1. Department of Plant and Microbial Biology, University of California, Berkeley, California 94720, and USDA/ARS Plant Gene Expression Center, 800 Buchanan Street, Albany, California 94710, USA

Correspondence to: Peter H. Quail¹ Correspondence and requests for materials should be addressed to P.H.Q. (e-mail: Email: quail@nature.berkeley.edu).

Abstract

The phytochrome photoreceptor family directs plant gene expression by switching between biologically inactive and active conformers in response to the sequential absorption of red and far-red photons¹,². Several intermediates that act late in the phytochrome signalling pathway have been identified, but fewer have been identified that act early in the pathway³,⁴. We have cloned a nuclear basic helix–loop–helix protein, PIF3, which can bind to non-photoactive carboxy-terminal fragments of phytochromes A and B and functions in phytochrome signalling in vivo⁵. Here we show that full-length photoactive phytochrome B binds PIF3 in vitro only upon light-induced conversion to its active form, and that photoconversion back to its inactive form causes dissociation from PIF3. We conclude that photosensory signalling by phytochrome B involves light-induced, conformer-specific recognition of the putative transcriptional regulator PIF3, providing a potential mechanism for direct photoregulation of gene expression.