Isopropyl-beta-D-thiogalactopyranoside(IPTG) is an S-glycosyl compound consisting of beta-D-1-thiogalactose having an isopropyl group attached to the anomeric sulfur. Isopropyl-beta-D-thiogalactopyranoside is a useful molecular biology reagent. Isopropyl-beta-D-thiogalactopyranoside is a molecular mimic of allolactose that initiates transcription of the lac operon[1]. The key feature of IPTG is that, unlike allolactose, is not hydrolyzed by β-galactosidase, therefore its concentration remains constant during an experiment. IPTG is therefore able to induce protein expression where the gene is under the control of the lac operator, when used as an effective inducer in the concentration range of 100μM to 1.5mM. A very strong inducer, it is very stable even in bacterial metabolism, so it is widely used in laboratories. 

Fig 1. Chemical structure formula and three-dimensional structure of Isopropyl-beta-D-thiogalactopyranoside

Isopropyl-beta-D-thiogalactopyranoside is beta - galactose glucoside enzyme activity of inductive substance. Based on this feature, as the carrier of pUC series of DNA (or other carrier with lacZ gene DNA) with lacZ lack for host cell transformation, or by the carrier of M13 phage DNA transfection, if add X - Gal in plate culture medium and Isopropyl-beta-D-thiogalactopyranoside, due to the alpha beta - galactose glucoside enzyme - complementary, can according to whether the present white colonies (or plaque) and easily pick out gene recombinant. In addition, it also can be used as a lac or tac promoter expression vector expression of the inducer to use[2].

Isopropyl-beta-D-thiogalactopyranoside is a gratuitous beta-D-galactosidase inducer. In Escherichia coli Isopropyl-beta-D-thiogalactopyranoside functions by binding to the lac repressor and altering its conformation. This inactivation prevents therepressor of binding to the operator of the lac operon and thus the repression of the beta-galactosidase coding gene lacZ. Isopropyl-beta-D-thiogalactopyranoside is a commonly used reagent in cloning procedures that require induction of beta-galactosidase activity and is used in conjunction with X-Gal (B-7150) or Lapis-beta-D-Gal (B-8900) in blue-white color selection of recombinant bacterial colonies.

References

[1] Hansen L H , Knudsen S , S?Rensen S J . The Effect of thelacYGene on the Induction of IPTG Inducible Promoters, Studied inEscherichia coliandPseudomonas fluorescens[J]. Current Microbiology, 1998, 36(6):341-347.
[2] Albers S V , Szabo Z , Driessen A J M . Archaeal Homolog of Bacterial Type IV Prepilin Signal Peptidases with Broad Substrate Specificity[J]. Journal of Bacteriology, 2003, 185(13):3918-3925.