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Journal of Bacteriology, March 2001, p. 2013-2024, Vol. 183, No. 6
0021-9193/01/$04.00+0   DOI: 10.1128/JB.183.6.2013-2024.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

Mobile Cytochrome c2 and Membrane-Anchored Cytochrome cy Are Both Efficient Electron Donors to the cbb3- and aa3-Type Cytochrome c Oxidases during Respiratory Growth of Rhodobacter sphaeroides

Fevzi Daldal,1,* Sevnur Mandaci,1,2 Christine Winterstein,1 Hannu Myllykallio,1,dagger Kristen Duyck,1 and Davide Zannoni3

Department of Biology, Plant Science Institute, University of Pennsylvania, Philadelphia, Pennsylvania 191041; Marmara Research Center, Research Institute for Genetic Engineering and Biotechnology, Gebze, Kocaeli 41470, Turkey2; and Department of Biology, University of Bologna, Bologna 40126, Italy3

Received 25 September 2000/Accepted 31 October 2000

We have recently established that the facultative phototrophic bacterium Rhodobacter sphaeroides, like the closely related Rhodobacter capsulatus species, contains both the previously characterized mobile electron carrier cytochrome c2 (cyt c2) and the more recently discovered membrane-anchored cyt cy. However, R. sphaeroides cyt cy, unlike that of R. capsulatus, is unable to function as an efficient electron carrier between the photochemical reaction center and the cyt bc1 complex during photosynthetic growth. Nonetheless, R. sphaeroides cyt cy can act at least in R. capsulatus as an electron carrier between the cyt bc1 complex and the cbb3-type cyt c oxidase (cbb3-Cox) to support respiratory growth. Since R. sphaeroides harbors both a cbb3-Cox and an aa3-type cyt c oxidase (aa3-Cox), we examined whether R. sphaeroides cyt cy can act as an electron carrier to either or both of these respiratory terminal oxidases. R. sphaeroides mutants which lacked either cyt c2 or cyt cy and either the aa3-Cox or the cbb3-Cox were obtained. These double mutants contained linear respiratory electron transport pathways between the cyt bc1 complex and the cyt c oxidases. They were characterized with respect to growth phenotypes, contents of a-, b-, and c-type cytochromes, cyt c oxidase activities, and kinetics of electron transfer mediated by cyt c2 or cyt cy. The findings demonstrated that both cyt c2 and cyt cy are able to carry electrons efficiently from the cyt bc1 complex to either the cbb3-Cox or the aa3-Cox. Thus, no dedicated electron carrier for either of the cyt c oxidases is present in R. sphaeroides. However, under semiaerobic growth conditions, a larger portion of the electron flow out of the cyt bc1 complex appears to be mediated via the cyt c2-to-cbb3-Cox and cyt cy-to-cbb3-Cox subbranches. The presence of multiple electron carriers and cyt c oxidases with different properties that can operate concurrently reveals that the respiratory electron transport pathways of R. sphaeroides are more complex than those of R. capsulatus.

* Corresponding author. Mailing address: Department of Biology, Plant Science Institute, University of Pennsylvania, Philadelphia, PA 19104. Phone: (215) 898-4394. Fax: (215) 989-8780. E-mail: fdaldal{at}sas.upenn.edu.

dagger Present address: Institut de Génétique et Microbiologie, Université Paris XI, 91405 Orsay Cedex, France.

Journal of Bacteriology, March 2001, p. 2013-2024, Vol. 183, No. 6
0021-9193/01/$04.00+0   DOI: 10.1128/JB.183.6.2013-2024.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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