Elucidation of the aerobic respiratory chains in mycobacteria

Abstract

The aerobic respiratory chain of mycobacteria consists of at least two branches, a cytochrome c branch terminating in an aa3-type cytochrome c oxidase, and a quinol branch terminating in cytochrome bd oxidase. The structure and function of the former branch, leading from the menaquinone-menaquinol pool to the cytochrome bc1 complex and terminating in the aa3-type cytochrome c oxidase, was characterized in Mycobacterium smegmatis. Allelic exchange mutants of M. smegmatis in the bc1 complex (ΔqcrCAB::hyg) and in subunit II of the aa3-type cytochome c oxidase (ΔctaC::hyg) were constructed and analyzed for growth, and gene expression using lacZ reporter assays and genome expression profiling by DNA microarray. Both mutants were found to be profoundly growth impaired. Disruption of this pathway resulted in an adaptation of the respiratory network that is characterized by a marked up-regulation of cydAB, which encodes the bioenergetically less-efficient and microaerobically induced cytochrome bd-type menaquinol oxidase that is required for the growth of M. smegmatis under O2-limiting conditions. Other adaptations to re-routing of the electron flux through the branch terminating in the bd-type oxidase were revealed by comparative expression profiling of the bc1-deficient mutant and its parental wild type strain using a partialgenome microarray of M. smegmatis that is enriched in essential genes. The majority of the genes up-regulated in the mutant are involved in intermediary metabolism and respiration. Also induced were several genes including, uspL and a homologue of Rv1592c, which were previously shown to be up-regulated by hypoxia in M. smegmatis (uspL) and M. tuberculosis (uspL and Rv1592). The cytochrome bc1-aa3 branch is required for growth of M. smegmatis under aerobic conditions and its disruption results in growth attenuation and up-regulation of cytochrome bd oxidase.