Phytochrome structure and photochemistry: recent advances toward a complete molecular picture

Phytochromes are nature's primary photoreceptors dedicated to detecting the red and far-red regions of the visible light spectrum, a region also essential for photosynthesis and thus crucial to the survival of plants and other photosynthetic organisms. Given their roles in measuring competition and diurnal/seasonal light fluctuations, understanding how phytochromes work at the molecular level would greatly aid in engineering crop plants better suited to specific agricultural settings. Recently, scientists have determined the three-dimensional structures of prokaryotic phytochromes, which now provide clues as to how these modular photoreceptors might work at the atomic level. The models point toward a largely unifying mechanism whereby novel knot, hairpin, and dimeric interfaces transduce photoreversible bilin isomerization into protein conformational changes that alter signal output.

► Structures of the photosensory module for several microbial phytochromes have been solved.
► A cryo-EM structure of a complete phytochrome is now available.
► The sum of these structural studies point to eventual light-driven changes in dimerization.
► Questions still remain about the initial events responsible for Pr to Pfr phototransformation.
► The structures obtained with microbial versions are likely pertinent to higher plant phytochromes.