Indigenous plasmids in a production line of strains for penicillin G acylase derived fromEscherichia coli W

Three indigenous plasmids designated pRK1, pRK2 and pRK3 were identified among producers of penicillin G acylase (PGA) derived from the strain Escherichia coli W ATCC 9637. Their size and copy number (CN) in E. coli W were determined (kb; CN): pRK1 (80; 3.4), pRK2 (5.1; 71), and pRK3 (4.8; 13.7). Strain E. coli RE2 harboring these plasmids was used for selection of strains with reduced number of plasmids: the strain RE3 without plasmid pRK1 and the plasmid-less strain cERE3 were isolated. Indigenous plasmids did not code for the resistance determinants against 23 antibiotics and 10 heavy metals.