This document describes the blue-white screening technique for identifying recombinant bacteria. Blue-white screening relies on the activity of β-galactosidase, an enzyme in E. coli that cleaves lactose. Plasmid vectors used in cloning carry a fragment of the lacZ gene. When the plasmid inserts foreign DNA at the multiple cloning site, it disrupts lacZ and prevents complementation, so the bacteria cannot metabolize lactose and appear white on an indicator plate. If no DNA inserts or inserts elsewhere, complementation occurs and bacteria appear blue. The technique allows rapid identification of bacteria that took up recombinant plasmid.

1. S.Y.B.Sc. - PRACTICAL
SESSION-3
(12th Aug.2021)
EXPERIMENT-2
EXPRESSION OF LAC EXPRESSION – BLUE WHITE
SCREENING

2. EXPERIMENT
-2 – STUDY of
LAC
EXPRESSION –
BLUE WHITE
SCREENING

3. BLUE-WHITE SCREENING & PROTOCOLS FOR
COLONY SELECTION
IDENTIFICATION OF
RECOMBINANT BACTERIA
Blue-white screening is a rapid
and efficient technique for the
identification of recombinant
bacteria. It relies on the activity
of β-galactosidase, an enzyme
occurring in E. coli, which cleaves
lactose into glucose and
galactose.

4. The method is based on the principle of α-complementation of
the β-galactosidase gene. This phenomenon of α-complementation
was first demonstrated in work done by Agnes Ullmann in the
laboratory of François Jacob and Jacques Monod, where the function
of an inactive mutant β-galactosidase with deleted sequence was
shown to be rescued by a fragment of β-galactosidase in which that
same sequence, the α-donor peptide, is still intact.[
Langley et al. showed that the mutant non-functional β-
galactosidase was lacking in part of its N-terminus with its residues
11—41 deleted, but it may be complemented by a peptide formed of
residues 3—90 of β-galactosidase.
BLUE-WHITE SCREENING & PROTOCOLS FOR
COLONY SELECTION

8. DISRUPTING
THE LACZ
GENE
The presence of lactose in the
surrounding environment triggers
the lacZ operon in E. coli. The
operon activity results in the
production of β-galactosidase
enzyme that metabolizes the lactose.
Most plasmid vectors carry a short
segment of lacZ gene that contains
coding information for the first 146
amino acids of β-galactosidase. The
host E. coli strains used are
competent cells containing
lacZΔM15 deletion mutation. When
the plasmid vector is taken up by
such cells, due to α-
complementation process, a

10. The plasmid vectors used in cloning are
manipulated in such a way that this α-
complementation process serves as a
marker for recombination.
A multiple cloning site (MCS) is present
within the lacZ sequence in the plasmid
vector.
This sequence can be nicked by restriction
enzymes to insert the foreign DNA.
When a plasmid vector containing foreign
DNA is taken up by the host E. coli, the α-
complementation does not occur,
therefore, a functional β-galactosidase
enzyme is not produced.
If the foreign DNA is not inserted into the
vector or if it is inserted at a location
other than MCS, the lacZ gene in the
plasmid vector complements the lacZ

11. VIDEO LINKS
https://www.youtube.com/watch?v=Y7gxELssMRw
https://www.youtube.com/watch?v=4fnS2xKjIbg