fosmid

A fosmid is a DNA cloning vector derived from the bacterial F factor, designed to propagate large DNA fragments in Escherichia coli. Fosmids combine features of plasmids and cosmids, offering stable maintenance with a relatively low copy number. They typically accommodate inserts of roughly 40 kilobases, enabling the cloning of sizable genomic fragments that exceed standard plasmid capacity.

Fosmids are maintained as low-copy-number plasmids, usually present at about one to two copies per cell, which reduces recombination and improves stability of large inserts. The vectors carry an origin of replication derived from the F factor (oriV) and a selectable marker such as chloramphenicol resistance; many also include lacZ alpha for screening. Construction of a fosmid library involves fragmenting genomic DNA, ligating fragments into the fosmid vector, and introducing recombinant molecules into E. coli. These libraries can be screened or mapped, and the clones can be pooled for sequencing. In some workflows, the fosmids can be packaged into lambda phage particles to generate high-density libraries.

Fosmids are used in genome sequencing projects, physical mapping, and functional studies where large inserts are advantageous but bacterial artificial chromosomes (BACs) are not necessary. They provide a balance between insert size, ease of handling, and host compatibility, and remain a common tool for isolating and studying large genomic regions before finishing assemblies.