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Evidence for a cytochrome bcc–aa₃ interaction in the respiratory chain of Mycobacterium smegmatis

James A. Megehee^1,

¹ Department of Microbiology, The University of Mississippi Medical Center, Jackson, 2500 North State Street, MS 39216-4505, USA
² Department of Biochemistry, The University of Mississippi Medical Center, Jackson, 2500 North State Street, MS 39216-4505, USA

Correspondence
Michael D. Lundrigan
mlundrigan{at}microbio.umsmed.edu

Spectroscopic analysis of membranes isolated from Mycobacterium smegmatis, along with analysis of its genome, indicates that the cytochrome c branch of its respiratory pathway consists of a modified bc₁ complex that contains two cytochromes c in its c₁ subunit, similar to other acid-fast bacteria, and an aa₃-type cytochrome c oxidase. A functional association of the cytochrome bcc and aa₃ complexes was indicated by the findings that levels of detergent sufficient to completely disrupt isolated membranes failed to inhibit quinol-driven O₂ reduction, but known inhibitors of the bc₁ complex did inhibit quinol-driven O₂ reduction. The gene for subunit II of the aa₃-type oxidase indicates the presence of additional charged residues in a predicted extramembrane domain, which could mediate an intercomplex association. However, high concentrations of monovalent salts had no effect on O₂ reduction, suggesting that ionic interactions between extramembrane domains do not play the major role in stabilizing the bcc–aa₃ interaction. Divalent cations did inhibit electron transfer, likely by distorting the electron-transfer interface between cytochrome c₁ and subunit II. Soluble cytochrome c cannot donate electrons to the aa₃-type oxidase, even though key cytochrome c-binding residues are conserved, probably because the additional residues of subunit II prevent the binding of soluble cytochrome c. The results indicate that hydrophobic interactions are the primary forces maintaining the bcc–aa₃ interaction, but ionic interactions may assist in aligning the two complexes for efficient electron transfer.

Present address: Center for Infectious Disease and Vaccinology, The Biodesign Institute, Arizona State University, PO Box 875401, Tempe, AZ 85287, USA.