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J. Bacteriol. doi:10.1128/JB.01249-06
Copyright (c) 2006, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

A conserved histidine in cytochrome c maturation permease CcmB of Shewanella putrefaciens is required for anaerobic growth below a threshold standard redox potential

School of Biology, Georgia Institute of Technology, Atlanta, GA 30332

^* To whom correspondence should be addressed. Email: thomas.dichristina{at}biology.gatech.edu.

	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 mid-point 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 [O₂, NO₃^-, Fe(III)-citrate, MnO₂ and Mn(III)-pyrophosphate] yet was severely impaired for growth on electron acceptors with low E'₀ [NO₂^-, U(VI), DMSO, TMAO, fumarate, -FeOOH, SO₃^2- and S₂O₃^2-]. 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 the wild-type strain. Periplasmic extracts of CCMB1 contained slightly greater concentrations of the thiol functional group (-SH) compared with 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.