FIG. 2.

Effects of fnr null alleles on expression of a ø(nifH′-′lacZ) fusion in heterologous E. coli strains carrying K. pneumoniae nifLA on a plasmid. The activity of β-galactosidase was plotted as a function of OD₆₀₀ for cultures grown at 30^°C in minimal medium under anaerobic conditions with 4 mM glutamine as a limiting nitrogen source. Differential rates of transcription from the nifH promoter, which reflect NifA activity, were determined from the slopes of these plots. All strains carried a single copy of a ø(nifH′-′lacZ) fusion at the trp locus (9) and plasmid pNH3 encoding NifL and NifA under the control of the tac promoter. (A) fnr null allele transduced from M182 (fnr::Tn10): wild-type NCM1528 (diamonds), the respective fnr null allele in NCM1528 (RAS7) (circles), and the complemented respective fnr mutant by constitutive expression of E. coli fnr on pACYC184 (RAS10) (triangles) are shown. (B) fnr null allele from RM101: wild-type RAS22 (diamonds), the respective fnr null allele in RAS22 (RAS14) (circles), and the complemented respective fnr mutant by constitutive expression of E. coli fnr on pACYC184 (RAS16) (triangles) are shown.