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Membrane-Spanning and Periplasmic Segments of CcmI Have Distinct Functions during Cytochrome c Biogenesis in Rhodobacter capsulatus

Carsten Sanders, Clémence Boulay, and Fevzi Daldal^*

Department of Biology, Plant Science Institute, University of Pennsylvania, Philadelphia, Pennsylvania 19104

Received 11 September 2006/ Accepted 13 November 2006

In gram-negative bacteria, like Rhodobacter capsulatus, about 10 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 or 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-R. capsulatus CcmH (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 the CcmF-CcmH_Rc couple and overproduction of the N-terminal membrane-spanning segment of CcmI (CcmI-1) have 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 apocytochrome c₂ also provides complementation abilities similar to those of 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.

Published ahead of print on 22 November 2006.

Present address: Centre d'études de Saclay, Direction des Sciences du Vivant, Département de Biologie Joliot-Curie, Service de Bioénergétique, 91191 Gif-sur-Yvette, France.