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. ATP + protein L-histidine = ADP + protein N-phospho-L-histidine. Contains one covalently linked tetrapyrrole chromophore. In the N-terminal section; belongs to the phytochrome family.