This is the correct recipe for Xgal plates. Don’t use any other.

Recipe for SD + Xgal Plates

(Ref: Rose, M. and D. Botstein. 1983. Construction and use of gene fusions to lacZ (b-galactosidase) that are expressed in yeast. Methods Enzymol. 101: 167-180.)

Note: The pH 7.0 of this medium enhances color formation.

Solutions Required:

10x pH 7.0 Salt Solution

1M KH₂PO₄ 68.1 g

0.15M (NH₄)SO₄ 9.9 g

0.75N KOH 21.1 g

DI H₂0 ~400 ml

Adjust pH to 7.0 with H₃PO₄, then add DI H₂O to a final volume of 500 ml.

Sterilize by autoclaving.

1000x Mineral Solution

2mM FeCl₃ 32 mg

0.8M MgSO₄ 19.7 g

DI H₂O 100 ml

Sterilize by autoclaving.

100x Vitamin Solution

200 mg/ml inositol 20 mg

40 mg/ml thiamine 4 mg

40 mg/ml pyridoxine 4 mg

40 mg/ml pantothenic acid 4 mg

2 mg/ml biotin 0.2 mg

DI H₂O 100 ml

Filter sterilize and store at 4°C in a bottle wrapped in foil (in fridge shelf).

Xgal Solution

20 mg/ml Xgal in dimethyl-formamide

SD-Xgal Recipe

For 500 ml (2x for 1L)

DI H₂O 389 ml

10x pH Salts Solution 50 ml

Tyrosine 15 mg

Agar 10 g

Autoclave for 20 min.

Allow liquid to cool to at least 50°C.

After cooling, add the following:

50% Glucose 20 ml

10% CAA 20 ml

100x Uracil Solution 5 ml

100x Adenine Solution 5 ml

100x Tryptophan Solution 5 ml

1000x Mineral Solution 0.5 ml

100x Vitamin Solution 5 ml

20 mg/ml Xgal 1 ml

The concentration of glucose and Xgal in this medium is 2% (w/v) and 40 mg/ml, respectively.

For Ura^- Xgal, leave out the 100x Uracil solution.

For Trp^- Xgal, leave out the 100x Tryptophan solution.