TABLE 3.

β-Lactamase activity

Host	Plasmid	Statusb	β-Lactamase activity (U/mg of protein)c
			Not induced	Induceda
ML4947 (AmpD wild type)	pKU403	Wild type	0.04 ± 0.01	1.94 ± 0.84
	pKU404	H-mut	18.86 ± 3.10	25.94 ± 5.53
	pKU405	L-mut	2.99 ± 1.09	8.26 ± 0.68
	pKU406	I-mut	6.34 ± 1.01	25.95 ± 4.01
	pKU407	I-mut	7.10 ± 0.60	30.95 ± 7.66
	pKU408	pKU403 ΔampR	0.14 ± 0.02	0.12 ± 0.01
	pKU409	pKU404 ΔampR	0.13 ± 0.01	0.12 ± 0.01
	pKU410	pKU405 ΔampR	0.13 ± 0.01	0.10 ± 0.01
	pKU414	pKU406 ΔampR	0.13 ± 0.01	0.12 ± 0.01
	pKU411	pKU403 ΔampC	0.01	0.01
	pKU412	pKU404 ΔampC	0.01	0.01
	pKU413	pKU405 ΔampC	0.01	0.01
	pKU415	pKU406 ΔampC	0.01	0.01
	pMW218	Vector	0.01	0.01
			0.01	0.01
ML4953 (AmpD mutant)	pKU403	Wild type	4.89 ± 1.28	37.68 ± 9.53
	pKU404	H-mut	43.77 ± 5.88	41.54 ± 13.82
	pKU405	L-mut	27.72 ± 5.11	48.48 ± 7.52
	pKU406	I-mut	35.06 ± 4.46	41.39 ± 5.21
	pKU407	I-mut	31.99 ± 2.97	38.28 ± 9.74
	pKU408	pKU403 ΔampR	0.11 ± 0.00	0.09 ± 0.01
	pKU409	pKU404 ΔampR	0.09 ± 0.01	0.06 ± 0.01
	pKU410	pKU405 ΔampR	0.11 ± 0.02	0.08 ± 0.01
	pKU414	pKU406 ΔampR	0.11 ± 0.01	0.07 ± 0.01
	pKU411	pKU403 ΔampC	0.01	0.01
	pKU412	pKU404 ΔampC	0.02	0.02
	pKU413	pKU405 ΔampC	0.02	0.02
	pKU415	pKU406 ΔampC	0.01	0.01
	pMW218	Vector	0.01	0.01
			0.02	0.01

^aOne to 4× the MIC of imipenem was used for induction of β-lactamase. 

^bL-, I-, and H-mut, mutant plasmids with low, intermediate, and high levels of β-lactamase activity, respectively. 

^cValues are means ± standard deviations of three independent experiments. Standard deviations for pKU411, pKU412, pKU413, pKU415, pMW218, and the host were <±0.001. One unit of β-lactamase activity was defined as the amount of β-lactamase that hydrolyzed 1 μmol of cephalothin in 1 min at 30°C.