Regulatory 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. Forms a two-component system with the Rrcp1 response regulator. ATP + protein L-histidine = ADP + protein N-phospho-L-histidine. Homodimer. Contains one covalently linked tetrapyrrole chromophore. The R form exhibits both ATP-dependent autophosphorylation and phosphotransfer to Rcp1 activities. Unlike the higher plants where Pfr is thought to be the active form. In the N-terminal section; belongs to the phytochrome family.