Phytochromes With Noncovalently Bound Chromophores: The Ability of Apophytochromes to Direct Tetrapyrrole Photoisomerization¶†

Abstract

Chromophore–apoprotein interactions were studied with recombinant apoproteins, oat phytochrome (phyA) and CphB of the cyanobacterium Calothrix PCC7601, which were both incubated with the bilin compounds biliverdin (BV) IXα, phycocyanobilin (PCB) and the 3′‐methoxy derivative of PCB. Previously it was shown that CphB and its homolog in Calothrix, CphA, show strong sequence similarities with each other and with the phytochromes of higher and lower plants, despite the fact that CphB carries a leucine instead of a cysteine at the chromophore attachment position and thus holds the chromophore only noncovalently. CphA binds tetrapyrrole chromophores in a covalent, phytochrome‐like manner. For both cyanobacterial phytochromes, red and far‐red light–induced photochemistry has been reported. Thus, the role of the binding site of CphB in directing the photochemistry of noncovalently bound tetrapyrroles was analyzed in comparison with the apoprotein from phyA phytochrome. Both the aforementioned compounds, which were used as chromophores, are not able to form covalent bonds with a phytochrome‐type apoprotein because of their chemical structure (vinyl group at position 3 or methoxy group at position 3′). The BV adducts of both apoproteins showed phytochrome‐like photochemistry (formation of red and far‐red–absorbing forms of phytochrome [Pr and Pfr forms]). However, incubation of the oat apophytochrome with BV primarily yields a 700 nm form from which the Pr–Pfr photochemistry can be initiated and to which the system relaxes in the dark after illumination. The results for CphB were compared with a CphB mutant where the chromophore‐binding cysteine had been introduced, which, upon incubation with PCB, shows spectral properties nearly identical with its (covalently binding) CphA homolog. A comparison of the spectral properties (Pr and Pfr forms) of all the PCB‐ and BV‐containing chromoproteins reveals that the binding site of the cyanobacterial apoprotein is better suited than the plant (oat) phytochrome to noncovalently incorporate the chromophore and to regulate its photochemistry. Our findings support the proposal that the recently identified phytochrome‐like prokaryotic photoreceptors, which do not contain a covalently bound chromophore, may trigger a light‐induced physiological response.

Assembly of Synthetic Locked Phycocyanobilin Derivatives with Phytochrome in Vitro and in Vivo in Ceratodon purpureus and Arabidopsis

“…When recombinant phyB-N651 was mixed with BV, a low concentration of the bilin remained bound to the protein after the NAP column separation; the adduct was photoactive (see Supplemental Figure 3H online), in agreement with earlier studies (Jorissen et al, 2002). Assemblies with PCB and locked chromophores were again characterized by spectral changes indicative of covalent bond formation.…”

Apo-bacteriophytochromes modulate bacterial photosynthesis in response to low light

“…However, in Deinococcus radiodurans BphP (DrBphP), although alanine substitutions at this position disrupted covalent attachment of BV, DrBphP was still able to interact with BV and maintain a relatively normal absorbance spectrum (21). Similar results have been reported for other BphPs and phytochromes as well (22,23).…”

Photophysical diversity of two novel cyanobacteriochromes with phycocyanobilin chromophores: photochemistry and dark reversion kinetics

“…Thermal reversion has also been observed with both red and blue shifted states containing 15 E chromophores; the kinetics are furthermore influenced by the environment and by domain interactions within the holoprotein [52–56]. The reversion of the 585 nm state of GAF3 from AphC is unusually fast, which may relate to a function for light intensity sensing (see below).…”