Gene expression of terminal oxidases in two marine bacterial strains exposed to nanomolar oxygen concentrations

The final step of aerobic respiration is carried out by a terminal oxidase transporting electrons to oxygen (O2). Prokaryotes harbor diverse terminal oxidases that differ in phylogenetic origin, structure, biochemical function, and affinity for O2. Here we report on the expression of high-affinity (cytochrome cbb3 oxidase), low-affinity (cytochrome aa3 oxidase), and putative low-affinity (cyanide-insensitive oxidase (CIO)) terminal oxidases in the marine bacteria Idiomarina loihiensis L2-TR and Marinobacter daepoensis SW-156 upon transition to very low O2 concentrations (<200 nM), measured by RT-qPCR. In both strains, high-affinity cytochrome cbb3 oxidase showed the highest expression levels and was significantly up-regulated upon transition to low O2 concentrations. Low-affinity cytochrome aa3 oxidase showed very low transcription levels throughout the incubation. Surprisingly, however, it was also up-regulated upon transition to low O2 concentrations. In contrast, putative low-affinity CIO had much lower expression levels and markedly different regulation patterns between the two strains. These results demonstrate that exposure to low O2 concentrations regulates the gene expression of different types of terminal oxidases, but also that the type and magnitude of transcriptional response is species-dependent. Therefore, in situ transcriptome data cannot, without detailed knowledge of the transcriptional regulation of the species involved, be translated into relative respiratory activity.