Table 1.

Regulatory properties of wild-type and hybrid lambda repressor proteins^a

Plasmid	Repressor	Tetracycline Susceptibility	β-Galactosidase Activity (units)	Repression (%)
pSH20	none	resistant	(1.1 ± 0.1) × 104	0
pSH26	NcI–PR	sensitive	(2.0 ± 0.2) × 103	82
pSH27	NcI–dPR2	sensitive	(2.8 ± 0.4) × 102	98
pSH28	cI	sensitive	(2.0 ± 0.1) × 102	98
pSH29	NcI–zip	sensitive	(1.7 ± 0.1) × 103	84

^aNcI-PR, a fusion of cI (residues 1–131) and D25N HIV-1 protease, forms a stable dimer and functionally represses the λP_R-lacZ-tet reporter cassette in vivo. The λP_R promoter in the cassette contains only a single copy of λO_R1 to avoid complexity arising from the cooperative binding of repressors to multiple copies of λO_R. Plasmids were transformed into E. coli strain MC1061 to test for in vivo binding of hybrid repressors to λO_R1. The low copy-number plasmid pSH20 contains the reporter cassette alone. pSH26–29 are derivatives of pSH20 with the expression modules of various repressor hybrids driven by the lacUV5 promoter. NcI–dPR2 is a hybrid protein of NcI and the N- and C-terminal segments of HIV-1 protease tethered by a pentapeptide linker (P¹Q²I³T⁴L⁵-GGSSG-S⁹⁵T⁹⁶L⁹⁷N⁹⁸F⁹⁹). NcI–zip is a hybrid of NcI and GCN4. The DNA of NcI and NcI–zip are from pJH157 and pJH370 (J. C. Hu), respectively²⁹. Susceptibility to tetracycline (10 μg/ml) and β-galactosidase activity were measured as indicators of binding to λO_R1²⁹,³⁰. Repression is calculated as: 1 − (β-galactosidase activity with repressor/β-galactosidase activity without repressor).