TABLE 5.

Expression of POR activity in E. coli BL21(DE3, pNJH301)^a

Growth stage	POR activity (nmol of methyl viologen reduced min−1 mg−1)
	Microaerobic induction		Aerobic induction (pNJH301)
	pET21a control	pNJH301
Pre-IPTG addition	7.5	6.2	6.2
Post-IPTG addition	5.8	27.4	6.0
Post-rifampin addition	5.9	38.4	5.8

^aCell extract was prepared from E. coli BL21(DE3) harboring either pNJH301, for expression of H. pylori POR, or the vector control, pET21a, at various stages during the induction of recombinant POR expression. Samples were taken immediately before the addition of 1 mM IPTG to the growth medium, 90 min after the addition of 1 mM IPTG, and 90 min after the addition of 200 μg of rifampin ml⁻¹ to inhibit host RNA polymerases. Following the addition of IPTG, cells were cultured under either aerobic or microaerobic conditions. A sevenfold overexpression of POR activity was achieved only in cells induced under reduced oxygen concentrations; under aerobic conditions, no increase in POR activity could be detected.