The construction of plasmid pBR322 was carried out by recombination of three plasmids, pSC 10 1, pSF2 124, and pMB 1, and others, such as Rl and ColEl, served as intermediaries. It was created in 1977 and named its creators, p standing for plasmid, and BR for Bolivar and Rodriguez. pBR322 is 4361 base pairs E. coli plasmid-cloning vector containing the origin of replication from pMB1 (a plasmid in the ColE1 compatibility group) and ampR gene, encoding ampicillin resistance protein (source plasmid RSF2124) and tetR gene, encoding tetracyclin resistance protein (source plasmid pSC101). Widespread use of pBR322 has prompted numerous studies into its molecular structure and function.

Figure 4: Diagrammatic representation of vector pBR322 with exemplified restriction sites.