This invention provides novel truncation mutants of a phytochrome from the bacterium Deinococcus radiodurans. When expressed either in bacteria or mammalian cells, these mutant phytochromes spontaneously incorporate biliverdin, a ubiquitous intermediate in heme catabolism, and become fluorescent in the infrared (IR) region. These phytochromes are the first genetically encoded labels that can be excited by far-red light and fluorescent in the true IR (>700 nm). If these mutants instead incorporate protoporphyrin IX, an intermediate in heme biosynthesis, illumination now generates significant amounts of singlet oxygen. Singlet oxygen is useful because it can be used to kill individual proteins or cells, detect long-range protein-protein interactions, or generate electron-microscopic contrast. The invention also relates to methods of making and using such proteins and protein variants.
