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pUC18-19 - It is a derivative of pBR322 and is much smaller (-2.7 kb); it has all the essential parts of pBR322, e.g.,
(1) ampicillin resistance gene and
(2) Col E1 origin.
The second scrobale marker is due to E. coli gene lacZα encoding the a fragment of β-galactosidase, the enzyme that hydrolyses lactose.
The E.coli strains, e.g., JMIO3, JM1O9, used as hosts for the pUC series vectors have the lacZα deleted from their lacZ genes. When pUC enters such an E.coli cell, the host genome and the plasmid encode for different parts of the p-galactosidase enzyme, which interact with each other to produce the active enzyme enabling these cells to hydrolyse lactose. β -galactosidase also hydrolyses X-gal (5-Bromo-4-chloro-3-indolyl-p-D-galactoside) to yield a blue dye.

Therefore appropriate lacZ- E. coli cells transformed by the pUC vectors behave as lacZ+ and produce blue coloured colonies on a X-gal containing medium. A poly linker sequence located within the lacZα provides several (10 in case of pUC18/pUC19) unique restriction sites for DNA insertion.

The polylinker sequence by itself does not interfere with lacZα ' expression. But when a DNA insert is placed within it, lacZα expression is prevented. Vectors pUC18 and pUC19 are identical, except for the orientation of the polylinker sequence, which is oriented in the opposite directions in the two vectors.

The unique restriction sites used for integration of DNA inserts into pUC vectors interrupt the lacZα fragment so that appropriate E. coli cells possessing recombinant pUC DNA are β-galactosidase deficient and, as a result, produce white colonies on X-gal medium.

Therefore, appropriate E.coli cells transformed with pUC recombinant DNA are grown on ampicillin, X-gal and IPG (isopropyl- β D-thiogalactoside; it serves as inducer of β -galactosidase, while X-gal itself can not) containing medium to eliminate non transformed cells.

The white colonies are selected as they contain the recombinant DNA (in contrast, blue colonies will contain the unaltered vector). The other vectors in pUC series are pUC 8, pUC 9, pUC 12, pUC 13, etc.

The pUC series vectors offer the following adantages over pBR322:
(1) each E. coli cell produces up to 700 copies without any treatment,
(2) cells containing recombinant DNA are selected in a single step,
(3) the sites for DNA insert integration are confined to the poly linker, which permits the use of two restriction anzymes to open the vector, and
(4) they also allow sequencing of the DNA insert.
 

 
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