pUC19

[3] This is in part because cells that have successfully been transformed can be easily distinguished from those that have not based on color differences of colonies.

[3] If pUC19 is inserted into E. coli JM109 and grown on agar media supplemented with IPTG and X-gal, then colonies will appear blue, as the plasmid encodes for the α-peptide required to make a functional form of β-galactosidase.

[3] The high copy number is a result of the lack of the rop gene and a single point mutation in the ori of pMB1.

[7][8] The lacZ fragment, whose synthesis can be induced by IPTG, is capable of intra-allelic complementation with a defective form of β-galactosidase enzyme encoded by host chromosome (mutation lacZDM15 in E. coli JM109, DH5α and XL1-Blue strains).

Thus bacteria carrying recombinant plasmids in the MCS cannot hydrolyse X-gal, giving rise to white colonies, which can be distinguished on culture media from non-recombinant cells, which are blue.