FIG 1.

Loss of fixT leads to derepression of FixLJ-dependent transcription, protoporphyrin accumulation, and a stationary-phase growth defect. (A) Activity of a P_fixK-lacZ transcriptional reporter in wild-type (WT) or ΔfixT mutant cells grown to high density with limited aeration (average ± SD, n = 3 biological replicates, each with 3 averaged technical replicates). *, P < 0.0005, Student’s t test. P_fixK is directly activated by FixLJ (12, 18, 19). (B) Cell pellets harvested from cultures grown as in panel A. Plasmid-free WT and ΔfixT mutant pellets are on the left. Strains carrying either empty pMT805 (EV) or xylose-inducible pMT805-fixT (fixT⁺⁺) for complementation are on the right. (C) Top, absorption spectra of cell lysates showing prominent Soret peaks (414 nm) and 4 Q bands. Strain genotypes are annotated as in panel B. Bottom, quantification of lysate absorbance at 414 nm (average ± SD, n = 3). *, P < 0.005, Student’s t test; **, P < 0.0001, one-way ANOVA followed by Dunnett’s posttest comparison to ΔfixT mutant EV. (D) Growth curve, measured by the optical density at 660 nm (OD₆₆₀), following entry into stationary phase for WT and ΔfixT mutant cells (top) and plasmid-bearing complementation strains (bottom). Points are the averages ± SD of the results from 3 biological replicates.