production. They are used primarily as phage display vectors, allowing insertion of a gene of interest in frame with a coat protein to present the encoded peptide or protein on the phage surface, while the plasmid remains inside the bacterial host.

In typical use, a phagemid is introduced into Escherichia coli and a helper phage provides the missing phage genes needed to assemble phage particles. The phagemid’s packaging signal directs encapsidation of its DNA into new phage particles, which display the encoded fusion on coat proteins such as pIII or pVIII. The result is a phage display library that can be screened by biopanning against a target to enrich high-affinity binders.

Phagemids support large libraries and are used to display peptides, antibody fragments, or other proteins, enabling selection, mutagenesis, and affinity maturation. They combine the convenience of plasmid propagation in bacteria with the surface display capabilities of filamentous phages. They are distinct from full phage genomes because they rely on a helper phage for assembly and cannot propagate independently; the packaged genome is typically single-stranded DNA.

Historically, phagemids became widespread with the development of M13-based display systems in the late 20th century and have since become a standard tool in molecular biology for display and selection experiments.