Structural requirement of bilin chromophore for the photosensory specificity of phytochromes A and B

Phytochromes are an important class of chromoproteins that regulate many cellular and developmental responses to light in plants. The model plant species Arabidopsis thaliana possesses five phytochromes, which mediate distinct and overlapping responses to light. Photobiological analyses have established that, under continuous irradiation, phytochrome A is primarily responsible for plant's sensitivity to far-red light, whereas the other phytochromes respond mainly to red light. The present study reports that the far-red light sensitivity of phytochrome A depends on the structure of the linear tetrapyrrole (bilin) prosthetic group. By reconstitution of holophytochrome in vivo through feeding various synthetic bilins to chromophore-deficient mutants of Arabidopsis, the requirement for a double bond on the bilin D-ring for rescuing phytochrome A function has been established. In contrast, we show that phytochrome B function can be rescued with various bilin analogs with saturated D-ring substituents.