A Conserved Histidine in Cytochrome c Maturation Permease CcmB of Shewanella putrefaciens Is Required for Anaerobic Growth below a Threshold Standard Redox Potential

Abstract

ABSTRACT Shewanella putrefaciens strain 200 respires a wide range of compounds as terminal electron acceptor. The respiratory versatility of Shewanella is attributed in part to a set of c -type cytochromes with widely varying midpoint redox potentials (E′ 0 ). A point mutant of S. putrefaciens , originally designated Urr14 and here renamed CCMB1, was found to grow at wild-type rates on electron acceptors with high E′ 0 [O 2 , NO 3 − , Fe(III) citrate, MnO 2 , and Mn(III) pyrophosphate] yet was severely impaired for growth on electron acceptors with low E′ 0 [NO 2 − , U(VI), dimethyl sulfoxide, TMAO (trimethylamine N -oxide), fumarate, γ-FeOOH, SO 3 2− , and S 2 O 3 2− ]. Genetic complementation and nucleotide sequence analyses indicated that the CCMB1 respiratory mutant phenotype was due to mutation of a conserved histidine residue (H108Y) in a protein that displayed high homology to Escherichia coli CcmB, the permease subunit of an ABC transporter involved in cytochrome c maturation. Although CCMB1 retained the ability to grow on electron acceptors with high E′ 0 , the cytochrome content of CCMB1 was <10% of that of the wild-type strain. Periplasmic extracts of CCMB1 contained slightly greater concentrations of the thiol functional group (-SH) than did the wild-type strain, an indication that the E h of the CCMB1 periplasm was abnormally low. A ccmB deletion mutant was unable to respire anaerobically on any electron acceptor, yet retained aerobic respiratory capability. These results suggest that the mutation of a conserved histidine residue (H108) in CCMB1 alters the redox homeostasis of the periplasm during anaerobic growth on electron acceptors with low (but not high) E′ 0 . This is the first report of the effects of Ccm deficiencies on bacterial respiration of electron acceptors whose E′ 0 nearly span the entire redox continuum.