TABLE 1.

Influence of aspartate, glutamate, and glutamine availability on urease operon transcriptional level of S. thermophilus DSM 20617, measured as GusA reporter activity

Culture conditions	GusA activitya	Urease activity (%)b
Milk, aspartate (2 mM)	862 ± 28	100
Milk, aspartate (2 mM), glutamate (0.75 mM)	579 ± 18	53
Milk, aspartate (2 mM), glutamine (0.75 mM)	649 ± 22	33
Milk, aspartate (2 mM), NH4Cl (0.75 mM)	634 ± 24	26

^aGusA activity was expressed as milli-OD₄₂₀ units per mg of protein as the mean of four independent determinations ± the standard deviation. Experiments were performed by incubating recombinant S. thermophilus MIM200 for 2 h in milk in the presence of aspartate at a growth-limiting concentration (2 mM), followed by 10 min in the presence of glutamate, glutamine, or ammonium chloride or without any other addition.

^bUrease activity was evaluated with the same conditions used for GusA activity experiments and was expressed as a percentage of urease activity measured in aspartate-supplemented milk as a mean of three independent determinations; standard deviations were always less than 12%. The pH of the cultures at the end of the incubation time was 6.6.