Color Tuning in Red/Green Cyanobacteriochrome AnPixJ: Photoisomerization at C15 Causes an Excited-State Destabilization

Cyanobacteriochromes (CBCRs) are cyanobacterial phytochrome-like photoreceptors that carry a single or several GAF (cGMP phosphodiesterase/adenylyl cyclase/FhlA) domains in a repetitive manner. Unlike phytochromes that photoswitch been red-absorbing 15Z Pr and far-red-absorbing 15E Pfr states, CBCRs exhibit a much wider spectral activity. One of the best-characterized CBCRs, the phototaxis regulator PixJ of Anabaena sp. FCC 7120, AnPixJ can adopt two thermally stable photoreversible states, a red-absorbing dark state (Pr) and a green-atisorbing photoproduct (Pg). Cross-polarization magic-angle spinning (Cp/MAS) NMR spectroscopy on AnPixJ assembled in vitro with uniformly C-13- and N-15-labeled phycocyanobilin (PCB) chromophore identifies changes of the electronic structure of the chromophore between the two states. Results are compared with the data from red-, and far-red-absorbing forms of the Complete sensory module of cyanobacterial phytochrome aiming at a conceptual understanding of the distinct photoproduct (Pg Vs Pfr) absorbances upon Pr photoconversion. The PCB chromophore in the Pr state of both photosensors exhibits very similar spectral features. The photo conversion of Cph1 and the red/green switching AnPixJ C15-Z/E photoisomerization result in a very similar chemical-shift difference (Delta delta) pattern having, however, opposite sign. The persistence of this pattern confirms the identity of the photochemical isomerilation process, while the difference in its sign demonstrates that the same electronic factors drive into opposite direction. It is proposed that the LUMO energy of the 15E photoproduct is stabilized in Cph1 but destabilized in AnPixJ leading to opposite color shifts upon phototransformation.