抄録
The plasmid pNR333 is a kanamycin-resistant, deletion derivative of pNR 113 with an extremely high copy number in Escherichia coli and in Proteus mirabilis. In order to determine the usefulness of pNR333 as a replication gene of vector, the genes encoding chloramphenicol acetyl-transferase (CAT) and β-galactosidase (β-gal) were cloned individually into both pNR333 and other low-copy-number plasmids. The expression of the cloned genes was compared by measuring the specific activity of each enzyme and the amounts of the proteins produced. A hybrid plasmid pNR333-cat expressed 53 times as much activity of CAT as the low-copy plasmid S-a which had a copy number of four. The lacZ gene cloned in pNR333 produced 17 times as much β-gal as in the low-copy-number plasmid pNR1150. These results suggest that pNR333 is a useful vector plasmid for producing a large amount of polypeptides in E. coli hosts.
