TABLE 1.

β-Galactosidase and relative repression activities of reporter plasmids^a

Promoter and lacZ and Cro variants	β-Galactosidase specific activity ± SEM	Relative repression	Colony phenotype
P2 lacZ′ cro+	0.58 ± 0.13	224	White
P2 lacZ′ croΔ	130 ± 22		Blue
P3 lacZ+cro+	154 ± 25	134	White
P3 lacZ+croΔ	20,634 ± 1,365		Blue
P2 lacZ+cro+	1,274 ± 258	43	Blue
P2 lacZ+croΔ	54,972 ± 4,236		Blue

^aβ-Galactosidase activities and relative repression ratios were determined on DH5α cultures with the indicated promoter-434 O_R3 operator-lacZ combinations coexpressed with either wt 434 Cro or the repressionally inactive CroΔ HTH deletion mutation. Specific activities of the β-galactosidase are expressed as nmol of ONPG hydrolyzed per min per mg of extract (13). The standard error of the mean is given. Each activity was determined at least four times. Relative repression ratios were calculated as previously described (13) as the quotient of the specific activity of β-galactosidase in cell extracts of unrepressed cultures (reporter plasmid coexpressed with CroΔ protein) over those determined from repressed cultures (reporter plasmid coexpressed with wt 434 Cro protein). The complete absence of repression would result in a relative repression value equal to 1. Colony phenotypes were assayed on agar plates containing IPTG and X-Gal as described in Materials and Methods.