Table 1.

Specificity of CTXφ RstR repressors for rstA promoters

Reporter	RstR repressor*
	Classical (pHK2)		El Tor (pHK1)		V. cholerae CTXφ lysogen†
	−Arabinose	+Arabinose	−Arabinose	+Arabinose	Classical	El Tor
Classical rstA-lacZ (pHK 101)	527	7	688	933	5	74
El Tor rstA-lacZ (pHK102)	157	167	255	1.2	260	1

β-Galactosidase units are reported as nmol of o-nitrophenyl β-d-galactoside hydrolyzed per min per OD₆₀₀. Assays were performed in triplicate, and the average value is presented. 

^*The arabinose-inducible classical (pHK2) or El Tor (pHK1) rstR plasmids were introduced along with an rstA-lacZ reporter plasmid into Escherichia coli strain CC118 (21). Cultures were grown in L broth (25) for 16 hr in the presence or absence of 0.05% arabinose, and the β-galactosidase activity was determined (25). 

^†The classical and El Tor rstA-lacZ reporters were introduced into lacZ⁻ derivatives of classical (0395) and El Tor (E7946) V. cholerae CTXφ lysogens, and the activity of β-galactosidase was determined (25).