A CYANOBACTERIAL PHYTOCHROME 2-COMPONENT LIGHT SENSORY SYSTEM

The biliprotein phytochrome regulates plant growth and developmental r esponses to the ambient light environment through an unknown mechanism . Biochemical analyses demonstrate that phytochrome is an ancient mole cule that evolved from a more compact light sensor in cyanobacteria. T he cyanobacterial phytochrome Cph1 is a light-regulated histidine kina se that mediates red, far-red reversible phosphorylation of a small re sponse regulator, Rcp1 (response regulator for cyanobacterial phytochr ome), encoded by the adjacent gene, thus implicating protein phosphory lation-dephosphorylation in the initial step of light signal transduct ion by phytochrome.