In bacteria, the PerR protein controls expression of genes that help the bacterium deal with peroxide stress. Peroxide (H2O2) generates damaging hydroxyl radicals, such as may be produced during a reactive oxygen burst, used by some immune cells to fight invading pathogens. This structure is PerR from Bacillus subtilis, a harmless soil-dwelling bacteria, used to study many different bacterial systems, including spore formation. PerR is thought to work by sitting on DNA, thus stopping certain proteins being made. When the protein is activated, by binding metal ions, the protein is released from the DNA, allowing expression of genes, in this case, to deal with peroxide stress. PerR is therefore an example of a regulatory repressor protein. The PerR protein is also found in Campylobacter jejuni, a pathogen responsible for thousands of cases of food-poisoning each year. The music is the PerR amino acid sequence, from Campylobacter jejuni, converted to a music format and played on a virtual synth. http://www.rcsb.org/pdb/explore/explore.do?structureId=3F8N http://www.ncbi.nlm.nih.gov/pubmed/19508285?dopt=Abstract