Cosmid

A cosmid, first described by Collins and Hohn in 1978, is a type of hybrid plasmid (often used as a cloning vector) that contains cos sequences, DNA sequences originally from the Lambda phage. Cosmids can be used to build genomic libraries.

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Cosmids are able to contain 37 to 52 kbp of DNA, while normal plasmids are able to carry only 1-20 kbp. They can replicate as plasmids if they have a suitable origin of replication: for example SV40 ori in mammalian cells, ColE1 ori for double-stranded DNA replication or f1 ori for single-stranded DNA replication in prokaryotes. They frequently also contain a gene for selection such as antibiotic resistance, so that the transfected cells can be identified by plating on a medium containing the antibiotic. Those cells which did not take up the cosmid would be unable to grow.

However, unlike plasmids, they can also be packaged in phage capsids, which allows the foreign genes to be transferred into or between cells by transduction. Plasmids become unstable after a certain amount of DNA has been inserted into them, because their increased size is more conducive to recombination. To circumvent this, phage transduction is used instead. This is made possible by the cohesive ends, also known as cos sites. In this way, they are similar to using the lambda phage as a vector, but only that all the lambda genes have been deleted with the exception of the cos sequence.

Cos sequences are ~200 base pairs long and essential for packaging. They contain a cosN site where DNA is nicked at each strand, 12bp apart, by terminase. This causes linearization of the circular cosmid with two "cohesive" or "sticky ends" of 12bp. The DNA must be linear to fit into a phage head. The cosB site holds the terminase while it is nicking and separating the strands. The cosQ site of next cosmid (as rolling circle replication often results in linear concatemers) is held by the terminase after the previous cosmid has been packaged, to prevent degradation by cellular DNases.Picture]

Because of the fixed size of the phage head, terminase can only package cosmids that are between 75% and 105% of the length of the normal phage. Thus the practical upper limit of the insert size is around 40kb, since there will also need to be origins of replication, selection genes and multiple cloning sites. To package even more DNA into a vector, bacterial artificial chromosomes or yeast artificial chromosomes can be used.

References

• Bruce A. Voyles (2002) The biology of viruses 2nd ed. ISBN 0-07-237031-9
• Stryer, Lubert (1995) Biochemistry 4th ed. ISBN 0-7167-2009-4

Eurekah Biosciences Collection: Viruses, @NCBI