Technical Abstract: The biochemical mechanism by which the phytochrome family of plant sensory photoreceptors transmit perceived informational light signals downstream to transduction pathway components is undetermined. However, the recent sequencing of the entire genome of the cyanobacterium Synechocystis has revealed a protein that has an NH2-terminal domain with striking sequence similarity to the photosensory NH2-terminal domain of the phytochromes, and that has a COOH-terminal domain strongly related to the transmitter histidine kinase module of bacterial two-component sensors. The Synechocystis protein can autocatalytically ligate chromophore and exhibits photoreversible light-absorption changes analagous to the phytochromes, indicating its capacity to function as an informational photoreceptor. Together with earlier observations that the COOH-terminal domain of the plant phytochromes also have sequence similarity to the histidine kinases, these data suggest that the cyanobacteria utilize photoregulated histidine kinases as a sensory system and that the plant phytochromes may be evolutionary descendants of these photoreceptors.