Bacterial artificial chromosomes (BACs)

In order to overcome the above difficulties associated with YAC vector, a bacterial cloning system based on E. coli F factor was designed which was capable of cloning fragments of upto 300-350kb. These were described as bacterial artificial chromosomes (BACs) and are 'user friendly' being a bacterial system. BAC vectors are superior to other bacterial systems, based on high to medium copy number of replicons, since they show structural instability of inserts, deleting or rearranging portions of cloned DNA.

Cloning segment includes the following sequences: (i) phage lambda cosN site (providing a fixed position for specific cleavage with lambda terminase) and loxP site (providing a position for cleavage due to PI Cre protein in presence of loxP oligonucleotide); these two sites allow generation of ends that can be used for restriction site mapping to arrange the clones in an ordered array.

(ii) two cloning sites (HindIII and BamHI) and (iii) several C + G rich restriction sites (NotI, Eagl, Xmal, Smal, Bg I1 and Sfil) for potential excision of inserts; the cloning site is flanked by T7 and SP6 promoters for generating RNA probes for chromosome walking and for DNA sequencing. BAC libraries have already been prepared in humans, mouse, rice, wheat, Lotus, etc.