Phytochrome and bacterial sensor proteins are related by functional and structural homologies. They are both sensors of environmental stimuli and share structural homologies which comprise a domain of about 250 amino acids (about 28 kg.mol-1). This domain is C-terminal in phytochromes and in several bacterial sensor proteins. In both groups of sensors this domain undergoes conformational changes which are caused by the N-terminal part sensing the stimulus. In the case of bacterial sensors, the conformational alteration is, regulated by additional proteins, conferred to a corresponding regulator protein which then acts on transcription. The coincidences between the two groups of sensors are striking enough to assume phytochrome to transduce signals in a way comparable to the bacterial two-component systems.