Biology Department Seminar

A complex array of photoreceptors coordinates the response of both prokaryotes and eukaryotes to their ambient light environment. One of the most influential is the phytochrome superfamily, a large and diverse group of photochromic photoreceptors that use a bilin (or linear tetrapyrrole) chromophore for light detection. These biliproteins sense red (R) and far-red light (FR) through two relatively stable conformations, a R-absorbing Pr form and a FR-absorbing Pfr form. By photointerconverting between Pr and Pfr, phytochromes act as light-regulated switches in various signaling cascades. Phytochrome-type photoreceptors were first discovered in higher plants by their ability to direct numerous R/FR photoresponses critical for agricultural productivity. More recently, they were also shown to exist in various cyanobacteria, proteobacteria, actinobacteria, fungi, and slime molds. Despite their agricultural importance and evolutionary conservation, we still do not fully understand at the molecular level how phytochromes photoconvert between Pr and Pfr nor how this switch initiates signal transduction. In the past few years, great strides have been made in determining how phytochromes function at the molecular level. Key was solving the first 3-D structures of the chromophore-binding domain (CBD) as Pr. These structures conclusively determined the conformation of the bilin linked to the apoprotein, revealed how the bilin is deeply buried in the CBD, showed that the CBD is uniquely folded into a rare figure-of-eight knot, identified a heretofore unknown dimerization contact in the CBD, and provided important clues for how plant phytochromes arose from their microbial progenitors. Work is now underway using novel thermostable phytochromes to solve the structure of Pfr. Eventually this work will provide a framework to redesign phytochromes for agricultural benefit.