TABLE 2.

Lactose transport and β-galactosidase activity in the S. xylosus wild type, C2a, and in the lacR mutant, TX258

Strain	Growth conditionsa	Lactose transportb (pmol of lactose transported/min/mg of protein)	β-Galactosidase activityc (nmol of nitrophenol produced/min/mg of protein)
C2a	B	371 ± 99	9 ± 2
	B + lactose	5,143 ± 187	98 ± 14
	B + galactose	578 ± 94	7 ± 2
TX258 (lacR)	B	78 ± 8	<1
	B + lactose	83 ± 8	<1

^aThe strains were grown in B medium (B) to an OD₅₇₈ of 1.2. The indicated sugar was added at 25 mM. After 1 h of growth, the cells were harvested. Cultures without additional sugar were grown and harvested accordingly. 

^bLactose transport was determined by using 200 μM [¹⁴C]lactose (7.12 mCi/mmol) and 1 ml of washed cells at an OD₅₇₈ of 3. The values (means ± standard deviations) represent the initial uptake rates within the first 4 min of the experiment. They were taken from two measurements, each from three cultures. 

^cThe values (means ± standard deviations) represent two determinations, each with bacterial extracts from three cultures.