Engineered bacteriophytochrome heterodimers for research and applications.

Many proteins are dimeric, functioning as complexes of two identical or different subunits. Bacteriophytochromes are homodimeric photoreceptor proteins that sense red/far-red light with a photosensory module (PSM) and convert it to a biological response via an output module, usually a histidine kinase (HK). Here, we generate monomeric bacteriophytochrome PSMs that form stable heterodimers once mixed by modifying two salt bridges at the dimerization interface of the Deinococcus radiodurans phytochrome (DrBphP). We confirm that these heterodimeric PSMs can control output HK module activity in response to red light and reveal that dimerization is required for kinase activity of the model HK FixL, but not necessarily for phosphatase activity of DrBphP. By applying the heterodimeric variants to a red light-regulated gene expression tool, we exemplify the combined control of cellular events using both heterodimerization and light. These results pave the way for new heterodimeric systems, for example in receptor protein research and optogenetics.