fosmids

Fosmids are cloning vectors derived from the Escherichia coli F-factor plasmid that enable stable propagation of large DNA inserts, typically about 35 to 40 kilobases in length. They occupy a niche between conventional plasmids and BACs in terms of insert size and stability.

The vector backbone carries an origin of replication from the F-factor, a selectable antibiotic resistance marker, and cloning sites for inserting DNA fragments. Inserts are maintained at low copy in Escherichia coli, which helps preserve large DNA fragments that may be unstable in higher-copy plasmids.

Fosmids are primarily used to construct genomic libraries and to enable large-scale mapping and sequencing projects. To build a fosmid library, genomic DNA is partially digested and size-selected to around 40 kb, ligated into the fosmid vector, and transformed into E. coli. Each clone typically contains a single large DNA fragment, and the collection expands to cover the genome with overlapping inserts.

Screening methods such as colony hybridization or PCR, together with pooling strategies, are used to identify clones of interest.

Advantages include the ability to clone relatively large fragments with good stability and ease of handling in standard bacterial culture. Limitations include insert sizes that are still smaller than those of BACs, and library construction can be more labor-intensive and time-consuming compared with high-throughput short-insert approaches. Fosmids remain a useful tool in functional genomics and comparative genomics, especially when maintaining long fragments is important for assembly and organization of complex genomes.