X-Gal (5-Bromo-4-chloro-3-indolyl-ß-D-galactopyranoside) is a chromogenic substrate for
ß-Galactosidase. It yields a blue precipitate, is soluble in DMF, and can be used to detect vectors, plasmids, or DNA fragments that contain the ß-Galactosidase gene.
IPTG (Isopropyl-ß-D-thiogalactopyranoside) may be used to maximize the expression of genes in expression vectors. When screening a lambda bacteriophage library with an antibody, the probability of success can be increased by preventing the expression of the fusion protein until the plaques are well established. By placing a nitrocellulose filter containing the inducer IPTG onto the plate and continuing growth at 37°C, the expression of the potential ß-galactosidase-cDNA fusion protein can be induced after 3-4 hours of plaque growth. The nitrocellulose filter then is screened with antibodies. IPTG is also used in combination with X-Gal to detect active ß-galactosidase, and it is practical in applications using Bacteriophage M13 and Igt11 vectors. (1)
References:
1. Maniatis, T., Fritsch, E. and Sambrook, J. (1982). Molecular Cloning: A Laboratory Manual.
New York: Cold Spring Harbor Laboratory.
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