TABLE 1.

β-Galactosidase expression in tyrR366 strains from lac fusions to proU and its variants^a

Plasmid	β-Galactosidase activity for:
	hns+		hns-205
	−NaCl	+NaCl	−NaCl	+NaCl
pMU6441	0.5	1,450	34	1,582
pMU6442	1	351	68	720
pMU6443	<0.5	135	33	624

^aCultures of strains JP8042 (tyrR366 hns⁺) and JP10939 (tyrR366 hns-205::Tn10) carrying the indicated plasmid derivatives were grown, for β-galactosidase assays, to mid-exponential phase in one-quarter-strength medium 56 (containing glucose [0.2%], thiamine [1 μg/ml], and trimethoprim [10 μg/ml]) without and with 0.3 M NaCl supplementation (−NaCl and +NaCl, respectively). Medium 56 was modified from the medium described by Monod et al. (25) and contains the following, per liter: K₂HPO₄ (10.6 g), NaH₂PO₄ · 12H₂O (6.1 g), (NH₄)₂SO₄ (2 g), MgSO₄ · 7H₂O (0.2 g), Ca(NO₃)₂ (10 mg), and FeSO₄ · 7H₂O (0.5 mg) (pH adjusted to 7.0). Enzyme specific activity values are reported in Miller units (24).