Phage artificial chromosome or P1-derived artificial chromosome (PAC) is a form of chromosome derived through biological manipulation and it originates from a ‘phage’ instead of a ‘plasmid’, as seen in the construction of many other artificial chromosomes.
Definition of a ‘phage’:
A ‘phage’ or a ‘bacteriophage’ is a virus that can enter a bacteria and replicate within the cell using one or more methods. There are many types of phages and P1 bacteriophage is a unique type of phage that exists in nature.
Mechanism of ‘phage’ replication:
When phages infect a bacterial cell, it can exist in two forms; either through ‘lysogeny’ or by ‘lysis’. In ‘lysogeny’, the phage exists in a similar manner to a bacterial plasmid and in a circular form. While existing in this form, the phage can replicate without destroying the cell. However, during the ‘lysis’ phase, the phage can destroy the cell it inhabits and will create new particles as well.
Uniqueness of P1-bacteriophage:
A P1 phage can exist in both forms as described earlier, but its unique feature lies in its existence as an independent entity within the cell, rather than incorporating itself with the host chromosomes during the phase of ‘lysogeny’. Thus, it acts like a plasmid during its existence and therefore can replace the function of a plasmid during processes, which entails this feature. However, the scientists consider P1 derived chromosome to contain features of both plasmids and ‘f’ factor, which is a unique plasmid like DNA sequence used in creating bacterial artificial chromosomes (BAC).
PACs can accommodate larger inserts of DNA than a plasmid or many other types of vectors. Sometimes, the number of inserts can be as high as 300 kilobase paires.
Construction of PACs through electroporation:
During the construction of PACs, P1 phage containing cells will undergo a process known as ‘electroporation’, which will increase the permeability of the cell membrane and allow DNA material to enter the cell and couple with the existing DNA. This process will give rise to PACs and from there onwards, the PACs can replicate within the cell through ‘lysogeny’, without destructing the cell or incorporating into rest of the chromosomes.
Uses of PACs:
PACs are in high demand when it comes to cloning important biomedical sequences, which are essential for many scientific functions. One of its main uses is the genome analysis and map based cloning of complex plants and animals, which requires isolation of large pieces of DNA rather than smaller segments. Furthermore, PAC based cloning is useful in the study of ‘phage therapy’ and in scientific studies focusing on how antibiotics act on a particular bacteria.
Although there are other forms of artificial chromosomes which can accommodate more base pairs than PACs, relative user friendliness of these vectors makes it a popular choice among many biomedical researchers.