X Vectors

The bacteriophage X genome comprises 48,502 bp. On entering the host cell, the phage adopts one of two life cycles: lytic growth or lysogeny. In lytic growth, approx 100 new virions are synthesized and packaged before lysing the host cell, releasing the progeny phage to infect new hosts. In lysogeny, the phage genome undergoes recom­bination into the host chromosome, where it is replicated and inherited along with the host DNA (3). Which of the two different life cycles is adopted is determined by the multiplicity of infection and the host cell nutritional status. The larger the multiplicity of infection and the poorer the nutritional state of the cell, the more lysogeny is favored (4).

Use of X as a cloning vector involves only the lytic cycle. This renders the middle third of the X genome, which encodes functions for gene expression regulation and establishment of lysogeny, redundant for these purposes. It is the ability to replace this portion of the genome with foreign DNA without affecting the lytic life cycle that makes X useful as a cloning vector. Insertion X vectors have the nonessential DNA deleted and contain a single site for insertion of DNA. Typically 5-11 kb of foreign DNA can be accommodated in these vectors