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. Both bilin lyase domains bind with the bilin tetrapyrrole chromophore precursor. The domain 1 shows red, far-red light photoreversibility. The domain 2 is photochemically inactive. Contains two covalently linked tetrapyrrole chromophores. Belongs to the phytochrome family.