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J. Bacteriol. doi:10.1128/JB.01441-06
Copyright (c) 2006, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Membrane-spanning and periplasmic segments of CcmI have distinct functions during cytochrome c biogenesis in Rhodobacter capsulatus

Department of Biology, Plant Science Institute, University of Pennsylvania, PA, 19104, USA

^* To whom correspondence should be addressed. Email: fdaldal{at}sas.upenn.edu.

	Abstract

In Gram-negative bacteria like Rhodobacter capsulatus, about ten membrane-bound components (CcmABCDEFGHI and CcdA) are required for periplasmic maturation of c-type cytochromes. These components perform the chaperoning and thio-oxidoreduction of the apoproteins as well as the delivery and ligation of the heme cofactors. In the absence of any of these components, including CcmI proposed to act as an apocytochrome c chaperone, R. capsulatus does not have the ability to produce holocytochromes c, and consequently to exhibit photosynthetic growth and cytochrome cbb₃ oxidase activity. Previously, we have demonstrated that null mutants of CcmI partially overcome cytochrome c deficiency phenotypes upon overproduction of the CcmF-CcmH_Rc couple in a growth medium-dependent manner, and fully bypass these defects by additional overproduction of CcmG. Here, we show that overproduction of either the CcmF-CcmH_Rc couple or the N-terminal membrane-spanning segment of CcmI (CcmI-1) has similar suppression effects of cytochrome c maturation defects in CcmI-null mutants. Likewise, additional overproduction of CcmG, the C-terminal periplasmic segment of CcmI (CcmI-2), or even of apocyt c₂, also provides similar complementation abilities to these mutants. These results indicate that the two segments of CcmI have different functions, and support our earlier findings that two independent steps are required for full recovery of the loss of CcmI function. We therefore propose that CcmI-1 is part of the CcmF-CcmH_Rc-dependent heme ligation, while CcmI-2 is involved in the CcdA- and CcmG-dependent apoprotein thioreduction steps, which intersect at the level of CcmI during cytochrome c biogenesis.

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