Table 3. Repression of P_R, P_L, and P_RM by Cro and CI.

	PR β-galactosidase, Miller units
Repression	OL+	ΔOL	PL luciferase OL+, luc units	PRM luciferase ΔOL, luc units
Repression by Cro
cro-	1,516	1,555	5.9 × 106	6.7 × 105
cro+	873	971	3.4 × 106	2.7 × 105
Fold repression by Cro (cro-/cro+ in the absence of Cl at 42°C)	1.7	1.6	1.7	2.5
Repression by Cl
42°C	1,516		5.9 × 106
30°C	4		1.4 × 104
Fold repression by Cl* (activity at 42°C/activity at 30°C)	380		420

For repression by Cro in the absence of Cl, cultures of NC414, NC415, NC416, and NC417 were grown at 30°C overnight, then diluted and shifted to 42°C to reach OD₆₀₀ = 0.3-0.4. β-galactosidase and luciferase activities were measured as described in Materials and Methods. For repression by Cl in the absence of Cro, cultures of NC417 were grown at 30°C overnight, then split, diluted 1:200, and incubated at 30°C and 42°C, respectively, to reach OD₆₀₀ = 0.3-0.4. β-Galactosidase and luciferase activities were measured as described.

^*Repression of P_L and P_R by Cl varies from experiment to experiment because of variations of the repressed levels.