Characterization of the regulatory and expression context of an alternative oxidase gene provides insights into cyanide-insensitive respiration during growth and development.

Alternative oxidase (AOX) is encoded in small multigene families in plants. Functional analysis of the Arabidopsis (Arabidopsis thaliana) alternative oxidase 1c (AtAOX1c) promoter, an AOX gene not induced by oxidative stress, indicated that regulation of expression was complex, with the upstream promoter region containing positive and negative response regions. Comparison to the promoter region of soybean (Glycine max) alternative oxidase 2b (GmAOX2b), another AOX gene not induced by oxidative stress, revealed that they contained seven sequence elements in common. All elements were active in the promoter region of AtAOX1c in suspension cells and in leaf tissue from Columbia and mutant plants, where a mitochondrial protein import receptor was inactivated. Analysis of coexpressed and putatively coregulated genes, the latter defined as containing five or more sequence elements functional in AtAOX1c, indicated that AtAOX1c was coregulated with components involved with cell division and growth. Consistent with this analysis, we demonstrated that site II elements, previously shown to regulate the proliferating cell nuclear antigen, are present in the upstream promoter region of AtAOX1c and were strong negative regulators of AtAOX1c expression. It was demonstrated that NDB4, a gene encoding an external NAD(P)H dehydrogenase, displayed strong coexpression with AtAOX1c. Overall, these results indicate that AtAOX1c is regulated by growth and developmental signals.