Dimers of the N-terminal domain of phytochrome B are functional in the nucleus

A plant modulates its developmental processes in response to light by several informational photoreceptors such as phytochrome. Phytochrome is a dimeric chromoprotein which regulates various aspects of plant development from seed germination to flowering1. Upon absorption of red light, phytochrome translocates from the cytoplasm to the nucleus2,3,4, and regulates gene expression through interaction with transcription factors such as PIF3 (refs 5–7). The phytochrome polypeptide has two domains1,8: the amino-terminal photosensory domain with a chromophore and the carboxy-terminal domain which contains signalling motifs such as a kinase domain9. The latter is widely believed to transduce the signal to downstream components1,5,8,9,10. Here we show that the C-terminal domain of Arabidopsis phytochrome B (phyB), which is known as the most important member of the phytochrome family1, is not directly involved in signal transduction. The N-terminal domain isolated from phyB, when dimerized and localized in the nucleus, triggered full phyB responses with much higher photosensitivity than the full-length phyB. These findings indicate that the C-terminal domain attenuates the activity of phyB rather than positively transducing the signal.