IPTG

Dissolve solid IPTG into dH2O to make 1 M IPTG solution. Store at -20 °C. More often, a 100 mM stock solution is more useful. IPTG needs to be thawed before use.

X-gal stock solution

Dissolve 20 mg solid x-gal into 1 mL DMF. Keep away from light and store at -20 °C. Alternatively, dissolve 40 mg x-gal powder into 1 mL DMF for a more concentrated stock.

• ex: for a 10 mL x-gal solution (20 mg/mL): 200 mg (0.2 g) x-gal into 10 mL DMF

Plates

To make plates, add IPTG and X-gal to final concentrations of 40 μg/ml X-gal and 0.1 mM IPTG. Add to LB agar that has been cooled down to 50°C. In other words, add 1 μL IPTG per mL LB agar and 2 μL X-gal per mL LB agar.

• ex: 200 μL IPTG and 400 μL X-gal for 200 mL LB agar (~10 plates).

For top agar, add 10 μL IPTG and 40 μL x-gal for 3 mL top agar.

Spread plates

Spread 40 μL of 100 mM IPTG solution and 40 μL of 20 mg/mL x-gal solution onto plate. Final amounts: IPTG = 1 mmol IPTG and 2 mg x-gal per plate. Incubate for 1 h prior to usage.