TABLE 2.

Relative levels of TonB expression^a

TonB source	Relative TonB level
	AroB+	AroB−
	M9	M9	T
Chromosomal	1	1	1
pKP299	8	2.5	9
pKP325, PcnB+	0.13	0.12	0.11
pKP325, PcnB−	0.004	0.004	0.005

^aValues are expressed as ratios relative to the level of chromosomally encoded TonB under the given genetic and culture circumstances. Note that for the AroB⁻ strains, the absolute values for chromosomally encoded TonB were about 2.5-fold that of the AroB⁺ strains. M9 medium was supplemented with 0.2% d-glucose, 0.4% vitamin-free Casamino Acids, and 40 μg of tryptophan ml⁻¹, 0.4 μg of thiamine ml⁻¹, 10 mM MgSO₄, 0.5 mM CaCl₂, 88 μM iron (provided as FeCl₃ · 6H₂O), and 34 μg chloramphenicol ml⁻¹. T medium was supplemented with 0.2% d-glucose and 34 μg of chloramphenicol ml⁻¹.