Table 2. Expression of P_toxA in the wild-type, ptxS and ptxR deficient backgrounds.

Strain	PtoxA:’lacZ		Pkgu::’lacZ
	Without ketogluconate	+ ketogluconate	Without ketogluconate	+ ketogluconate
wt	415±40	1545±90	300±50	850±100
ΔptxS	1630±40	1390±50	875±50	800±100
ΔptxR	175±60	210±25	230±5	250±50

The promoter region of toxA or kgu was cloned into pMP220 (Tc^R) and the resulting plasmid electroporated into the indicated strains. Cells were grown on M9 minimal medium with citrate (15 mM) and overnight cultures were diluted 50-fold in the same medium in the absence or in the presence of 2-ketogluconate (5 mM). β-galactosidase activity was determined in cells in the exponential phase of growth after 3 hours of incubation. Data are the average of 3 independent assays each performed in duplicate.