Wavelength-Dependent Photoreaction of PYP from Rhodobacter capsulatus

PYPs (photoactive yellow proteins) are blue light sensor proteins found in more than 100 species. Compared with the extensive and intensive studies of the reactions of PYP from Halorhodospira halophila (Hh-PYP), studies of the reactions of other PYPs are scarce. Here, the photoreaction of PYP from Rhodobacter capsulatus (Rc-PYP) was studied in detail using ultraviolet-visible absorption and transient grating methods. Rc-PYP exhibits two absorption peaks at 375 and 438 nm. By using the transient absorption and the temperature-dependent absorption spectrum, the absorption spectra of two forms, pUV and pBL, were determined. Upon photoexcitation of pBL, two intermediates are observed before returning back to the dark state, with a time constant of 1.2 ms, which is 3 orders of magnitude faster than the dark recovery of Hh-PYP. Upon photoexcitation of pUV, two intermediates are observed to produce a long-lived final product, although one of the processes is spectrally silent. The diffusion coefficients decreased transiently for both pBL and pUV reactions, suggesting a relatively large conformational change during the reactions. It is particularly interesting to observe that the blue light irradiation of the long-lived product of pUV returns the product to the dark state. This result suggests different opposing responses of the biological function due to photoexcitation by ultraviolet and blue lights.