Specific Chromophore-Protein Interactions in Bacteriophytochromes Rpbphp2 and Rpbphp3 from Rhodopseudomonas Palustris

Various organisms can sense light through a large family of signaling proteins known as photoreceptors. Upon absorption of a photon in the appropriate wavelength range, photoreceptors undergo structural changes in the chromophore, an organic pigment embedded in the photosensory module of the protein. Phytochromes are red-light photoreceptors originally discovered in plants and more recently in bacteria. They are unique in their ability to undergo reversible photoconversion between two photoisomerizable states, Pr (red light ∼ 700 nm) and Pfr (far-red light ∼ 750 nm). In the photosynthetic bacterium, Rhodopseudomonas palustris, a pair of bacteriophytochromes, RpBphP2 (P2) and RpBphP3 (P3) modulate synthesis of a light-harvesting complex. P2 and P3 have the same biliverdin chromophore (BV) and share 52% amino acid sequence identity, yet they have distinct photoconversion properties. P2, similar to classical bacteriophytochromes, alternates between Pr and Pfr states. P3 is unusual since it alternates between Pr and a unique Pnr (near-red light ∼ 650 nm) state. Our experimental goal is to identify amino acid residues that form specific interactions with BV during photoconversion and as such play key roles in forming distinct photoisomerizable states of P2 and P3. Through site-directed mutagenesis of conserved amino acids informed by structural and sequence analysis of bacteriophytochromes, we created a single-amino acid mutant variant of P3 T480P that undergoes classic reversible photoconversion between Pr and Pfr states. We also report on P2 and P3 mutants that show minimal absorption in the red region of visible light spectrum or are naturally more fluorescent than wild-type proteins. Currently, we are investigating chemical mechanisms that justify observed P2 and P3 mutant phenotypes.