Distinct classes of red/far-red photochemistry within the phytochrome superfamily

Abstract

Phytochromes are a widespread family of photosensory proteins first discovered in plants, which measure the ratio of red to far-red light to control many aspects of growth and development. Phytochromes interconvert between red-absorbing P r and far-red-absorbing P fr states via photoisomerization of a covalently-bound linear tetrapyrrole (bilin) chromophore located in a conserved photosensory core. From recent crystal structures of this core region, it has been inferred that the chromophore structures of P r and P fr are conserved in most phytochromes. Using circular dichroism spectroscopy and ab initio calculations, we establish that the P fr states of the biliverdin-containing bacteriophytochromes DrBphP and Pa BphP are structurally dissimilar from those of the phytobilin-containing cyanobacterial phytochrome Cph1. This conclusion is further supported by chromophore substitution experiments using semisynthetic bilin monoamides, which indicate that the propionate side chains perform different functional roles in the 2 classes of phytochromes. We propose that different directions of bilin D-ring rotation account for these distinct classes of red/far-red photochemistry.