Preparation of 1M IPTG Solution

Overview:

  • In molecular biology laboratories, IPTG is commonly used to induce the expression of recombinant proteins in E. coli and to identify positive clones (blue/white selection) containing an insert in molecular cloning experiments. To be responsive to IPTG, the target gene must be under the control of lac or tac promoter/operator sequences.
  • In absence of IPTG, lac repressor (lac I gene product) inhibits the transcription by binding to specific sequences, the lac operator, and inhibits the transcription of downstream genes. 
  • IPTG activates the transcription by binding and removing the lac repressor from the lac operator, thus allowing transcription of downstream genes by RNA polymerase.
  • IPTG is commercially supplied as a white to off-white crystalline powder which is readily soluble in water. Generally, a 1 M IPTG solution in water is prepared that can be stored at -20°C for many months. 
  • The molecular weight of IPTG (CAS 367-93-1) is 238.3 g/mol which means you need to dissolve 238.3 grams of IPTG in water to prepare 1 L of 1M solution.

Requirements:

Reagents
♦ IPTG powder
♦ Deionized / Milli-Q water

Equipment and disposables
♦ Falcon® Conical Tubes, 15 mL

Composition:
1M IPTG in water

Objective:  
Preparation of 10 ml of 1 M IPTG solution

Preparation

Step 1: Weigh out 2.38 grams IPTG (Molecular Weight 238.3) and transfer to 15 ml Falcon® conical tube. Add 8 ml deionized / Milli-Q water. Tighten the cap and dissolve IPTG by inverting the tube several times.

Note: 
IPTG dissolves quickly in water. Generally inverting the tube several times is sufficient to dissolve IPTG. Alternatively, one can use a magnetic stirrer to dissolve it.

Step 2: Adjust the volume 10 ml with deionized / Milli-Q water. Mix by inverting the tube a few times.

Step 3: Sterilize the solution by passing through a filter unit containing 0.22 µm filter paper.

Note:
Filter sterilization removes all suspended particles with a size of more than 0.22 μ which includes most bacteria and their spores but not mycoplasma. However, it does not inactivate enzymatic activities (e.g., DNases).

Precaution:
Never autoclave IPTG solution.

Storage:
The solution can be stored at -20 °C for several months in small aliquots (0.5 ml or 1.0 ml). 

Applications:
♦ Protein expression and purification in E. coli
♦ Molecular cloning – Identification of positive clones using a blue-white selection

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