Photoreceptor which exists in two forms that are reversibly interconvertible by light: the R form that absorbs maximally in the red region of the spectrum and the FR form that absorbs maximally in the far-red region. Has also a slight blue shift for the far-red maximum. Could also absorb green light. May participate in regulating pigment synthesis like the carotenoid deinoxanthin which could protect the bacterium from intense visible light. ATP + protein L-histidine = ADP + protein N-phospho-L-histidine. Contains one covalently linked tetrapyrrole chromophore. Lacks the cysteine conserved in plant phytochromes (at the position of Met-259) that binds chromophore. An engineered sequence used for X-ray crystallography forms a thioether link to biliverdin through Cys-24. The natural sequence can bind phycocyanobilin and phytochromobilin in vitro, but the identity of the natural chromophore is unknown. In the N-terminal section; belongs to the phytochrome family.