Does Galactose Inhibit the Lac-Operon in Addition to Lactase Itself?
This question is easily answered by looking inside of the phenotypically lac- cells to see if lactase is present or absent. If present, then galactose does not affect the expression of the lac-operon. Supplies • The plates made in the previous section. Better yet, a parallel set of growths on plates made with nutrient agar plus 0.3% lactose and the range of galactose concentrations. (It might be wise to add sucrose to the plates so that they are all of equal osmolarity. There is a hint that cells grown under osmotic stress also hyperexpress their lac-operons. What is the value of lacA? Hint: project idea?) The NA plates allow the growth of the bacteria without an accumulation of dyes that might confuse subsequent ONPG analysis. • Flat toothpicks • 0.1% SDS (sodium dodecylsulfate) • Chloroform • 12 ml ONPG solution (0.1 g ONPG/12 ml distilled water) • 2 eyedroppers Semi-quantitative Protocol • Using the toothpicks scrape up equal quantities of cells from each of the gal- growths on 11 p
