Bacterial artificial chromosomes are engineered pieces of DNA usually from Escherichia coli that can replicate and divide. Scientists can insert DNA of interest into the BAC and as the E. coli grows and divides, the DNA is amplified allowing large amounts of it to be isolated for sequencing (1). This molecular tool was used primarily for the Human Genome Project in which human genomic DNA libraries were constructed and sequenced (2).
BACs were created by Hiroaki Shizuya from California Institute of Technology. Creation of the BAC was done with three goals in mind: 1) create a stable cloning system that was easy to manipulate and map, 2) the ability to insert large pieces of DNA for sequencing, and 3) to develop a molecular cloning tool that could be used even after an organism's sequence was determined (3).
Components of BACs:Edit
BACs are comprised of several components that are needed for replication much like the pieces of a bacterial cloning vector. The difference between a cloning vector like pUC19 and a BAC is that BACs can carry up to 400kB of inserted DNA while traditional vectors can only hold up to 100kB (3). Additionally, traditional vectors are engineered with promoters to express a specific protein while the BAC contains the endogenous regulatory elements (4).
The machinery of a BAC is based on the E. coli F plasmid, which is a low copy number (1-2 copies per cell) plasmid. The advantage of the low copy number is to reduce recombination events from the inserted DNA and the vector itself. The first BAC (pBAC108L, Figure 1 ) contains four restriction enzyme sites, a chloroamphenicol resistance site (CmR), the origin of replication site oriS, and the parA and parB partitioning genes that are responsible for segregation of the bacterial plasmid when the bacteria grows and divides (3, 4, 5, 6).
New Applications for BACsEdit
Once the human genome was complete, scientists found new and interesting ways of using BACs both in vitro and in vivo. In vivo, BACs are used extensively for the creation of transgenic mice. For example, tagging the central nervous system cholinergic elements with GFP to follow patterns of neurotransmission (JAX Mice Database, 7). More recently, BAC vectors are being used to make recombinant proteins in vitro due to their open chromatin regions. In fact, Mader et al. showed that CHO cells transfected with recombinant protein BAC vectors versus plasmid vectors enhanced the transcription rate of the proteins of interest (8).
1. National Human Genome Research Institute. Talking Glossary of Genetic Terms. 2013
2. Wikipedia. "Bacterial Artificial Chromosomes." http://en.wikipedia.org/wiki/Bacterial_artificial_chromosome
3. Shizuya, H; Kouros-Mehr, Hosein (2001). "The development and applications of the bacterial artificial chromosome cloning system." Keio J Med. 50(1): 26-30. PMID: 11296661
4. Armstrong, J and Hirschi, K. (2011). "Recombineering of BAC DNA for the generation of transgenic mice". http://cdn.intechopen.com/pdfs/24401/InTechRecombineering_of_bac_dna_for_the_generation_of_transgenic_mice.pdf
5. Schumacher, MA. (2007). "Structural Biology of Plasmid Segregation Proteins." Curr Opin Struct Biol. 17(1):103-109 PMID 17161598
6. Wikipedia. "Segrosome." http://en.wikipedia.org/wiki/Segrosome
7. JAX Mice Database: http://jaxmice.jax.org/strain/007902.html
8. Mader et al. (2013). "Exploration of BAC versus plasmid expression vectors in recombinant CHO cells." Appl Microbiol Biotechnol. 97(9): 4049-54 PMID: 23081777