Cytochrome oxidase assembly, and cellular and genomic effects of nitric oxide, studied in yeast
Cytochrome oxidase is the terminal proton pumping enzyme of the respiratory chain, catalysing the reduction of oxygen to water. This complex enzyme is composed of up to thirteen subunits of both nuclear and mitochondrial genetic origin, but the order in which they assemble is not fully understood. To investigate assembly I utilised blue native gel electrophoresis to analyse Saccharomyces cerevisiae strains with mutations in cytochrome oxidase causing assembly defect. I identified novel subcomplexes including, for the first time, a subcomplex containing mitochondrially encoded subunits. The respiratory chain is a target of the free radical nitric oxide (NO), which reversibly inhibits cytochrome oxidase through competition with oxygen at its active site. NO also has a myriad of other targets, and many of its actions are mediated through reactive nitrogen species (RNS) formed on reaction of NO with other species. NO is released as part of the immune response to infection and has antimicrobial action. Here, I found that prolonged incubation of yeast with an NO donor inhibited growth and caused a decrease in cytochrome oxidase content in dividing cells, which may be due to NO interfering with the assembly of the enzyme. I have also analysed gene expression in yeast after short NO exposure, to gain insight into the stress sensed by the cell and the transcription factors involved. The data suggest that NO causes a general stress response, in addition to specific effects such as repression of respiratory chain genes and activation of antioxidant/detoxification systems. Many genes known to be regulated by Haplp were repressed, suggesting that NO might interfere with this transcription factor.