Blue-White Select^™ Screening Reagent

Sigma′s Blue-White Select^™ Screening Reagent is a chromogenic substrate for β-galactosidase, used to determine the presence or absence of a cloned DNA insert in bacteria growing on agar plates. Blue-White Select^™ Screening Reagent is designed for blue-white selection of recombinant bacterial colonies with the lac+ phenotype.

40 mg/mL IPTG
40 mg/mL X-Gal
(prepared in DMSO)

When Blue-White Select^™ Screening Reagent is cleaved by β-galactosidase, the resulting product will form a blue precipitate. Lac+ colonies grown in the presence of Blue-White Select^™ Screening Reagent remain white in color, allowing for easy differentiation between lac+ and lac- colonies.

The technique is based on vectors such as the pUC and the M13mp series that carry a fragment of the β-galactosidase gene encoding an a-fragment of β-galactosidase. Exploitation of these vectors requires use of a bacteria strain carrying the complementing gene fragment to allow the assembly of an active complex, resulting in the formation of blue colonies. Disruption of the ß-galactosidase gene by insertion of a DNA fragment into the vector′s multiple cloning site results in the loss of functional β-galactosidase activity; these colonies remain white, allowing for easy differentiation between lac+ and lac- colonies.

Blue-White Select is a trademark of Sigma-Aldrich Co. LLC